Preventing Mold and Other Moisture...

Preventing Mold and Other Moisture Problems

William W. Hill, Department of Urban Planning, Ball State University, Muncie, IN 47306 1

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Preventing Mold and Other Moisture ProblemsA Community Workshop

presented by

William W. Hill, Ph.D.Department of Urban Planning, Ball State University

Sponsored by the Community Outreach Partnership Center and East Central Reinvestment Corporation

December 12, 2002 2

Topics we will cover

• Mold• Fundamentals of moisture movement

– Relative humidity, absolute humidity & temperature

– Moisture transport mechanisms• Bath fans • Recessed lights• Crawl spaces

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Common causes of moisture problems we will not cover

Leaking pipes Roof leaks4

Peeling paint is often, but not always, the result of a moisture problem

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Recommended reading• Builder’s Guide: Cold Climates, by Joseph

Lstiburek. (1998). Published by the Energy and Environmental Building Association and available from the EEBA bookstore at EEBA.org

• Moisture Control Handbook, by Joseph Lstiburek and John Carmody (1993). Published by Van Nostrand Reinhold and available from the EEBA bookstore at EEBA.org

• Building Science Corporation web site http://www.buildingscience.com

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Mold -- the mother of all callbacks

• Mold is a large and growing problem in homes across the U.S.

• Not at all unexpected by building scientists



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Necessary but not sufficient conditions for mold growth

• Mold spores• Nutrient base• Temperature between 40oF and 100oF• Relative humidity greater than 70%

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Mold is a moisture problem

• Mold is not contagious• Mold does not come from “infected”

building materials• Mold does not come from the factory

or the sawmill

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Building Science

It’s not rocket science But it is science

Necessary but not sufficient conditions for a problem

• A source• A hole• A driving force

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Necessary but not sufficient conditions for a moisture problem

• Source• Hole• Driving force



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Moisture fundamentals

• Moisture moves in response to well understood physical forces. We call these “driving forces”

• The physics is the same no matter where you are

• The physics is the same no matter what the building

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Essential definitions• Absolute humidity = actual amount of water vapor in

air (See also humidity ratio, vapor pressure)

• Relative humidity (RH) = ratio of amount of water vapor in air to the amount that the air could hold at that temperature

• Dew point = temperature at which RH = 100% and condensation occurs (See also saturation temperature)

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Temperature and relative humidity

• As air is heated, the amount of water vapor it can hold increases. If the absolute humidity remains constant, the RH will decrease.

• As air is cooled, the amount of water vapor it can hold decreases. If absolute humidity remains constant, the RH will increase. If the air is cooled enough, it will hit dew point at the water vapor will condense into liquid water.

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Psychrometric Chart

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Two questions we can answer with the psychrometric chart

• Why is the air inside many homes dry in winter?

• How do walls get wet as a result of air leaking out of houses in winter?

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Why interior air is dry in winter



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The usual “band-aid” solution to dry winter air: a humidifier

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A better solution to dry winter air: seal the air leaks and stop

the infiltration

Other benefits of sealing air leaks:• No moisture being dumped into

the attic• More comfortable home• Lower heating bills

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How walls get wet from leaking air

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Insulation without an air barrier is asking for trouble

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Bath fans are needed for point source ventilation

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Ducts for bath fans need to carry the moisture all the way to the outside



Moisture transport mechanisms

#1 Liquid flow#2 Capillarity/surface tension#3 Air movement#4 Vapor diffusion

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Moisture transport mechanism #1

Bulk moisture movement by gravity and momentum

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Moisture transport mechanism #1 in a nutshell

• Buildings get wet when it rains• It rains!• Drain the rain off the building• Direct it away from the building• Provide a way for the building

components to dry when they get wet

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35[Carmody & Anderson]

Got to get these basics right!

36[Carmody & Anderson]

Poor guttering worse than none at all



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41 42

We’ve known about the importance of perimeter foundation drains for many years



43[Moisture Control Handbook, Fig. 2-21.]

For homes in this area, the perimeter

drain is quite possibly the most

important moisture control detail

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Incorrectly installed housewrap

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What’s the purpose of the housewrap?

• Air barrier• But it’s also the drainage plane!

