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Shallow Foundation Design, Construction and Repair in Colder

Climates

Fernando Pages

12/20/2012

A frost protected shallow foundation (FPSF) represents an alternative to deeper, more-costly foundations in cold regions with seasonal ground freezing and the potential for frost heave.

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Contents Shallow Foundation: Design, Construction and Repairs in Colder Climates ................................................. 3

US Acceptance of the FPSF Method ......................................................................................................... 3

A Brief History of Frost Protected Shallow Foundations .......................................................................... 5

FPSF Chronology ................................................................................................................................... 6

2012 SFPF Today ................................................................................................................................... 7

FPSF in Nebraska ................................................................................................................................... 9

Demonstration Projects .......................................................................................................................... 10

Green Rating Systems and FPSF .......................................................................................................... 11

Evaluating Building Sites ............................................................................................................................. 11

Shallow Foundation Design ......................................................................................................................... 13

Design Basics ....................................................................................................................................... 13

Simplified Design Method ....................................................................................................................... 14

Insulation ............................................................................................................................................ 16

Typical installation of the vertical insulation ...................................................................................... 17

The Dangers of UV Rays and Weed Whackers .................................................................................... 18

Designing and Installing Frost Protected Shallow Foundations for Unheated Buildings ........................ 19

Types of Protection for Insulation Used On Frost-Protected Foundations ............................................ 24

Acknowledgements ..................................................................................................................................... 25

Bibliography ................................................................................................................................................ 25

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Shallow Foundation: Design, Construction and Repairs in Colder

Climates Fernando Pagés Ruiz

A frost protected shallow foundation (FPSF) represents an alternative to deeper, more-costly

foundations in cold regions with seasonal ground freezing and the potential for frost heave.

The FPSF method incorporates strategically placed insulation to raise the frost depth around a building,

thereby allowing foundation depths as shallow as 14 inches, even in the most severe climates. The most

extensive use has been in the Nordic countries, where well over one million FPSF homes have been

constructed successfully over the last 50 years. The FPSF is considered standard practice for residential

buildings in Scandinavia, and has gained wider acceptance in the US through promotion within the green

building movement and acceptance in all of the model codes.

Most builders approach FPSF as a practical method to realize cost savings when constructing a slab on

grade foundation. A monolithic slab may still be the most affordable foundation, but if using

conventional techniques, the cost difference between slabs and basements and remains marginal—the

standard approach requires digging below the frost line (40-in or more in most areas of Nebraska) to

pour a deep footing before forming and pouring the slab in a two or sometimes three step process.

Considering excavation, forming and concrete costs, many designers and builders – including the author

-- reason there is more value in a building a basement than there are savings in slab on grade

construction. At least this was so until municipalities began to accept the frost protected shallow

foundation (FPSF) method with the adoption of the 2000 IRC and 2003 IBC (note: FPSF preexisted the

IRC under the 1995 CABO code).

US Acceptance of the FPSF Method Although pioneered in the US, the method actually developed in the colder regions of Europe and

Canada for more than 50 years before making it back to our shores, where it has been in practice

informally in parts of the US for over 25. In fact, Architect Frank Lloyd Wright designed and built a type

of FPSF intended to achieve affordability in his Usonian homes.

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Figure 1 shows an example of a frost protected shallow foundation and a conventional foundation

designed for a climate with an Air-Freezing Index (AFI) of 2,000°F (typical most of Nebraska) with a 100-

Year Return (winter) Period. Illustration courtesy National Association of Homebuilders Research Centeri

The principle of FPSF is simple, and yet counterintuitive, so in many areas of the US, including Nebraska,

it has remained a misunderstood technology, meeting with both builder and regulatory skepticism. To

make it easy to understand, the analogy of a down comforter is one way to explain how foam insulation

can change the frost depth under a building from about 40-in to 14-in. On a frigid winter night your body

loses internal heat and you feel chilly. Pull a comforter snugly under your chin and the quilt preserves

your body heat until you feel toasty warm. A FPSF works in exactly the same way as a comforter, except

using rigid foam insulation as quilt, and the heat emanating from the earth along with warmth

irradiating a heated building to keep the soil under the foundations from freezing.

By adding insulation around the exterior perimeter of the foundation wall, and in some climates

extending it beyond the perimeter horizontally at the base of the foundation wall, FPSF technology

captures the geothermal and conditioned building heat to effectively reduce the depth that frost can

penetrate around the perimeter of a structure. Even in the coldest climates, a typical FPSF only requires

between 1 in. and 2.5 in. of extruded polystyrene insulation installed on the face of the footing and

foundation wall, and in the coldest areas outward 12 in. to 36 in. horizontally at footing depth.

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Figure 2. From Building an Affordable House, Fernando Pagés Ruiz (The Taunton Press 2005)ii

The system can also be used to build unheated structures, as design methods exist to capture and

concentrate ground temperatures sufficiently to use FPSF in outbuildings. But perhaps the most

practical application of this technology in our region comes with eliminating the need for frost depth

(frost free) footings around an attached garage or other slab on grade structure (such as a porch, or

unheated sunroom) connected to a traditional basement foundation, such as occurs on the open side of

a walkout basement.

In addition to substantial construction cost savings, reduced excavation and concrete use, FPSF also

provide significant energy savings as the insulation required typically exceed existing code requirements,

including those of the 2009 IECC, now effective throughout Nebraska.

The subtleties of the FPSF require careful study. It’s important to select the right insulation, because

only a few products can maintain an effective R-value throughout the expected lifespan of a building,

and the design of FPSF is dictated by climate zone. Prescriptive design requirements are provided in the

International Residential Code (CHAPTER 403-03), and guidelines available from the NAHB Research

Center’s Revised Design Guide for Frost-Protected Shallow Foundations, both of which are useful design

resources. Once you learn the method for your region, it’s as easy to specify and install a FPSF as any

other foundation method.

