Design Considerations for an Air Barrier System

8/21/2019 Design Considerations for an Air Barrier System

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QBS 00c20a1-7 November 11, 2000 page 1 of 20

Design Considerations for

an Air Barrier Systemby

Rick Quirouette

Sandra Marshall

Jacques Rousseau

Abstract

Moisture problems in the cavities of exterior walls and roofs are often caused by the exfiltration of humidindoor air. Serious building envelope problems can result from moisture condensation within exteriorwalls and roof assemblies. They include efflorescence on brick and stone, spalling of brick and stone,corrosion of anchors and brick ties, paint peeling, icicles on the exterior facade, the rotting of sheathing

and wood framing, the production of mold and mildew and more. Indirectly, it also causes increasedrain penetration, high energy costs and low indoor humidity in winter.

The moisture problems occur when the humidity of the exfiltrating air condenses in a construction cavity,most often on the backside of a sheathing or cladding. The cause of this condensation can beprevented or at least controlled by the installation of an air barrier system, by limiting the indoor relativehumidity level in winter and by balancing the building pressure through its ventilation systems.

This article examines the problems of air leakage control, the concepts of air leakage paths and airpressure differences, the design requirements of air barriers systems, the properties of air barriermaterials and systems, the building code requirements, the design of air and vapour barriers. It alsoprovides notes on the preparation of drawings and specifications and site review considerations.

Objectives

After reading this article you should understand: 1. The types of problems caused by air leakage through a building

envelope 2. The nature of holes, openings and air leakage paths in the building

envelope 3. The concepts of air flow and pressure difference


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4. The structural loading effects of an air pressure difference on airbarrier materials and systems

5. Where the air barrier should be positioned or located in a wall, roof, orwindow 6. How to distinguish between air barrier materials, vapour barrier

materials and air/vapour barrier materials

7. How to interpret and comply with the building code requirements 8. On-site methods to test and commission air barrier

Introduction

Wetness in wood framed walls and wood rot inthe exterior sheathings of small buildings is oftencaused by air leakage. It is now known that airleakage also causes frost in attics, ice dammingat the edge of roofs, efflorescence on brick andstone facades, spalling of bricks and mortar,corrosion of anchors, the formation of frost in

cavities, icicles on facades, high energy costs, lowindoor humidity and more recently the cause ofmold and mildew in buildings.

During the early to mid eighties, it was realizedthat air sealing was not the same as vapourdiffusion control. It was discovered that airsealing required a different approach, to includespecific air sealing (air barrier) designrequirements, new materials and equallyimportant, revisions to the National Building Codeof Canada (NBCC).

Fig. 1 Efflorescence due to air leakage

Fig. 2 Ice on Faade


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In the 1985 edition of the NBCC, the airbarrier requirements were introduced asseparate and distinct requirements from thevapour barrier requirements. In 1986, theInstitute for Research in Construction (IRCand formerly DBR) presented the basicdesign and performance requirements for air

barriers to building professionals acrossCanada. At that time, the informationincluded a recommended maximum airleakage rate (air permeance) for the buildingenvelope of 0.1 Litres/(second x metre

2

)(L/s.m2) at 75 Pascals (Pa) pressuredifference across the construction sample foran average winter indoor relative humidity(RH) between 20% and 30%.

Recently, the Canadian Construction MaterialCentre (CCMC) established four permissibleair leakage rates, from 0.05 to 0.2 L/s.m

2

corresponding to various indoor humiditylevels and various exterior wall drying rates.The CCMC relates the maximum permissibleair leakage rate to the range of water vapourpermeance (WVP) of the outer layers ofconstruction material which are located on theexterior side of the air barrier plane. Forexample, a maximum air leakage rate of 0.2L/s.m2 at 75 Pa is permitted if no layer ofmaterial on the exterior side of the air barrier

plane has a WVP less than 800ng/(Pa.s. m2)

(16)

In the early 90s, Canada Mortgage and Housing Corporation (CMHC) undertook a study of11 high-rise buildings across Canada to determine how the air leakage rates of exterior walls

and windows of apartment buildings compared to the IRC target requirements. Specifically,air leakage rates for these buildings were determined by conducting suite, floor and wholebuilding depressurization tests. These buildings ranged in age from 2 to 33 years old. Theoverall leakage rates per unit of exterior wall area found was in the range of 0.7 to 7.8 L/s.m

2,

at a pressure difference of 50 Pa (standard pressure difference for a whole buildingdepressurization test) across the exterior wall (see Table 1). When the above leakage ratesare adjusted for a 75 Pa air pressure difference, the overall leakage rate for the exterior wallsis approximately in the range of 0.9 to 10.3 L/s.m

2. Therefore, the range of air leakage rates

from the exterior walls of high-rise apartment buildings was found to be between 9 and 103times larger than the target values recommended by IRC or CCMC. *

*The change in air pressure differences between 50 Pa and 75 Pa relates to whole buildingvs. exterior wall test methods. Houses tested in the late 70s used the 50 Pa differencecompared to curtain wall testing for large commercial buildings which used 75 Pa.