– It took the place of building paper– Best attribute of building paper is that

installers are thinking “moisture control” when they install it

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Think “shingles” when you install housewrap

• Pieces above must overlap those below, always directing water to the outside

• Details around windows are easy to get wrong

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Capillarity and surface tension

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Drip loops are important

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With no drip loop, the

antenna lead acts like a pipe,

directing water into the house

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55 56[Moisture Control Handbook, Fig 2-20]

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59 60[Moisture Control Handbook, Fig 2-24



61[Moisture Control Handbook, Fig 2-26] 62

63[Moisture Control Handbook, Fig 2-29]

Or Cedar BreatherTM or similar product

64[Moisture ControlHandbook, Fig 2-19]

Capillary rise in foundations

65[Moisture ControlHandbook, Fig 2-21]

Preventing capillary rise in foundations

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Air movement



The fundamentals: Necessary but not sufficient conditions for moisture

transport by air movement

• A source• A hole• A driving force

Moisture sources for transport by air movement

• Exterior: Hot humid air in cooling climates

• Interior: Wet crawl spaces, bathing, cooking, etc

Principal driving forces for air movement in buildings

• Wind-induced pressure (positive on windward side, negative on leeward side)

• Stack effect (warm air rises)• Pressure differences caused by the HVAC

system

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Stack effect -- a 24/7 driving force

• Warm buoyant air produces a pressure difference(driving force) 24 hours a day all winter long

• Creates the following pressure distribution between inside and outside of house:– Positive pressure at top of house– Negative pressure at bottom– Zero pressure difference halfway between top and

bottom (the neutral pressure plane)

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Stack effect fundamentals

• Pressure is directly proportional to height and temperature difference

• The greater the pressure, the greater the driving force

• The greater the driving force, the more infiltration and exfiltration

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Stack effect demonstration box

• Can be used to model any building• The “heating system” is a 250 W light bulb• Set to model a single story home

on a day when the temperature difference between inside and out is 40oF

e.g., Tinside = 70o F Toutside = 30o F

Stack effect demo -- questions

• Most important air leakage sites?• Most important doors in a house?• Solution to cold floors, frozen pipes in crawl space

or basement?• Solution to radon in the basement?• Why does the CO from the attached garage get

drawn into the house?• How can failing to air seal the bottom plate result

in an expensive call back?


How air pressure leads to wetting

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Recessed lights: Big holes in the worst possible place, the top of the house



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Recessed light as seen from the attic

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Recessed lights in cathedral ceilings

cause major moisture problems

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Results of 1993 Penn State tests on air leakage through recessed light fixtures• Energy costs: $5 to $30 per light fixture per year• Moisture: About 1/3 gallon per day• Assumptions:

– Indoor air at 70oF and 40% RH– Attic at 32oF– 10 cfm continuous air leakage

• 20 fixtures => 180 gallons per month!

Source: EDU, January 199482

One solution: Airtight fixturesCost: About $5 more per fixture

New construction Remodel

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Airtight fixture for sloped ceilings

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Back to our mold-growing second story window



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Source: Interior humidity condensing on cold storm

Hole: Air leaks around primary window

Driving force: Stack effect

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Vapor diffusion

Moisture transport by vapor diffusion

• Sources: Same as for air transport• Holes: Not that critical; a vapor diffusion

retarder that is 5% holes is still 95% effective

• Driving force: Vapor pressure

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Vapor diffusion & vapor barriers

• It’s what builders and architects have been taught to think is important

• It’s the least important of the moisture transport mechanisms

• I have never seen a moisture problem resulting from the lack of a vapor diffusion barrier

• I have, however, seen problems caused by a vapor barrier

89Source: Builder’s Guide, Cold Climates, Fig III.1

90Builder’s Guide, Cold Climates, Fig III.3

Vapor diffusion is far less important than air pressure

in moving moisture



Terminology is important: What’s the purpose of that poly?

• Vapor diffusion retarder– Does not have to be continuous or perfect– Vapor diffusion = vapor pressure x area x

permeability

• Air barrier– Should be as continuous and perfect as possible– Air under pressure will find the “weak links” -- the

holes in the air barrier

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Drying potential

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Walls and other building components need to be able to dry

• Assume they will get wet• The primary moisture transport mechanism

for drying is vapor diffusion• Thus, while walls seldom get wet from

diffusion, they mostly dry via diffusion• They must be able to dry to either the inside

or the outside94

Vapor barriers in the wrong place are worse than none at all

• A vapor barrier (i.e., a vapor impermeable material) on both sides of a wall gives the wall no place to dry

• When in doubt, leave it out

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A wall designed to dry to the outside

[Builder’s Guide, Cold Climates, Fig III.4]96

A wall designed to dry to the inside

[Builder’s Guide, Cold Climates, Fig III.5]



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A wall with no place to dry

[Builder’s Guide, Cold Climates, Fig III.17]98

Vapor diffusion from the outside can cause problems

• This is a primary moisture transport mechanism in the South (cooling climate)

• It can be a problem anywhere with brick veneer walls built with a vapor diffusion retarder on the inside (as per code!)