A Brief History of Frost Protected Shallow Foundations In the US, slab-on-grade houses were constructed around the turn of the century in cold climates near

Chicago, Illinois. Frank Lloyd Wright designed and built a type of FPSF to meet affordability needs during

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the Depression, and used FPSF techniques in his "Usonian" style homes with shallow slab-on grade

foundations. However, the technology was never developed or applied to the degree realized in Europe.

In the 1950s, Swedish and Norwegian researchers constructed the first experimental houses using

insulated shallow foundations. These homes provided practical experience and empirical data on the

FPSF technology. By 1972, nearly 50,000 slab-on-grade foundations had been built in Sweden, and the

FPSF technology had gained wide acceptance. In the 1970s, the Scandinavian countries consolidated

their research efforts in an attempt to address cold regions engineering topics. The effort led to the

1976 publication of Frost I Jord ("Frost Action in the Ground")iii. Scandinavian engineers consider Frost I

Jord a reliable guide for design against frost action in soils. Based on the results of the Frost I Jord

Project, the Norwegian Building Research Institute started publication in 1978 of "Building Details"

specifically detailing FPSF design and construction methods. The FPSF technology is now considered

standard practice in Norway, Sweden, and Finland and throughout Northern Europe.

Even before the wide adoption of FPSF technology through code acceptance, it is estimated that several

thousand homes and other structures were built in the United States using the FPSF technology, but

only in circumstances where building codes are non-existent, or when special engineering was

performed, or where local conditions (e.g. Alaska) necessitate consideration of insulated foundations

placed at depths above the maximum seasonal frost penetration. Research in FPSF was conducted by

the National Association of Homebuilders Research Center (NAHB-RC) in Spirit Lake, Iowa with builder

Bill Eich, precisely because this community lacked regulatory obstacles. Spirit Lake did not, and still does

not have building code enforcement.

The major model building codes in the U.S. did not specifically address the FPSF technology until the

2000-2003 ICC model building codes, although both the CABO One and Two Family Dwelling Code and

the BOCA National Building Code recognized performance-based criteria for frost protection of

foundations since 1995. The ICBO Uniform Building Code (common to northern states) and the SBCCI

Standard Building did not permit any alternate methods of protecting foundations from frost heave—a

tradition continuing today in much of Nebraska.

FPSF Chronology

1930 Frank Lloyd Wright builder first FPSF in US

1953 1st FPSF House - Sweden

1968 Swedish Code Approval 1.5 million built since 1953

1984 NAHBRC Begins Research Norwegian Guide Translation, Study

1992 HUD Demos, NOAA work HUD Reports, AFI Maps

1994 NAHB/RC writes guide FPSF Design Guide

1995 US Code Approval CABO 1&2 Family Dwelling Code 18

1995 PATH Field Evaluations Reports on web

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2000 IRC Approval NAHB Publications, NOAA on web

2001 ASCE 32-01 Published Referenced in IRC & IBC

2002 IRC Amended Additions Garages OK

2012 SFPF Today

Presently, the 2006 and 2009 editions of the IRC and IBC, now effective in most jurisdictions of

Nebraska, provides a prescriptive basis for FPSF in single and two-family homes, and allows an

engineered approach in the IBC in compliance with the ASCE 32-01 performance standard. However,

some cities, such as Lincoln and Omaha have amended or deleted this section of the residential code.

Prescriptive code langue and figures excerpted from 2009 IRCiv:

R403.3 Frost protected shallow foundations. For buildings where the monthly mean temperature of

the building is maintained at a minimum of 64°F (18°C), footings are not required to extend below the

frost line when protected from frost by insulation in accordance with Figure R403.3(1) and Table

R403.3(1). Foundations protected from frost in accordance with Figure R403.3(1) and Table R403.3(1)

shall not be used for unheated spaces such as porches, utility rooms, garages and carports, and shall

not be attached to basements or crawl spaces that are not maintained at a minimum monthly mean

temperature of 64°F (18°C).

Materials used below grade for the purpose of insulating footings against frost shall be labeled as

complying with ASTM C 578.

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a. See Table R403.3 (1) for required dimensions and R-values for vertical and horizontal insulation

and minimum footing depth.

FIGURE R403.3 (1) INSULATION PLACEMENT FOR FROST PROTECTED FOOTINGS IN HEATED BUILDINGS

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TABLE R403.3 (1) MINIMUM FOOTING DEPTH AND INSULATION REQUIREMENTS FOR FROST-

PROTECTED FOOTINGS IN HEATED BUILDINGS.

AIR FREEZING

INDEX (°F-days)b

MINIMUM

FOOTING

DEPTH,

D(inches)

VERTICAL

INSULATIONR-

VALUEc, d

HORIZONTAL INSULATION

R-VALUEc, e

HORIZONTAL INSULATION DIMENSIONS

PER FIGURE R403.3(1) (inches)

Along walls At corners A B C

1,500 or less 12 4.5 Not required Not required Not required Not required Not required

2,000 14 5.6 Not required Not required Not required Not required Not required

2,500 16 6.7 1.7 4.9 12 24 40

3,000 16 7.8 6.5 8.6 12 24 40

3,500 16 9.0 8.0 11.2 24 30 60

Table 1. Highlighted: Most of Nebraska falls under the 2,000 freezing degree day index with some areas

of northern Nebraska in the 2,500 degree index.

FPSF in Nebraska

When adopting the 2009 IRC, the City of Lincoln amended this section of the code as follows:

20.12.400 Section R403.1.4.1 Amended; Frost Protection.

Section R403.1.4.1 of the International Residential Code is amended to read as follows:

R403.1.4.1 Frost protection. Foundation walls, piers and other permanent supports of buildings

and structures shall extended below the frost line specified in Table R301.2. (1).

Exceptions:

1. Protection of freestanding accessory structures with an area of 400 square feet or less, of

light-framed construction, with an eave height of 10 feet or less shall not be required.

2. Protection of freestanding accessory structures with an area of 500 square feet or less, of

light-framed construction, with an eave height of 10 feet or less shall be allowed to be a

monolithic slab as shown in Figure R403.3(1).