TABLE 1 - CMHC AIR LEAKAGE SURVEY(10)

No. Region BuildingNo.

Year Air Leakage (L/s.m2) @50 Pa.

1 Atlantic 1 1982 7.8

Fig. 3 Spalling Brick


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2 2 1983 4.1

3 Quebec 1 1991 2.2

4 2 1960 4.6

5 Ontario 1 -- 2.3

6 2 -- 2.0

7 Prairies 1 1986 7.7

8 2 1970 2.89 B.C. 1 1984 1.9

10 2 1991 0.7

11 3 1991 1.1

The leakage of air occurs when there is a hole, opening or air leakage path through theenvelope, and an air pressure difference applied across the envelope. There are threesources of air pressure difference which cause air leakage. These are stack effect, windandfan pressurization. Wind and stack effect are natural phenomenon related to outdoortemperature and barometric pressure variations. These phenomena cannot be changed ormodified. However, fan pressurization is caused by the building ventilation systems whichcan increase or decrease the indoor building pressure. In recent years, building pressure

due to ventilation design has increased resulting in increased air leakage problems.

Holes, openings and air leakage paths occur through all building envelopes to a greater orlesser degree.. This is the principal subject of this article - the design and construction of anair barrier system for the building envelope of new buildings. Todays challenge forarchitects and other building designers is threefold. First, air barrier systems must bedesigned to be more robust against wind pressures. Second, designers must develop betterdetails to improve the continuity of the air barrier function at joints, junctions and connections.Thirdly, HVAC systems must be designed to provide balanced building pressures and or toprovide control margins allow building operators to change the indoor building pressure asrequired.

Air LeakageAir leakage is the flow of indoor air or outdoor air through a hole or path leading from oneside to the other. The magnitude of air leakage can be determined for various situations.

For example, if an exterior wall has a hole, 25.4 mm by 25.4 mm (1 inch square) through the

wall and there is an air pressure difference of 10 Pa (0.04 in. water) from one side of the hole

to the other, air leaks through the hole at approximately 2.4 L/s. (4 cfm). While this does not

seem like much, the leakage of air through the same hole over one day would be (2.4 L/sec. x

60 sec./min. x 60 min./hr. x 24 hr./day or) 208,224 L/day. This represents a volume of air

measuring 216 m3*

(or 8000 cu ft.,) passing through the one inch square hole in one day. This air can carry a

great deal of water.*6 m x 6 m x 6 m or 20ft x 20ft x20 ft

Air Leakage and Condensation


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If this air were to leakthrough an exterior wall, itcould pass under thegypsum board edge at afloor wall junction and into a

wall cavity. From there itcould pass between the topplate and the interiorgypsum, move into the atticand then out through a roofvent. If this air were humid,it would transfer a greatdeal of moisture to theoutside or deposit it ascondensation.

For example, if the indoor

air were at 21oC (70

oF) and

30% relative humidity (RH),

it can be determined that

530 grams (approximately1 lb.) of water would escape to the outside in one day. However,

if the outdoor temperature was -10oC (14

oF) and the roof underside temperature was below

the dew point temperature of the leakage air, 3oC (37

oF), it is entirely possible that a

substantial portion of the 530 grams would condense on the underside of the roof sheathing

and possibly turn into frost. This is the process that is usually associated with the formation

of condensation and frost in attics and in exterior wall cavities.

Air Leakage and Energy

When air exfiltrates through the building envelope to the outside, the building loses heatenergy in winter and air conditioning energy in summer. The heat energy of exfiltration air isusually composed of the sensible heat (the energy required to raise the temperature of agas) of both the moist air and the latent heat of vapourization (the energy required to changea liquid to a vapour. When humid exfiltrating air loses moisture through condensation, it alsoreleases heat. This is the heat of fusion as water vapour (humidity) changes from a vapourto a liquid (condensation). When dry air is used to ventilate a wet area, the moisture mustabsorb heat from the surrounding air to vapourize the water in the area of wetness.

For example, if outdoor air at -10oC, 80% RH and the outdoor air infiltrates into a small

commercial building at the rate of 250 L/s, it will require approximately 9.38* kW of power toraise the temperature of the outdoor air to 21

oC. Also, If the infiltration air is humidified to

30% RH at 21oC from 80% RH at 10

oC, it will require approximately 2.48** kW power to

produce additional water vapour to raise the RH to 30% at 21 oC. Over 24 hours, the total

sensible and latent heat required is 285 kWh per day or ((9.38 +2.48) kW x 24.