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Sun on brick => inward vapor diffusion

[Moisture Control Handbook, Fig. 3-22]100

Why the increase in mold problems behind brick veneer in new homes?

• Polyethylene vapor barriers installed under drywall as per code

• New exterior sheathings are more permeable than plywood

• Poor detailing in brick work

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Latest recommendation on poly vapor retarders from Lstiburek

• Polyethylene should not be installed on the interior of any assembly – with the exception of above grade walls and ceilings in locations with 8,000 heating degree days or greater (Muncie has 5900 HDD)

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There is one place where a vapor diffusion retarder is absolutely essential -- as a ground cover in

the crawl space



Foundations

The cause of many moisture can be traced

to the foundation --crawl space or basement

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Why crawl spaces are a problem more often than basements

• While homeowners wouldn’t tolerate a wet basement, crawl spaces are ...– Out of sight, out of mind

• Building codes, code interpretation and common practice do not properly address moisture control in crawl spaces

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A damp foundation can provide an almost unlimited source of moisture for distribution throughout the house

Let’s apply our understanding of building science fundamentals to understand how poor foundation design and detailing can result in mold in the attic (and elsewhere)

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Moisture transport, foundation to attic• Source of moisture: rain, snow melt and

groundwater [Bulk or liquid moisture]• Poor site drainage, no perimeter drain allows

wetting of foundation [Liquid flow]• No capillary break allows groundwater to

wick to surface of soil and concrete/block wall in crawl space [Capillarity]

• No vapor barrier on soil and blocks allows moisture to enter air in crawl [Diffusion]

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Foundation to attic, continued

• Lack of continuous air barrier allows moist air to be carried by stack effect to the attic [Air transport]

• Moist air cools in attic, condenses on roof sheathing [Condensation and/or elevated relative humidity]

• Conditions are ripe for mold growth and rotting of roof sheathing, etc.



The bottom line on foundations --both crawl spaces and basements

1 Pay careful attention to site grading, guttering and downspouts

2 Install a good perimeter drain, surrounded by pea gravel and filter fabric

3 Install a capillary break on top of the footing (damp proofing) and over the ground (4 inches of pea gravel)

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Bottom line, continued

4 Install a quality vapor diffusion retarder (6 mil poly) on the ground and up the inside walls

5 Insulate and air seal the perimeter walls, including the access door

6 Provide a continuous air barrier in the house above

7 Run all condensate lines to outside

111[Builder’s Guide, Cold Climates, Fig. 4.17] 112[Moisture Control Handbook, Fig 5-15]

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Moisture control details, “short basement”

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Short basement with concrete floor



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Perimeter drain must drain to daylight or a sump

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What’s the difference between a crawl space and a basement?

Answer 1: About 4 feet

Answer 2: Building code requires vents in the shorter one

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Crawl space venting is not a solution

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No amount of venting can deal with poor surface water drainage

Crawl space vents: Conflicting code requirements

• Thou shalt install operable vents in crawl spaces -- CABO One and Two-Family Dwelling Code– Requires minimum net free area of 1/150 of crawl

space area, 1/1500 with an approved vapor retarder

• Thou shalt insulate crawl spaces -- CABO Model Energy Code (MEC)– Requires either R-19 floor insulation or R-10 walls



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The “science” behind crawl space venting requirements

• Requirement dates to 1942 FHA Property Standards• None of the research in Britton’s [1948] seminal

paper appears to support the recommendation for venting

• Conclusion: “There is no technical basis in the literature for current or past crawl space ventilation requirements”Source: Bill Roses’ 1994 ASHRAE paper

Ventilation cannot do the job

• Evaporation cannot remove the amount of water that comes into a crawl space that has poor site drainage and is lacking a perimeter drain

• Worse, venting provides the appearance of a solution, which prevents builders, architects and code officials from focusing on real solutions

Crawl space ventilation does not work and can make matters worse

• In summer, venting crawl spaces will add moisture to the house– Outside air in summer is hot and humid.– Use the psychrometric chart to see why that air can

only add moisture to the crawl space• In winter, vents, if not closed and made airtight,

will result in higher heating bills

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Psychrometric Chart

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Effect of crawl space vent position on space heating (research on one house

over seven winters)

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