3. Decks less than 400 square feet not supported by a dwelling need not be provided with

footings that extend below the frost line. (Ord. 19674 §79; February 13, 2012).v

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Omaha, which still enforces the 2006 IRC and IBC, deleted Section R403.3 Frost protected shallow

foundations in its entirety. The City of Norfolk explicitly permits it, “Section R403.1.4.1 Frost Protection.

Amend exceptions to read as follows: 1. Frost-protected footings constructed in accordance with

Section R403.3 and footings and foundations erected on solid rock shall not be required to extend below

the frost line. 2. Accessory buildings less than one hundred eighty (180) square feet shall not be required

to be constructed with footings which extend below frost line. Concrete for slab on grade shall be a

minimum of four (4) inches thick and a grid work of number 4 rebar four (4) foot on center. S. Sioux City

permits FPSF. Freemont permits it when designed by a registered architect or engineer; similarly in

Papillion when sealed by a structural engineer.

In most instances, Nebraska building departments will require, and accept an engineered approach

based on ASCE 32-01 (American Society of Civil Engineers, Design and Construction of Frost-Protected

Shallow Foundations, 2001), stamped by a Nebraska licensed professional structural engineer or

architect. In Lincoln, the author has constructed 26 buildings and one commercial remodel using the

method.

Demonstration Projects Leading up to the first code approvals of the FPSF method, the NAHB-RC undertook construction and

monitoring of several homes in northern US climates using alternate methods of foundation frost

protection.

Phase I: One home was constructed and instrumented in each of three sites -Fargo, North Dakota, Spirit

Lake, Iowa, and Williston, Vermont. Phase II: During the 1993-94 winter season two additional homes

were built -one in Fargo and another in Palmer, Alaska to test the technology under the harshest

available winter conditions in the US.vi All of the homes were built on slab-on-grade foundations. The

styles of these homes varied from affordable townhomes and detached residences to custom built

homes. The FPSF design for each site was based on the established practice in Norway. Four sites were

constructed with extruded polystyrene (XPS) insulation and one site with expanded polystyrene (EPS).

Temperature sensors and an automated data loggervii were installed at each site during the construction

of the homes. Sensors were placed to obtain foundation temperatures, ground temperatures, indoor

and outdoor air temperatures and slab surface temperatures. Soil type, moisture content, and other

parameters were recorded at each site. Monitoring proceeded over two winters (1992-1994). Additional

homes (Phase 2) were monitored in the season 1993-1994, one of the coldest on record.

The frost heave potential at all of the sites was successfully mitigated for both the heated and unheated

areas of the homes and none of the homes experienced freezing of the subgrade soils supporting the

foundation. Test areas on two homes were purposely under-designed to subject the technology to a

severe test as would be expected should a 100-yr return period winter event occur. These designs were

also successful in preventing the potential for frost damage, and further validate the integrity of the

design recommendations. Cost savings, the driver for this technology in the early days, ranged from 1.1

to 3.8 percent of the total home sales price.

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Today, concerns with climate change and the general trend toward green building systems has renewed

interest in SFPF as a means to combine higher insulation values for energy efficiency, with low impact

site development (reduced excavation) and material sparing techniques (less concrete).

Green Rating Systems and FPSF

Purely from an embodied energy standpoint, shallow frost-protected and slab-on-grade foundations

have better environmental performance because they use less material. The leading green building

rating systems recognize this. LEED credits and NAHB points are available. In LEED, prerequisites include

slab insulation, an integral component of FPSF, as well as available credits for innovation, site

stewardship, and environmentally preferable products (when using 30% fly ash). ANSI National Green

Building Standard points are available for reduced site disturbance (Chapter 5), resource efficiency,

(with FPSF called out explicitly in Chapter 6), foundation insulation (Chapter 7), and foundation drainage,

a strongly recommended practice when using FPSF (Chapter 8).

Evaluating Building Sites The duty of the foundation designer is to establish the most cost-effective design suitable to the

structure and site conditions. FPSF are most suitable for slab-on-grade structures on sites with moderate

to low sloping grades. Although design methods developed for FPSF specify thermal resistances and

foundation depths that ensure protection against frost heave damage in most types of soils, frost

protected foundations are not a structural system — structures must still comply with building code or

accepted practice.

As with any foundation, organic soil layers (top soil) should be removed to allow the foundation to bear

on firm soil or compacted fills. By design, the proposed insulation requirements are based on the worst-

case ground condition of no snow and organic cover on the soil. The recommended insulation will

effectively prevent freezing of all frost-susceptible soils prevalent in our region.

Typical to Nebraska, most of the region has silty soil conditions with sufficient moisture to allow frost

heave but not so much as to cause the soil itself to drastically resist the penetration of the frost line.

Actually, a coarse grained soil (non-frost susceptible), which is low in moisture, will freeze faster and

deeper than silty soils, but with no potential for frost damage. Thus, the proposed insulation

recommendations effectively mitigate frost heave for all soil types under varying moisture and surface

conditions.

Although highly uniform in comparison to other regions where FPSF are commonplace (Wyoming, for

example), soil conditions do vary across Nebraska, and even within counties. Regional inexperience with

shallow foundations (not exclusive to the FPSF method) dictates caution, so the designer should

evaluate all available subsurface information, including site investigation and laboratory testing data,

when available, and choose a design method commensurate with the quality of available data. The

evaluation of soil conditions should be geared to determine bearing capacity and soil consistency. By

definition, shallow foundations transmit loads to the near-surface soils, the level most prone to have

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organic soils, fill, and disturbed soil with debris and / or preexisting structural remnants. It is also the

area most susceptible to wetting if expansive soils are present.

Regionally, less than 50% of Nebraska soils are underlain by soils with clays of high swelling potential,

except in the extreme southeast, where less than 50 percent of these areas are underlain by soils with

abundant clays of slight to moderate swelling potential. Scattered areas, particularly in the west, do

contain soils with abundant clays of high swelling potential, per the US Geological Survey.viii

Expansive soils contain minerals such as smectite clays that are capable of absorbing water. When they

absorb water they increase in volume. The more water they absorb the more their volume increases.