*(1.21 x 250 x (21-(-10)))kW(14)

** (3000 x 2.3 x (0.0046-0.0013)(14)

Fig. 4 Attic frost due to air leakage


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If the leakage rate and outdoor temperature remain constant for a month, a smallcommercial building requiring 250 L/s of fresh air (infiltration +supply) would require 8682kWh to heat and humidify the infiltration and supply air. Commercial energy audits haveidentified air leakage as a major component of energy consumption, varying between 15%

and 35% of the total annual energy cost in commercial buildings equipped with heating

and cooling systems. The absence of an air barrier can result in excessive energy costsfor heating and cooling.

Holes, Openings and Air Leakage Paths

Air may leak through a building envelope via numerous types of holes, openings or paths.For example, it may infiltrate into an exterior wall at the weep holes of a brick cladding,through the imperfections of the brick ties and into the insulation cavity. It may exit into aroom from an electrical outlet or from under the gypsum board finish at the exterior wall.

Alternately it may exfiltrate through imperfections of a window frame or below a sill into awall cavity and up into a parapet to exit from the underside of a metal cap flashing. In theseexamples, it is difficult to estimate the effective cross sectional area and the flow path lengthbut it is certain that air leakage will occur and that condensation may accumulate in exteriorwalls during the winter.

Air leaking through the building envelope must find an opening or path to the other side.Openings or leakage paths may be classified as direct, diffuse or channel.

A direct openingisusually characterized as a hole,crack or joint through which youcan see to the other side (Fig. 5

Direct flow). The flow path isshort and direct. In this casethe leakage of air may passdirectly to the outside withoutleaving much condensation inthe roof or exterior wall butthere will be a loss of heatenergy and moisture from theinside. If the air escaping hasmore moisture than can besupported by the outdoor air atthat temperature, it will appearas fog.

Diffuse air flow is characterized by thescreen effect whereby air permeates throughthe body or surface of a construction materialto pass from one side to the other. Forexample air may be forced to pass through aconcrete block wall, glass fiber insulation,

Fig. 5 Direct Flow

Fig. 6 Diffuse Flow


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some building papers and fiberboard sheathing (Fig. 6 Diffuse flow).


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Channel flow is the most common type of opening or leakage path. It is characterized bya tortuous path from an opening on one side of a wall or roof to an opening on the other.Paths of this type include air leakage into outlets, wiring holes, the base of gypsum boardfinishes, through a cladding vent hole, along steel deck flutes and numerous other flow pathexamples. Some openings or leakage paths may develop after construction because of

differential shrinkage of lumber, as it dries out, thermal expansion and contraction of claddingelements, or the deflection of panelsunder wind load. Channel flow airleakage paths are the most difficult tofind. When they are found they areoften sealed at the entrance or exitopenings of the leakage path.

Air Pressure Difference

There are three principal sources of air

pressure difference in buildings. Theseare stack (or chimney) effect, wind andventilation systems. The net air

pressure difference across awall or roof may be acombination of all three, andit may vary from one locationof the building envelope tothe other. The magnitude ofthis difference can also varyconsiderably depending onthe shape of the building, theexposure, the height and

local conditions of outdoortemperature and humidity(Fig. 8).

Fig. 7 Channel Flow

Fig. 8 Stack Effect


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Stack effect is a condition which occursin all buildings in winter and summer. It

is the tendency for a mass of air at onetemperature (building air volume ) torise or fall in another mass of air at adifferent temperature (the outdoor air).In winter, the tendency is for building airto rise up and out of buildings. Thisresults from the difference in density ofthe indoor and outdoor air whichgenerally causes a slight outwardpositive pressure at the top of abuilding, while exerting a slight inwardnegative pressure at the base. As aresult, air will tend to infiltrate at thelower levels of the building whileexfiltrating at the upper levels. Insummer the effect is opposite. The

location in the building where the air pressure difference diminishes to zero (Fig. 9) is termedthe neutral plane (Np).

Typically, the stack effect pressure difference at the top of a reasonably air tight 10 storey

building, 30 m high (approximately 98 ft), with the neutral plane just above the 1st storey, 3

m above grade (10 ft), will be about 50 Pa (1 lb./ft2) of positive air pressure difference

(outwards) at the roof line when the outdoor temp is -20oC and the indoor air is +20oC. This

pressure difference is sustained against the roof and exterior wall as long as the temperaturedifference remains and the air leakage through the building envelope remains small. As theoutdoor temperature changes, the stack effect pressure also changes. It will becomesmaller as the outdoor temperature warms up and larger as the outdoor temperature falls toa lower level. Generally, stack effect is the most significant force causing air leakage inbuildings because the size and direction of the pressure difference is sustained for

months.

