Editor s note: Introduction Wind 32 Interface October 2003

Editor’s note: This article was developed from a paper originally published as part of the Proceedings of the RCI 18th Inernational Convention & Trade Show in Tampa, Florida, on March 13-18, 2003.

Introduction

This paper reviews the design considerations of air barriers, including the air pressures on buildings, the fundamentals of con­ trolling those pressures, air barrier properties, “air and vapor bar­ riers,” and the required properties of air barriers. Specific designs will be reviewed, and warm-side air and vapor barriers vs. cold-side air barrier systems compared. The complexities of the “air­ tight drywall approach,” or “ADA,” will be discussed. Finally, the paper will review roof air barrier concepts.

Figure 1

Unlike the moisture transport mechanism of diffusion, air pressure differentials can transport hundreds of times more water vapor through air leaks in the envelope over the same period of time (Quirouette, 1986). This water vapor can condense within the envelope in a concentrated manner, wherever those air leaks may be (Figure 1).

There are three major air pressures on buildings that cause infiltration and exfiltration:

• Wind Pressure • Stack Pressure • HVAC Fan Pressure

There are two other insignificant pressures, namely: • Changing atmospheric pressure

• Changes in temperature causing changes in air volume. Wind

The annual average wind pressure on buildings is of signifi­ cance in performing energy or moisture-related air leakage calcu­ lations for buildings. When averaged out over the course of the year, it is about 10-15 mph (0.2-0.3 psf) (10-14 Pa) in most loca­ tions in North America. Wind pressure tends to pressurize a building positively on the façade it is hitting, and as the wind goes around the corner of the building, it cavitates and speeds up con­ siderably, creating especially strong negative pressure at the cor­ ners and less strong negative pressure on the rest of the building

walls and roof (Figures 2 and 3), (Hutcheon and Handegord, 1983).

Stack Pressure

Stack pressure is caused by a dif­ ference in atmospheric pressure at the top and bottom of a building due to the difference in temperature and, therefore, the weight of the columns of air indoors vs. outdoors in the winter. Stack effect in heating climates can cause infiltration of air at the bottom of the building and exfiltration at the

Figure 3. Plan view of roof with contours showing negative pressure distribution. From Leutheusser, H.J., University of Toronto, Department of Mechanical Engineering, TP 604, April 1964, Fig. 15) (10.7)

Figure 2

Figure 4 top, as seen in Figure 4. The reverse

occurs in cooling climates with air conditioning indoors.

Fan Pressure

Fan pressure is caused by HVAC system pressurization, usu­ ally positively, which is fine in cooling climates but can cause incremental envelope problems to wind and stack pressures in heating climates. HVAC engineers tend to do this to reduce infil­ tration (and with it pollution) and disruption of the HVAC system design pressures relationships.

Figure 5 shows each of these pressures separately and a com­ bined diagram.

Energy codes in the U.S., including ASHRAE Standard 90.1 and the IECC, go to painstaking lengths in trying to identify all the instances where air leakage can happen through the building envelope and require that all such holes be sealed or gasketed. While this is a gallant attempt by code writers who have the right idea about air-tightening the envelope, this approach invites designers, builders, and enforcement officials alike to ignore such instructions. It is a crisis (Anis, 2001).

The code authorities have focused on quantifying the allowable air leakage of glazed openings, establishing standards and

lating organizations, and putting in place methods of testing and verification. On the other hand, the opaque envelope has been ignored. Today, in an average office building with the exterior walls at 50% glazing, the windows are responsible for 1.5% of the total infiltration, while the opaque envelope is responsible for a whopping 98.5%.

Opaque assemblies of a building 100 x 100,

two stories high, 50% glazing.

Width 100 Length 100 Height 28 Roof area: 10000 Slab area 10000 Gross Walls 11200 Windows 5600 Net walls 5600

Total envelope: walls, slab and roof 31200

Office Building

1.2 cfm/sf @ 75 Pa Total envelope leakage 37440 cfm @ 75 Pa Window leakage @ 0.1 cfm/sf 560 1.50% Opaque assembly air leakage 36880 cfm @ 75 Pa

(Proskiw, Phillips, 2001) The US Department of Energy reports that up to 40% of the energy used by buildings for heating and cooling is lost due to infiltration (BETEC/DOE/ORNL Spring

Symposium: Air Barrier Solutions, Pollock, 2001). Using CONTAM modeling shows that in newer buildings, infiltration is responsible for about 25% of the heating load and 4% of the cooling load (Emmerick, S.J. and Persily, A.K., 1998). The Canadian National Building Code, and more recent­ ly Massachusetts, take a more comprehensive and conceptual approach, namely requiring an air barri­ er “system” in the building envelope. A continuous air barrier system is the combination of intercon­ nected materials, flexible sealed joints, and compo­ nents of the building envelope that provide the

air-tightness of the building envelope and separations between conditioned and unconditioned spaces (Figure 6, Lux and Brown, 1986).

Air Barrier code requirements are summarized as follows: • A continuous plane of air-tightness must be traced

throughout the building envelope with all moving joints made flexible and sealed.

• The air barrier material in a system must have an air per­ meability not to exceed 0.004 cfm/sf at 0.3" wg (1.57 psf) [0.02 L/s.m2 _{@ 75 Pa].}

• The air barrier “system” must be able to withstand the maximum design positive and negative air pressure to be placed on the building and must transfer the load to the structure.

• The air barrier must not displace under load or displace adjacent materials.

• The air barrier material used must be durable or able to be maintained.