Expansions of ten percent or more are not uncommon. This change in volume can exert enough force on

a building or other structure to cause damage. This will occur with shallow and deep footing alike, but

shallow footings are more susceptible because they bear on soil closer to the surface and more likely to

get wet.

All construction projects should include a soil analysis to identify the types of soil present and determine

their expansive properties. Despite regional trends, local occurrences of expansive soils can be found in

all soil categories. On-site testing and observation may include sampling for laboratory tests performed

to define soil properties and identify those soils that do not conform to project specifications. For

moisture content, strength and stability, the early identification of issues helps avoid future problems

and allows for the correction of problems during construction.

Most contractors will use a backhoe or trencher to excavate the foundation. If a backhoe is used, the

bucket’s teeth will create a one-to-three inch layer of loose soil that should be compacted using a

vibratory plate on sandy soil, or impact, such as a mechanical whacker, on silty or clay soil. Once the

excavation and compaction/consolidation is complete, the designer or a soils consultant should inspect

the footings to verify that soil conditions match design assumptions. Inspectors sometimes check the

firmness of the soil using a 3/8-in diameter steel rod as a probe, or a soil penetrometer. On larger

projects with soils engineering and heavier loads, such as parking garages and roadways, a Proctor

compaction test may be appropriate. If the soil conditions are not as anticipated, especially if soft or

stratified, the area in question may require deeper excavation to reach good soil with adequate bearing

capacity. If the questionable area is small, sometimes the designer can use a grade beam approach to

span the deficient gap.

The general principles of foundation design and construction when concerns exist about site soil

conditions include the following:

Test to identify any problems.

Design to minimize moisture content changes and, when necessary, insulate structure from soil

volume changes.

Maintain a constant moisture environment after construction through proper drainage structure

and moisture management.

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You can find general information about Nebraska soils on a county-by-county basis, and specific

information on some cities, such as Lincoln and Omaha, at the US Department of Agriculture, National

Resources Conservation Services, web soil survey, Nebraska:

http://www.ne.nrcs.usda.gov/technical/soil_surveys.html

Shallow Foundation Design A frost protected shallow foundation allows builders to construct a structurally sound foundation that is

more resource efficient and less costly than a conventional foundation. Although typically associated

with slab on grade construction, the method may also be applied to stem wall foundations, and

unventilated crawlspace foundations, and used effectively with walkout basements by insulating the

foundation on the downhill side of the house. FPSF are also useful for remodeling projects and infill

construction sites because their installation minimizes site disturbance. In addition to residential,

commercial, and agricultural buildings, the technology has been applied to highways, dams,

underground utilities, railroads, and earth embankments.

There are two types of FPSF and two design approaches. The most common FPSF technology is for use

with heated structures, recognizing the thermal interaction of building foundations with the ground.

Heat input to the ground from a conditioned building, when captured by insulation, effectively raises the

soils temperature, and hence the frost depth at the perimeter of the foundation. This type of FPSF can

be designed according to the prescriptive method contained in the IRC R403.3 for single family and

townhouse construction and by reference to IRC R403.3 for low rise and multifamily buildings.

The second type of FPSF is for use with non heated structures, recognizing geothermal heat captured

under the building and foundations with insulating materials. This method, as well as all none-residential

construction categories using FPSF under the IBC, requires a licensed designer in compliance with the

ASCE 32-01 performance standard.

Design Basics

As stated, designers of SFPF can take two approaches: A simplified prescriptive design or a detailed

design. The simplified method adopted by the model building codes streamlines the design and material

selection process of FPSFs for heated buildings, but in consolidating the design steps for the simplified

method, R-values for insulation were established so that performance levels under various conditions

and slab surface temperatures are conservatively accommodated. Therefore, more economical

construction may be obtained when detailed design procedures are followed. The detailed design

procedure should always be used when buildings include unheated areas such as attached garages,

unless conventional footings are used for the garage.

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Figure 3. Frost Penetration into the Ground in Various Conditions.

Frost depth varies by ground cover and soil composition. Note how the dotted line, representing frost,

varies across the landscape. You will see that humus (the principle of mulching) retains geothermal heat

very well, and the frost depth in the humus area remains relatively shallow compared to the gravel,

which allows cold to penetrate and does not retain geothermal heat.

Simplified Design Method The IRC provides a simplified, prescriptive method of determining depth of your footing, as well as the

type of insulation require and its installed location.

To start, reference the Air-Freezing Index (AFI) for the region of your construction site. In most areas of

Nebraska, this will be 2,000. The AFI is an indicator of the combined duration and magnitude of below-

freezing temperature occurring during any given freezing season. The IRC provides a color coded, AFI

contour map (see Figure 4.), which works well in most areas except transitional areas where the AFI

range takes you from one set of specifications to another.

You can generally obtain accurate information at the local building department, or find more

comprehensive charts online at the National Climate Center (NCDC) website:

http://lwf.ncdc.noaa.gov/oa/fpsf .

The IRC also provides a state-by-state guide with county-by-county references R403.3 (2), available at:

http://publicecodes.citation.com/icod/irc/2012/icod_irc_2012_4_sec003_par020.htm.

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Figure 4. Air-Freezing Index Map (Estimated 100 Year Return Period)

Source: http://lwf.ncdc.noaa.gov/oa/fpsf

Once you know the AFI for the area you will build, look up the IRC Table R403.3(1), Minimum Footing

Depth and Insulation Requirements for Frost-Protected Footings in Heated Buildings (see table 1, page

6), and then cross-reference your AFI to the prescribe the minimum footing depth, as well at the R-Value

and placement of the insulation required. You will note most areas with an AFI from 2,500 to 4,000 only

require a 16-in footing depth, areas below 2,500 AFI require even less depth, saving considerable

excavation and concrete. In most areas of Nebraska, where the AFI is 2,000, the footing depth is 14-in

and only vertical insulation is required. But keep in mind, this applies only to frost protection, other

conditions, such as building loads and soils may require a deeper footing.