Wind pressure occurs on a building when the wind stagnates at a surface or facade. Whenthe wind impinges on a building, it causes an increase in air pressure on the windward sideof the building or facade and a decrease in pressure on the leeward and sides of the building.Similarly a flat roof will generally experience a decrease in pressure (or uplift pressure)

induced by the wind washing over the roof surface area.

When wind is blowing at a building surface, some of the wind will penetrate (infiltrate) to theinside on the windward side while some of the indoor air will escape (exfiltrate) to the outsideon the leeward and sides of the building and possibly from the roof surface. As the air leaksin and out of the building, the indoor pressure rises or falls until an equilibrium is reachedbetween the total infiltration and the total exfiltration. The indoor pressure is the resultant

Fig. 9 Stack Effect


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balance of the air infiltration and exfiltration. Generally, the indoor pressure falls slightly withrespect to wind pressures around and over a building.

Wind produces the largest air pressure differences on the exterior walls and roof of abuilding. It is not unusual to select a wind pressure base load of 0.5 kPa to 1.0 kPa (10 to 20lbs/ft

2) for most buildings in Canada. Wind gusts can be 2.5 times higher than the base load.

While the cladding and exterior walls are expected to support this load, it will be the airbarrier system which will support the majority of this load if it is continuous and air tight

and the exterior cladding is vented to the outside. However, for the air barrier to resistsuch loads, it must be structurally supported or attached. Wind loads can displace air barriermembranes or cause membranes and boards to rupture and leak permanently.

Fan pressurization. Ventilation air for a building is provided by fans. They are usually calledupon to introduce, exhaust and circulate the air in a building. The makeup and exhaust fansof a building may be set positively (make-up greater than exhaust) to pressurize the building.This configuration is provided in most commercial buildings including medium to high-risebuildings to minimize infiltration at lobby levels caused by stack effect. The fans may also beconfigured for negative pressure (exhaust greater than make-up) to prevent moist indoor airfrom exfiltrating through a roof or exterior walls. For example this configuration may be usedfor an indoor swimming pool in a cold climate.

Ventilation fans produce a small but significant air pressure difference across the buildingenvelope. This air pressure difference must be considered during the design of the buildingenvelope. Since the building requires an air barrier system, the HVAC designer must beadvised that ventilation balance must not only include consideration of the mass flow balanceof air within the building, but also consideration of the building pressure balance if themakeup and exhaust air are at unequal rates. Unusually high building pressure due toventilation system design and operation is becoming increasingly problematic with respect tocondensation problems in roofs and exterior walls. The condensation is produced by theexfiltration of indoor air through minor discontinuities in the air barrier system due to theresultant high building pressurization.

Design Requirements

Air barrier system must meet four (4) requirements. These are continuity, air impermeability,strength and durability.

Continuity requires that the air barrier of a wall must be continuous with the air barriersystem of a roof, window or below grade components. This continuity need not be achievedby the same materials throughout, but each material involved in the control of air leakagemust be connected to the others into a continuous plane of air tightness. For example, theair barrier of the roof at the perimeter must connect to the air barrier of the exterior wall.

Air Impermeabilitymeans the air barrier materials and system must be virtually air tight.Typically an air barrier system should not leak in excess of 0.1 L/s.m

2at 75 Pa air pressure

difference for modest humidity buildings. To achieve this level of performance, a designermust choose materials with an air permeability rating that is less than 0.1 L/s.m2at 75 Pa.This data is now available for many typical construction materials.


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Canada Mortgage and Housing Corporation undertook several studies to develop a testmethod and to obtain the air permeability of various construction materials. Forty to fiftytypical construction materials were conditioned at a standard temperature and humidity.They were then attached to a testing apparatus, essentially a metal box that could bepressurized or depressurized to force air through the material being tested in either direction.

The amount of air leakage that resulted was converted into an air permeability rating (Table2).


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Table 2. Air Permeability of Selected Building Materials Where Rate = L/s-m2at 75 Pa and Nml = No measurable leakage.No. Material Rate

1 smooth surfaced roofing, 2 mm Nml

2 modified torch-on membrane (glass matt) Nml

3 modified torch-on membrane (polyester matt) Nml

4 modified peel and stick membrane, 1.3 mm Nml5 polyethylene film, 1.5 mm (6 mil ) Nml