Trade tip, winter construction: As you study the FPSF method in preparation for your first

project, you will read warnings about freeze protection during construction. In theory, your

foundation should be completed and the building enclosed and heated prior to the first frost.

It’s a good rule to follow, but don’t panic if your foundation gets caught out in the cold—

everything will be okay. The system is designed very conservatively, and the author has built

many foundations in late autumn that were not covered, and much less heated before winter

came on, and none suffered any damage whatsoever. Other builders have experienced the

same, and this is not an exception. Again, the simple method for building a FPSF under a heated

structure is designed to benefit from the heat generated by the building. It is best to complete

the foundation, enclose the building and heat it prior to winter, but the design is robust and

FPSF are designed with a significant margin of safety.

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Insulation

It is important to select the right, rigid insulation, because only a few products can maintain an effective

R-value below grade and throughout the expected lifespan of a building. The author has not found

suitable products at the local home improvement center and had to special ordered the material from a

building material supplier instead (Millard Lumber). Ridged insulation comes in 4x8 sheets, so you must

to cut to size. Insulation suitable for footings must be labeled as complying with ASTM C 578, Standard

Specification for Rigid, Cellular Polystyrene Thermal Insulation. The actual design values for FPSF

insulation materials as prescribed in the IRC were calculated conservatively, 10% less than the nominal

R-value for extruded polystyrene (XPS) and 20% less than nominal values for expanded polystyrene

(EPS), to compensate for any degradation that might occur over time due to moisture and exposure. The

author has always used XPS on recommendation of a structural engineer. Extruded polystyrene (XPS)

insulation is suitable for the vertical and horizontal below grade application, while expanded polystyrene

insulation can only be applied to the vertical wall.

To find what R-value you’ll need, cross reference the IRC Table R403.3(1), Minimum Insulation

Requirements for FPSFs in Heated Buildings, and you will find the insulation R-values required for the

vertical insulation on the third column, Vertical Insulation R-Value, and, if needed, horizontal insulation

in column four. For example, looking at the AFI map, you will notice that most of the Front Range of

Colorado, all of Nebraska, Illinois, and even northern Michigan lie in the 1,000-to-2,000 AFI or warmer.

If you work in one of these areas, by cross-referencing the IRC Table R403.3 (1), you will find you will

need a minimum R-5.6 value for the vertical insulation with no horizontal insulation required (NR).

Trade tip, IECC: One thing to consider, if your are working under the 2009 International Energy

Conservation Code (IECC), the foundation insulation requirements may meet or exceed those of

the FPSF method, meaning there’s no good reason to dig below the frost line protect your

foundations from frost, since the insulation you’re using already may be sufficient.

On the other hand, if you’re working in northern Minnesota, where the AFI ranges from 2,000-to-3,000,

you will have to obtain specific information about your job site location from the local building

department (or the National Climate Center website) and use the vertical and horizontal insulation

specified (ranging from R-6.7 to R-7.8). You will also find values in columns A, B, and C for horizontal

insulation. These values specify the width of the horizontal insulation from the perimeter of the

foundation. They are separated into columns because different widths apply within a foundation, one

for the straightaway with a wider dimension at the corners. Column A describes the basic width along

the straight runs, and column B, the width at the corners, with column C indicating how long each

corner needs to extend from the angle. If horizontal insulation is required, all of this does increase the

cost and complication of using the FPSF method, so you will run a cost comparison to see if it makes

sense on your project. See FIGURE R403.3 (1) page 6.

Trade tip, installing horizontal insulation: If required, horizontal insulation must bedded firmly

on smooth ground and buried a minimum of 12" below grade. It’s easiest to over-excavate and

then backfill the area under the horizontal insulation with a granular base (sand or pea gravel).

When horizontal insulation extends more than 24", it must be protected by a hard surface, such

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as hard plastic, sheet metal, or even plywood, with backfill carefully placed to assure positive

drainage of water away from the foundation. It also follows that any landscaping planed should

avoid digging above the horizontal insulation.

Typical installation of the vertical insulation

Because in Nebraska foundation contractors generally install full basements and use aluminum forms,

they have little experience with slab on grade. The easiest (although not the least costly) method for

installation considering the local subcontractor skill set involves a three step process: After digging

shallow footings, form the exterior perimeter of the footings, right up against the excavation trench,

with XPS boards cut to width, place vertically, and stake to hold the insulation boards in place. It’s

important to avoid gaps between boards, especially at corners and joints. To secure the ridged

insulation, weave tie-wire through the foam and round the wooden stakes, tightening it with a quick

twist. When pouring concrete, a slightly dryer mix works best, to avoid getting a sloppy mix under the

insulation, which can cause it to lift.

Once the footings have set, install a stem wall as normally done on any footing, in our case using 2-foot

tall aluminum basement wall forms, or in other areas, concrete masonry block or wood forms. Once the

forms are striped, or the block grouted, laminate ridged insulation onto the exterior of the forms using

an adhesive marked “suitable for use with foam board,” or, more specifically, suitable for use with

polystyrene foam board. Avoid solvent adhesives, because they will dissolve the polystyrene insulation

boards. You will have to install an XPS ripping over top of the exposed edge of the footing, taking car to

secure until backfill is in place.

To a contractor experienced with slab on grade construction, all of this will sound like a lot of extra

work—and you’re right. The easiest and most economical use of the FPSF method comes with a

monolithic slab poured against a formed exterior perimeter edge lined with foam. To make sure the

foam sticks to the concrete and not the forms, the author has tacked the foam onto wood forms with

double headed nails and a washer, similar to those used to hold roofing felt, and tacked the nails onto

the inside of the foam just enough to hold the insulation in place.

Others have used a different approach, over-excavating the entire building pad, forming the building

perimeter with slab-on grade forms, filling the sub-slab area with structural fill and compacting, and

then hand excavating the perimeter and interior footings into the fill, thereafter pouring the entire

foundation monolithically.

Trade tip, termites: Anyone that builds in a heavily termite-infested area will have squirmed at

suggestions the author made in the preceding paragraphs. Termites love burrowing behind

foam insulation and find ready access the yummy parts of the building without anyone noticing

the tell-tale termite tunnels along the foundation wall. Unfortunately, this is one weakness of

the FPSF, and why the method is proscribed in areas of the country with significant termite

infestation risk, such as South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, the

eastern half of Texas, and most of California.

18

The Dangers of UV Rays and Weed Whackers

Since the insulation on the surface of the stem wall above grade will remain exposed to the

degenerative effects of ultraviolet light and the potential mechanical assaults of lawn service

equipment, you have to find a way to protect it. Many builders employ the same process and materials

used to finish exterior insulation and finish systems (EIFS), beginning with a polymer-cement basecoat

troweled over the foam insulation panels, followed by a glass fiber reinforcing mesh laid over the

polystyrene insulation panels and fully embedded in the base coat, and then finished with an acrylic

color-integral finish coat troweled over the base coat and reinforcing mesh. The only problem with this

system (although used frequently) is that the materials are not recommended for contact with grade,

and certainly not below grade. (I don’t know of any problems resulting from this application, but the

author developed a method that he felt more comfortable with, and seemed to provide a higher level of

protection for the foam.)

To protect the stem wall insulation, the author used aluminum coil, the same as used to wrap fascia.

To create a watershed over the insulation: Using a siding-break bend a “Z” shape counter-flashing

pattern along the top of the coil and then run the aluminum over the face of the insulation and six

inches below grade — much like you would over fascia, but upside down.

Photo 1. View of aluminum coil bent like “Z” flashing over top of FPSF vertical insulation to protect it

from UV rays and impact damage.

19

Trade tip, caulk the seams: Be aware that the protruding edge of the foam creates a ledge on

which water can become trapped, and may even drain back into the building beneath the sill

plate. The “Z” flashing method described will keep water out of the building, but you have to be

careful to caulk the laps, otherwise water can back up between the joints, as it can on any

horizontal flashing.

Designing and Installing Frost Protected Shallow Foundations

for Unheated Buildings Although the IRCC and ICC do not provide a prescriptive path, design criteria does exist for FPSF use in

unheated buildings—including garages and porches attached to a heated structure. The standards for

unheated buildings were developed by the American Society of Civil Engineers (ASCE), as the “Standard

for the Design and Construction of Frost-Protected Shallow Foundations, ASCE 32-01,” available for

purchase at: www.asce.org .

The use of FPSF for unheated buildings provides the greatest advantage when used for either large,

detached structures, such as a warehouse (most codes allow small detached structures on a floating

slab), or un-heated and semi-heated structures attached to a heated structure, such as an attached

garage or front porch. Note that the IRC specifically disallows this combination unless designed by a

professional engineer in compliance with ASCE 32-01. The added insulation and excavation required can

diminish some of the cost advantages of this approach, too, but in some climate zones it still makes

sense, and builders use it because it provides a more reliable and better insulated base under slabs.

In an unheated building (or the unheated portion of a heated building), the insulation must cover the

entire foundation floor area, and depending on the climate zone, extend beyond the perimeter of the

building up to four feet. In addition to the insulation, the subsurface fill must allow moisture to drain

away from the foundation, because the conditions required to create frost heave include cold, moisture,

and frost susceptible soil. Dry gravel, for example, will not heave in frost. So the combination of an

insulating blanket with proper sub grade preparation, which includes none frost-susceptible soils, often

conspires to make this technology more expensive than a conventional foundation. However, when

building over very hard, rocky ground, such as found in areas of the northeast US, or when desiring to

maximize insulation, or minimize excavation and ground disturbance, this alternative becomes a viable

alternative.

The insulation chosen for frost protection in unheated buildings must be adequate for subsurface

placement and meet R-value requirements. Since it will lie underneath the footings, the insulation must

also be able to support light structural loads. Guidance for all insulation selection and none frost

susceptible fill are offered in ASCE 32-01.

20

Figure 5. Detail of typical FPSF design for unheated building. Drawing courtesy Jay Crandell, PE with

Advanced Residential Engineering Services, a Nebraska licensed structural engineer.

Figure 6. Alternate view of unheated building section using FPSF. Drawing courtesy Jay Crandell, PE with

Advanced Residential Engineering Services, a Nebraska licensed structural engineer.

Other uses of this geothermal-capture technology include column-base footings, utility trenches (a

potentially money-saving application for sewer and water service), driveways and patios, and for

21

building projections, such as stairways (see illustration). These applications, while useful, are not

referenced in the IRC or ASCE 32-01. However, you can obtain guidance from a HUD translation of the

Norwegian guidelines (http://www.huduser.org/portal/publications/destech/frostprt.html )for FPSF, or

a monograph of the US Army Cold Regions Research and Engineering Laboratory titled, “European

foundation designs for seasonally frozen ground” by Omar Farouki (Cornell University 2010).

Figure 7. Insulation under stairways, driveways, and even over utility lines provides a number of cost-

effective means to prevent frost heave in secondary applications. Drawing courtesy Jay Crandell, PE

with Advanced Residential Engineering Services, a Nebraska licensed structural engineer.

Diagnosing and Repairing FPSF Problems

In preparing this monograph, the author searched for problems related to use of the FPSF method and

found none. The search included the internet and calls to builders using the method in various parts of

the country. Because no problems were identified, the material in this section is hypothetical. Two

reasons for the lack of reported problems may relate to the limited use of the method and the

conservative design parameters promoted by the model codes. In the words one Iowa builder the

author interviewed, Rollie Peschon, “The [FPSF] method is bullet-proof.”