6 aluminum foil, Nml

7 plywood sheathing, 9.5 mm (3/8 in.) Nml

8 foil backed gypsum board Nml

9 extruded polystyrene insulation, 38 mm Nml

10 foil backed urethane insulation, Nml

11 cement board, 12.7 mm, Nml

12 gypsum board, (FB), 12.7 mm, Nml

13 THERMOLITE Insulation 0.0036

14 TYVEK HomeWrap 0.0039

15 plywood sheathing, 8.0 mm, 0.0067

16 waferboard, 16 mm 0.006917 gypsum board, (MR), 12.7 mm, 0.0091

18 Waferboard, 11 mm 0.0108

19 ISOCLAD insulation, 0.0114

20 spunbonded olefin film 0.0130

21 particle board, 12.7 mm 0.0155

22 gypsum board, 12.7 mm, 0.0196

23 particle board, 15.9 mm, 0.0260

24 tempered hardboard, 3.2 mm, 0.0274

25 TYVEK CommercialWrap 0.0400

26 expanded polystyrene insulation., type II, 0.1187

27 roofing felt, 30 lb., 0.1873

28 non-perforated asphalt felt, 15 lb., 0.2706

29 perforated asphalt felt, 15 lb., 0.3962

30 GLASCLAD Insulation 0.4480

31 fiber board, 11 mm 0.8223

32 fiber board with asphalt, 11 mm, 0.8285

33 perforated polyethylene #1 3.2307

34 perforated polyethylene #2 4.0320

35 expanded polystyrene insulation., type I, 12.2372

36 tongue and groove planks, 19.1165

37 glass-fiber insulation, 36.7327

38 vermiculite insulation, 70.4926

39 cellulose insulation 86.9457

Strengthmeans the air barrier system must be attached to a supporting structure and it mustresist excessive deflection, cracking, rupture or pull through at fasteners. The air barriersystem must withstand the highest expected air pressure load, usually wind, inward oroutward, without detaching from its support. It must also resist peak wind loads, a sustained


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stack effect or a sustained pressurization load without exhibiting creep load failure (gradualunbonding of a membrane from its support).

Durability requires that an air barrier system and its components be designed andconstructed to perform their intended function for the life of the building but more particularlythe life of the building envelope. It must be made of strong and robust materials with

adequate resistance to various environmental loads. Alternately, it must be positioned in thebuilding envelope so that it may be serviced as required or maintained at a reasonable cost.

An Air Barrier System

An air barrier system must bean integral part of the roof, anexterior wall or windows ofthe building envelope (Fig.10). It is not the combinationof materials comprising these

assemblies that is the airbarrier system but rather it isa specific plane of materialswithin the roof, exterior wall orwindow designated to performthe tasks of the air barrier ineach assembly. For example,the concrete of a cast in placeconcrete exterior wall havinginsulation and a metalcladding may be designated

as the air barrier component of this exterior wall. This is because the concrete is continuous,air impermeable, structurally adequate and durable. Similarly, an exterior sheet of metal with

a peel and stick membrane over the joints and the whole fastened to a steel stud frame maybe designated as the air barrier system for the same reasons as noted above (Fig. 11).

Polyethylene ilm is notsuitable as a air barriermaterial for commercialbuildings which areconstructed with non-combustible materials suchas concrete, steel, glass andaluminum. This is because itis too fragile, continuity is

difficult to achieve atpenetrations and joints (Fig.12) but more importantlybecause there is inadequatestructural support of themembrane against windpressure deformations.However, it is perfectly

Fig. 10 Complex Requirements for AB Continuity

Fig.11 Sheet Metal Air Barrier SystemFig. 12 Failed Poly Air Barriers


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suitable as a vapour retarder.

The air barrier function of the exterior wall must be joined with the air barrier function of theroof. This is sometimes difficult to achieve correctly particularly if the function of vapourretarder is confused with the function of the air barrier. For example, the waterproofmembrane of a roof is most likely to perform the function of the air barrier as it must also be

impervious to rain and melt water. In this type of building envelope, the roof membrane (airbarrier) must now be connected to the exterior wall air barrier (perhaps the concrete of ashear wall). However, because most exterior walls of commercial buildings terminate in aparapet, the exterior wall air barrier must extend to the roof membrane or the roof membranemust extend to the exterior wall air barrier.

In some cases, the two functions and the different types of materials may be joined by a thirdmaterial such as sheet steel, aluminum, elastomeric membrane or a caulked joint. In thissituation we refer to the third material as a link element. Note that a link element is alsosubject to the four (4) design requirements of air barriers. The link is sometimes designedas an accessory structural component to be installed during the structural part of theconstruction so that the other air barrier materials, roof membrane and/or exterior wallmembrane can be attached to the link during the later phases of construction.

An air barrier system for the building envelope is not an option. It is a mandatoryrequirement of the building envelope to limit uncontrolled air leakage. It must be designedusing robust materials, placed in a protected part of the roof or exterior wall or madeaccessible for maintenance and repair.

Air Barriers and Vapour Retarders

The function of the air barrier and the function of the vapour retarder are sometimesconfused. It is important to note that there is a significant and important difference betweenthese two functions and between the materials and systems providing these functions.

However, it is noted that these functions may also be provided by a single material or systemprovided the functional differences are well understood and the materials properties areselected as required. Specifically, a material or system functioning as a combined air/vapourbarrier must exhibit a low air permeance to resist the passage of air from an air pressuredifference and a low vapour permeance to resist the passage of vapour by diffusion when itis subject to a difference in humidity (water vapour pressure) between one side and theother.

In a building envelope design where both functions are to be provided by a single material,the plane of the air/vapour barrier materials must be on the warm side (or high vapourpressure side) of the building envelope for most types of buildings in Canada. If the airbarrier and vapour retarder functions are to be located at separate locations, the vapour

retarder function and material must be installed on the high vapour pressure side of the wallor roof while the air barrier function and material may be placed anywhere in the wall or roofsystem. However, when the air barrier is placed outside the insulation plane, the air barriermaterial selected must be permeable to water vapour or the design must ensure that watervapour will diffuse to the outside based on exact known conditions of indoor temperature,humidity and building pressure before use at that location.


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It is noted that vapour permeance is the rating of a material that determines the amount ofwater vapour which will pass through the body of a material by diffusion. This property ismeasured using a test procedure known as ASTM E-96. Within the ASTM E-96 procedure,there is a wet cup (A) or dry cup (B) test which may be specified. Both test proceduresmeasure the weight of water vapour which passes through a given area of material over aspecified interval of time. The result is usually expressed as water vapour transmission

(WVT), g/h.m2

, (grains/h.ft2

) and vapour permeance , g/Pa.s.m2

, (Perms, in.lb).

Air permeance is the rating of a material that determines the flow rate of air through the bodyof the material under a prescribed air pressure difference. This rating is measured using atest procedure known as ASTM E-283 and a pressure enclosure capable of receiving 1metre square samples of construction materials. By applying a pressure difference acrossthe sample and repeating the test several times in both directions at varying pressuredifferences, the leakage of air through the sample is measured and reported as airpermeance, usually in L/s.m

2 at 75 Pa. The air permeance values of Table 2 above were

obtained using this procedure.

The Building Code

Requirements for air barriers are stipulated in both Part 5 and Part 9 of the National BuildingCode of Canada (NBCC). The requirements are stipulated under Section 9.25.3 of Part 9(Housing and Small Buildings) of the 1995 NBCC and under Section 5.4 in Part 5(Environmental Separation) of the 1995 NBCC. The 1995 Edition of he NBCC has beenadopted by most Provinces in Canada. Professionals should verify the code requirements ofeach jurisdiction in which they practice. The 1997 Ontario Building Code has similarrequirements described in Section 5.4 Air Leakage.

The NBCC stipulates In Part 9, under Sub-section 9.25.3.1, sentence 1), Thermally insulatedwalls, ceiling and floor assemblies shall be constructed so as to include an air barrier system

which will provide a continuous barrier to air leakage. Further, the properties of air barrier

systems and the requirements for continuity are prescribed in sub sections 9.25.3.2 and9.25.3.3.

In Part 5, under Section 5.4, Sub-section 5.4.1, Air Barrier Systems, Sub-sub section5.4.1.1., Required Resistance to Air Leakage, under Sentence 1), the NBCC stipulates thatExcept as provided in Sentence (2), where a building component or assembly separatesinterior conditioned space from exterior space, interior space from the ground, or

environmentally dissimilar interior spaces, the component or assembly shall contain an airbarrier system.

It is further stipulated under Sentence 2); An air barrier is not required where it can be shownthat uncontrolled air leakage will not adversely affect any of

a) the health or safety of building users,

b) the intended use of the building, orc) the operation of building services.

Further, air barrier system properties are described in greater detail in sub-sub section5.4.1.2, Sentence 1 through 11 of the 1995 NBCC.

The building code requires an air barrier system for the building envelope for most types ofbuildings in Canada. The construction of an air barrier system and its particular prescriptive


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or performance requirements are generally provided in the design documents. It is thearchitect who prepares and produces the architectural documents. It is the architectsresponsibility to interpret the requirements of the Air Barrier Section of the applicable buildingcode and to apply them to the project under consideration. In this regard, it is most importantthat the architectural drawings, the wall sections and details and the specifications bedeveloped fully.

Drawings and Specifications

Working or productiondrawings typically illustrateplans, elevations, sections anddetails of a building project.Plans and elevations usuallypresent space layout andgeneral appearance of theexterior cladding systems.The section and detailsprovide information onassemblies with dimensions orwithout. Also, sections anddetails provide writtendescriptions of materialscomposing the variousassemblies and details.