The author has seen field results of defective installations both through observation of projects in

Colorado, Iowa, Montana, Wyoming and Nebraska, and his own early efforts to work out the installation

process. Problems would relate to either poor soil, such as highly expansive soils that damaged the

foundation, defective design, such as specifying inadequate insulation values for the climate, or more

likely, defective construction, including the common error of allowing cold bridging, see photo 2, page

20.

22

Photo 2. Cold bridging: The gap between insulation boards at the corner of this installation, as well as

the long, horizontal ledge of exposed foundation represent cold bridge defects in the FPSF installed in

2005 in Denver, Colorado. Despite these defects, the foundations have not suffered from frost heave

damage.

Areas of potential failure and how to mitigate them follow:

Inadequate insulation. Insulation installed below ground must meet ASTM C-578 requirements.

If insulation materials do not, and degrade over time through exposure to UV radiation and

moisture, the insulative quality of the material may be compromised. To repair, remove

inappropriate material and replace with approved insulation. If this proves impracticable,

consider the use of spray foam faced foundation material.

Undetected termite infestation. This type of foam should be carefully used on termite-prone

areas. If termite conditions are foreseen, the area must be treated against termite decay before

construction. In the event termite infestation exists with tunnels under the foam insulation,

treat ground around foundation with none repellant liquid termiticides designed for

subterranean termite control.

23

Subgrade Failure. Frost-protected shallow foundation must be built on soil with sufficient

bearing capacity. If foundation cracks due to settling of stratified or improperly compacted soils,

or when infiltration has caused erosion under foundations, repairs would entail the same

approaches used when similar circumstances occur on traditional foundations.

o The first step is determining the cause of the foundation structural failure.

o If the cause is subsidence, the usual repair entails underpinning, sometimes for a point

load, using a commercial system, such as Steel push piers, designed to give new

support to structures that have lost their original supporting soils. Like stilts, these

underpinning products will not only stabilize a sinking foundation but they also

can lift and hold the structure at its originally designed elevation. If length of

foundation support is compromised, such when the downslope side of a site

erodes, collapsing soil under the foundation, the repair can entail a new, concrete

grade beam pinned to the structure supported by piers or caissons if the soil

defect is deep enough to warrant.

o If the cause of failure is expansive soil, then either a chemical soil stabilization method,

such as the injection of lime and cement (or fly ash), widely used in stabilization and

improvement of the expansive subsoil. Other methods entail reinforcing the footing

with a grade beam strong enough to resist cracking and pinned to resist upward

movement. Since the forces at work are exceptionally powerful, this usually also entails

the removal of problem soil under the foundation and its reinforcement and the

installation of a flexible material that can absorb ground movement.

o In the unlikely event that the foundation failure is due to cracking or uplift from frost

heaving, the first solution is to improve foundation drainage and insulating

horizontally above footings.

Differential settlement. Frost-protected shallow foundation can be safely attached to deep

foundations following specific guidelines, but when these are not observed, differential

movement can cause structures resting on both foundations to crack, such as a wall connecting

an unheated garage and house. The general repair for this construction defect is to underpin

and attach the non-heated structure foundation to a new footing below the frost level.

Cold bridges. Cold bridges are created when building materials with high thermal conductivity,

such as concrete, are directly exposed to outside temperatures. Foundation insulation should be

placed such that continuity is maintained with the insulation of the house envelope. Although

frost heave damage is rare in FPSF, cold bridges may increase the potential for frost heave, or at

the least, create localized lower temperatures or condensation on the slab surface. Care must

be taken during construction to ensure proper installation of the insulation. If a cold bridge is

identified after construction, repairs would entail identifying the defect location and retrofitting

insulation. If necessary, remove concrete-to-concrete contact between insulated and none

24

insulated portions and fill with elastic insulation, such as below grade spray foam, if rigid foam

cannot be replaced in the gap.

Types of Protection for Insulation Used On Frost-Protected Foundations The most common problem with FPSF comes with damage to the vertical and / or horizontal insulation

due to lawn maintenance equipment and planting adjacent to the home. While shallow rooting plants

can be placed over horizontal insulation, it is best to keep this area free of vegetation and unwatered. To

avoid detrimental effects in areas where the insulation will be on direct contact with outdoor conditions,

there exist several alternatives that you can use to protect the insulation.

Treated plywood (foundation grade) can be used over both vertical and horizontal insulation.

Layers of stucco applied over wire mesh provide a strong and durable surface, if suitable with

the architectural finishes. Using a two or three coat method, it is possible to carve the color coat

to resemble brick, which is most often a desirable finish with most American vernacular home

styles.

Elastomeric stucco and elastomeric coatings are often used, but generally these products are

not recommended for contact with grade or below grade applications.

Cement siding or cement board replaces foundation grade plywood as an effective, durable

cladding that can take both weathering and mechanical assault. However, if painted, these

surfaces due absorb water and will cause the paint to blister.

Fiberglass-reinforced panels are often sold for the explicit purpose of protecting the top of

horizontal insulation and the exposed vertical insulation. These panels resemble those used to

inexpensively laminate tub and shower enclosures. Make sure the panels will not be susceptible

to UV radiation damage or yellowing.

25

Acknowledgements This course and monograph would not be possible without the significant contributions of Jay Crandell,

PE, ARES, and the work done by the NAHB Research Center (NAHB-RC) under grant from the US

Department of Housing and Urban Development (HUD), as well as information provided by Bill Eich of

Eich Construction and Rollie Peschon, both of Spirit Lake, IO, and pioneers of the FPSF method, having

participated in the construction of the first research home in the US under the auspices of the NAHB-RC.

All photographs in this document were taken by and belong to the author. Illustrations and tables were

adopted from US government publications available under public domain.

Bibliography American Society of Civil Engineers, SEI/ASCE 32-01, Design and Construction of Frost-Protected Shallow

Foundations, 2001. http://www.PUBS.ASCE.org

American Society for Testing and Materials. Standard Specification for Rigid, Cellular Polystyrene

Thermal Insulation. ASTM C 578 -92 Philadelphia, PA (1992).