With respect to air barriers, itis best to indicate the airbarrier function in a sub text, inbrackets, below a materials

description (Fig. 13 Detailindicating the air barrierfunction). In this way, thebuilder is alerted to theparticular function and itslocation within a part of thebuilding envelope. As thevapour retarder material andfunction may be confused withthe air barrier materials and

function, it is best to indicate both functions in the drawings and particularly if the functionsare to be provided by two different materials.

For any project, the performance requirements of the building envelope and the air barriersystem in particular must be described. The particulars of the air barrier systems of a projectshould include general descriptions, related works, materials, assemblies and standards ifapplicable and installation requirements. General specifications may be found in thefollowing Sections of the National Master Format: Section 07271, Air Barriers, Prescriptive orProprietary or Section 07272, Air Barriers by Performance.

Fig. 13 Detail Indicating Air Barrier Function


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Reviews, Mockups and Commissioning

The construction of an air barrier system is an integral part of the building envelope, but it isnot necessarily a specific product, an assembly or the responsibility of a particular sub tradewithin a project. The architect or designer must work with the builder to determine the most

appropriate sequence of construction to allow for the installation and testing of the air barriermaterials and system components. Design reviews with the responsible trades will alertthem to the importance of the air barrier installation quality and to the possibility of air barrierdiscontinuities. It will also reveal the absence of specific details, reveal unbuildable detailsand provide an opportunity to review protocols to determine compliance with the applicablebuilding codes.

Where a new assembly of materials is to be used or a new method of construction is to beundertaken, it is recommended that the specifications ask for a site mockup to verifyconstruction quality and performance values. If problems are determined early inconstruction, minor design changes may be undertaken at minimal cost and with a betterunderstanding of the final product performance and durability. Site mockups and testing

include smoke tracings for major leak locations and pressure testing for air leakage rates.

Commissioning the air barrier system is complicated by the fact that there are few rated airbarrier assemblies for buildings. If commissioning is to be done, it is best to start with areview of the air barrier design, followed by mockup testing of assemblies on site and wholebuilding testing with air leakage tracing using smoke. Quantitative measurements may beobtained through the application of two procedures and these are the ASTM E 283 and the

ASTM E 330 procedures. They involve the measurement of the air leakage rates through thebuilding envelope under a specified air pressure difference and the measurement of thestructural deformations under peak design air pressure loads. ASTM E-330 may be appliedto sample area of the air barrier system, but it must NOT be applied the whole building.

CCMC

The Canadian Construction Material Centre (CCMC) is a part of the Institute for Research inConstruction of the National Research Council of Canada. It offers an evaluation service,Registry of Product Evaluation for new and innovative construction materials, products,systems and services to all members of the Canadian Construction industry. It does so byresearching, investigating and testing the properties of a materials or system. The focus isperformance and durability for products systems and services and it issues CCMCEvaluation reports on a quarterly basis. These reports include various manufacturer airbarrier materials and systems. For a complete list of products, you should visit the followingweb site, http://www.nrc.ca/ccmc.

To obtain more information on the CCMC Evaluation process for air barriers, you mustprocure a copy of IRCs publication, Air Barrier System for Walls of Low-Rise Buildings:Performance and Assessment, NRCC 40635.

Questions

Question 1- How does air exfiltration result in the formation of icicles on claddings?


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Question 2- What was the recommend maximum air leakage rate for building envelopes inthe 1986 IRC Seminar for average humidity buildings?

Question 3- The CCMC has suggested four permissible air leakage rates for buildings. Thelargest rate is 0.2 L/s.m2. What is the minimum drying rate (perm rating) of the materials

outside of the insulation plane associated with this flow rate of air?

Question 4- If air leaks at 2.4 L/s for a day, what is the air change rate for a 100 m 2housewith 2.4 m high ceilings, 100 m

2of floor area and a heated basement with a ceiling height of

2.1 m?

Question 5- If air exfiltrates at 2.4 L/s for a day, how much water vapour is lost to theoutdoors if the indoor conditions are 21oC and 30% RH ?

Question 6- If air infiltrates at 250 L/s for a day and the air must be heated from -10oC, 80%

RH to 21oC, 30% RH, how much heat energy is required to heat and humidify the air to

indoor conditions for a month?

Question 7- What is the most common type of hole, opening or air leakage path through abuilding envelope?

Question 8- In which direction does air leak through the upper parts of an exterior wall of abuilding due to a stack effect in summer when there is no wind?

Question 9- Why is it necessary to attach or support an air barrier system in a wall or roofassembly?

Question 10- A building ventilation system may pressurize a building (indoor pressuregreater than outdoor pressure) to a level that is too high. What effect can this have on thebuilding envelope?

Question 11- What is the air permeability of 16 mm wafer board?

Question 12- In which Part and Section of the 1995 NBCC will you find the requirements ofair barriers for commercial buildings ?

Question 13- What Sections of the Master Format deal with air barriers?

Question 14- What are the units of vapour permeance and what are the units of airpermeance?


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Answers

Answer 1 - Icicles on cladding occur during cold periods when the sun thaws frost(condensation) behind a cladding which occurred because of air exfiltration and the meltwater drains to the outside through the weep holes to freeze as an icicle on the outsidesurface in the below freezing outside air temperature.

Answer 2 - The recommended maximum leakage rate for a building envelope in the 1986IRC Seminar was 0.1 L/s.m

2at 75 Pa.

Answer 3 - The minimum CCMC recommended drying rate for a permissible air leakage rateof 0.2 L/s.m2 is 800 ng/Pa.s.m2.

Answer 4 - The house will experience 0.46 air changes per day.

Answer 5 - The exfiltration air may lose up to 530 grams or approximately 1 lb. of water perday.

Answer 6 - The infiltration air may require up to 8682 kWh of energy to heat and humidify theinfiltration air to the desired indoor conditions for a month.

Answer 7 - Channel leakage.

Answer 8 - The outdoor air infiltrates or leaks in.

Answer 9 - Wind pressures can displace, detach or rupture the air barrier material if it is notstrong enough or adequately attached or supported.

Answer 10 - Excessive indoor building results when the makeup air supply rate exceeds theexhaust air discharge rates. This may cause an unnecessarily high air exfiltration ratethrough the joints and minor openings in the building envelope.

Answer 11 - 0.0069 L/s.m2at 75 Pa.

Answer 12 - Part 5, Section 5.4.

Answer 13 - Master Format Sections 07271 and 07272.

Answer 14- nanograms/[Pascal x seconds x metres square] (ng/Pa.s.m2). The air

permeance units in metric format is Litres/[second x metres square] (L/s.m2).


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References

1-Air Leakage in Buildings, Canadian Building Digest No. 23 of theNational Research Council of Canada, by A.G. Wilson, 1961.

2-Control of Air Leakage is Important, Canadian Building Digest No. 72of the National Research Council of Canada, by G. K.

Garden, 1961.3-Stack Effect in Buildings, Canadian Building Digest No. 104

of the National Research Council of Canada, by A. G. Wilson andG. T. Tamura, 1968.

4-An Air Barrier for the Building Envelope, Proceedings of the BuildingScience Insight 86, National Research Council of Canada,NRCC 22943, ISSN 0835-653X, Proceeding No. 13, Ottawa,January 1989.

5-The Difference Between a Vapour Barrier and an Air Barrier, by R. L.Quirouette, Building Practice Note No. 54, (BPN 54), of theNational Research Council of Canada, July 1985, ISSN 0701- 5216.

6-Air Permeance of Building Materials: Summary Report, a CMHC

research report prepared by Air Ins Inc., June 17, 1988.7-Airtightness Tests on Components Used to Join Different or SimilarMaterials of the Building Envelope, a CMHC research reportprepared by Air Ins Inc., Sept. 27, 1991.

8-CMHC Research Project: Testing of Air Barriers Construction Details II,a CMHC research report prepared by Morrison Hershfield Ltd.,March 31, 1993.

9-Commissioning and Monitoring the Building Envelope for AirLeakage, a CMHC research report prepared by MorrisonHershfield Ltd., Nov. 29, 1993.

10-Field Investigation Survey of Air Tightness, Air Movement and IndoorAir Quality in High-Rise Apartment Buildings: Summary Report,a CMHC research report prepared by Wardrop Engineering

Inc., July 1993.11-Air Leakage Control: Guidelines for Installation of Air Leakage

Control Measures in Commercial Buildings, a Public WorksGovernment Services Canada research report prepared byCanam Building Envelope Specialists Inc., et al.

12-The Development of Test Procedures and Methods to Evaluate AirBarrier Membranes for Masonry Walls, a CMHC researchreport prepared by ORTECH International, Nov. 2, 1990.

13-Structural Requirements for Air Barriers, a CMHC research reportprepared by Morrison Hershfield Ltd., August 13, 1991.

14-Air Leakage and Ventilation, Chapter 11 of a Building Science for aCold Climate by N. B. Hutcheon and G. O. Handegord, 1983.

15-Ventilation and Infiltration, Chapter 25 of the ASHRAE FundamentalsVolume, 1997.16-Air Barrier Systems for Walls of Low-Rise Buildings: Performance and

Assessment, published by the Institute for Research in Constructionof the NRCC, NRCC 40635, March 1997.

Design Considerations for an Air Barrier System

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