Comite Europeen de Normalisation (CEN). Building Foundations--Protection against Frost Heave.

Preliminary draft for proposed European Standard N185, CEN TC 89/WG5 (August 1992).

Crandell, Jay H., Peter M. Steurer, and William Freeborne. Demonstration, Analysis, and Development of

Frost Protected Shallow Foundations and Freezing Index Climatography for Residential Construction

Applications in the United States.

Farouki, Omar. European Foundation Designs for Seasonally Frozen Ground.

Frost I Jord (Frost Action in Soil). Nr. 17, Oslo, Norway (November 1976); in Norwegian.

http://www.huduser.org/publications/destech/frostprt.html

International Code Council, www.iccsafe.org

Jones, C.W., D.G. Miedema, and J.S. Watkins. Frost Action in Soil Foundations and Control of Surface

Structure Heaving. U.S. Department of the Interior, Bureau of Reclamation, Engineering Research

Center, Denver, CO (1982).

Labs, Kenneth, et al. Building Foundation Design Handbook. Prepared for the Oak Ridge National

Laboratory by the University of Minnesota/Underground Space Center (May 1988); distributed by NTIS,

Springfield, VA.

McFadden and Bennett, Construction in Cold Regions: A Guide for Planners, Engineers, Contractors, and

Managers, J. Wiley & Sons, Inc., (1991).

Moisture Control in Buildings Chapter 4: Effects of Moisture on the Thermal Performance of Insulating

Materials. ASTM Manual Series: MNL 18. Heinz R. Trechsel, Editor. Philadelphia, PA (1994).

26

Morris, Richard A. Frost-Protected Shallow Foundations: Current State-of-the-Art and Potential

Application in the U.S. Prepared for Society of the Plastics Industry, Inc. NAHB Research Center, Upper

Marlboro, MD (August 1988).

http://www.nahb.org/generic.aspx?genericContentID=3326

NAHB Research Center for U.S. Department of Housing and Urban Development, Design Guide for Frost-

Protected Shallow Foundations, Second Edition. www.nahbrc.org

NAHB Research Center for U.S. Department of Housing and Urban Development, Frost Protected

Shallow Foundations in Residential Construction, (1994).

National Oceanic and Atmospheric Association (NOAA), Climatic Data for Frost Protected Shallow

Foundations Publications and Data. http://lwf.ncdc.noaa.gov/oa/fpsf

NOAA, Mean Annual Temperature,

http://www.ncdc.noaa.gov/oa/climate/research/cag3/cag3.htmlCommittee on Frost Actions in Soils.

Norwegian Building Research Institute. Frost-Protected Shallow Foundations for Houses and Other

Heated Structures, Design Details. Forskningsveien 3b, Postboks 322, Blindern 0314, Oslo 3, Norway.

Translated by the NAHB Research Center (January 1988).

Proceedings of the 7th International Cold Regions Engineering Specialty Conference. Edited by D.W.

Smith and D.C. Sego, Canadian Society for Civil Engineering, Montreal Quebec, (1994).

U.S. Army Corps of Engineers, Cold Regions Research & Engineering Laboratory, Monograph 92-1,

Hanover, NH (March 1992).

Steurer, Peter M. and Jay H. Crandell. Comparison of the Methods Used to Create an Estimate of the Air-

Freezing Index. National Oceanic and Atmospheric Administration, National Climatic Data Center,

Asheville, NC (March 1993).

Steurer, Peter M. Methods Used to Create an Estimate of the lOO-Year Return Period of the Air-Freezing

Index. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National

Climatic Data Center, Asheville, NC (1989); Appendix A of SPI Phase II report.

End Notes i U.S. Department of Housing and Urban Development, Office of Policy Development and Research: FROST-PROTECTED SHALLOW FOUNDATIONS Phase II - Final Report (HUD 1994) Instrument No. DU100K000005897. ii Building an Affordable House: Trade Secrets to High-Value, Low-Cost Construction, Fernando Pagés Ruiz (The Taunton Press, 2005). iii HeIersted, R.S. Frost I Jord (Frost Action in Ground), Nr. 17. CommIttee on Frost Action in Soils. Prepared In NorregIan for the Royal NorregIan Council for Scientific and Industrial Research (Pslo, Norray). 1976. iv International Residential Code for One- and Two-Family Dwellings, 2009, Chapter 4. Foundations: R403.3 Frost protected shallow foundations.

27

v LMC Chapter 20.12 - City of Lincoln & Lancaster County Chapter 20.12 LINCOLN RESIDENTIAL BUILDING CODE 20.12.400 Section R403.1.4.1 Amended; Frost Protection. vi Because the foundations in Phase I were designed for a 100-year return period winter freezing event, the average winters experienced during 1992-93 did not provide a stringent performance test of the FPSF designs. Consequently, two additional demonstration homes were built in Phase II with the intention of subjecting the FPSF designs to a severe design event. The foundations were designed using the normal 100 year return period, with exception of a test zone created in each design. The insulation in the test zone was sized for an average winter event for that climate, instead of the 100-yr return period as normally required in the design guidelines. vii Each of the five demonstration homes were outfitted with automated data acquisition systems to record and download performance data to the NAHB Research Center in Upper Marlboro, MD. Temperature sensors were installed in the ground and around the foundations during construction in the summer of 1992 (Phase I homes) and the summer/fall of 1993 (Phase II homes). Other sensors were placed to obtain indoor air temperatures and slab surface temperatures. A weather station that measured ambient air temperature, wind speed, and frost penetration into undisturbed soil was also deployed at each site. Both thermocouple and thermistor sensors were used and, at some locations, sensors were duplicated to allow simple data quality checks. Automated dataloggers were installed when the homes were completed. viii "Swelling Clays Map of the Conterminous United States" by W. Olive, A. Chleborad, C. Frahme, J. Shlocker, R. Schneider and R. Schuster. It was published in 1989 as Map I-1940 in the USGS Miscellaneous Investigations Series.