• Connections between roof air barrier, wall air barrier, win­ dow frames, door frames, foundations, floors over crawl-spaces, and across building joints must be flexible to withstand thermal, seismic, moisture, and creep building movements; the joint must support the same air pressures as the air barrier material without displacement.

• Penetrations through the air barrier must be sealed. • An air barrier must be provided between spaces that have

either significantly different temperature or humidity requirements.

• Lighting fixtures are required to be airtight when installed through the air barrier.

Figure 7

• To control stack pres­ sure transfer to the envelope, stairwells, shafts, chutes, and elevator lobbies must be decoupled from the floors they serve by providing doors that meet air leakage crite­ ria for exterior doors, or the doors must be gasketed (Figure 7). • Functional penetra­

tions through the envelope that are nor­ mally inoperative, such as elevator shaft louvers and atrium smoke exhaust systems, must be dampered with airtight motorized dampers connected to the fire alarm system to open on call and to fail in the open position (Massachusetts Energy Code for Commercial Buildings 780 CMR 13, 2001).

In addition, there are other pressure differentials within build­ ings that should be controlled by the following methods:

• Compartmentalizing and sealing garages under buildings by providing airtight walls and vestibules at building access points.

• Compartmentalizing spaces under negative pressure, such as boiler rooms, and providing make-up air for combus­ tion.

• Decoupling supply or return floor and ceiling plenums from the exterior envelope. If these are connected, serious consequences will arise that should be considered; the exterior walls become ducts with air forced through them, potentially causing severe condensation, microbial growth, and deterioration.

• Controlling convection currents within envelope assemblies caused by connecting air on the cold side to air on the warm side of insulation.

• Generic materials that meet the air leakage requirements stated above are as follows (Bombaru, Jutras, and Patenaude, CMHC, 1988):

If housewraps and other film membranes are not fully sup­ ported on both sides, as is the case in a brick cavity wall, they cannot support negative wind loads without tearing at the staples and brick anchors or rupturing under load (Bosack and Burnett, 1998). Housewraps in brick cavity walls displace under negative wind pressure and “pump” building air into the assembly, poten­ tially causing condensation in cold climates. One manufacturer of spunbonded polyolefin, in pre-qualifying its membrane for use as an air barrier material, discovered that to withstand negative wind pressures, a fastener with a 1" diameter washer or a brick tie must be installed 6" o.c. (150 mm), and 16" (400mm) apart (Figure 8). Alternatively, continuous strapping with a fastener every 12" (300mm) would work. It is also important to know that products sold in Canada and the U.S. with the same name do not have the same air leakage properties.

Polyethylene is even harder to make into an air barrier. It lacks structural support when it is against glass-fiber batts and has the inherent quality of displacing, stretching, and even

rup-Material Air Leakage

Thickness

of Non-Measurable Air Flow Measurable Air Flow @ 0.3" wg CFM L/(s/

2₎ @ 75 Pa 0.006" 0.060" 0.106" 0.001" 0.060" 0.374" 1" 1" 0.5" 0.5" *Polyethylene Roofing membrane

Modified asphalt torched -on

*Aluminum foil

Sheet asphalt peel and stick Plywood

Extruded polystyrene Foil-backed urethane Cement board

Foil-backed gypsum board

0.315" Plywood 0.63" waferboard 0.5" Exterior gypsum 0.433" waferboard 0.5" Particle board

*Non-perforated spun-bonded polyolefin

0.5" Interior gypsum board

0.001 0.001 0.002 0.002 0.003 0.004 0.004 0.0067 0.0069 0.0091 0.0108 0.0155 0.0195 0.0196

*Membranes must withstand air pressures in both directions without displacement or damage. If not fully adhered, they must be sandwiched between two board materials.

would be a low-perm vapor barrier material. In that case, it is called an “air and vapor barrier.” If placed on the predominantly cool, drier side (low vapor pressure side) of the wall, it should be vapor permeable (5-10 perms or greater). Figures 10A-H show a few examples of air barrier system design continuity and structur­ al integrity, excerpted from the reference details published by the Commonwealth of Massachusetts at

http://www.state.ma.us/bbrs/energy.htm.

Finally, the complexity of trying to air-tighten a structure on the interior face of the wall is worth highlighting (Figure 11). This approach, using the interior drywall as the airtight plane, is only useful in residential work where renovation is not expected for many years. In commercial work, however, the intent of the designer will most likely be lost to renovation. Also, rewiring for data lines happens so often that the drywall’s air tightness will be compromised as the data contractor punches holes above the ceil­ ing. Electric back-boxes must be also used to connect to the dry­ wall, caulking the wiring as it comes through. It is a very complex, three-dimensional problem, and this author’s best advice is, “Don’t go there.”

Figure 9 Figure 8

turing under high wind loads. It is also diffi­ cult to seam to itself or other materials (Figure 9). Fastener holes through polyethyl­ ene also compromise its air-tightness and can stretch (Shaw, 1985).

Materials that do not qualify as air barri­ er materials without additional coatings are (Bombaru, Jutras and Patenaude, CMHC, 1988):

• Concrete block

• Plain and asphalt impregnated fiberboard

• Expanded polystyrene

• Batt and semi-rigid fibrous insulation • Perforated house-wraps

• Asphalt impregnated felt, 15 or 30 lb. • Tongue and groove planks

• Vermiculite insulation • Cellulose spray-on insulation Of course there are many products for­ mulated to qualify as air barrier materials. Some of these, as well as specifications and technical help, may be found at

http://www.airbarrier.org.

Location of the air barrier

The air barrier, unlike the vapor retarder (since its function is to stop air, not control diffusion), can be located anywhere in the envelope assembly. If it is placed on the pre­ dominantly warm, humid side (high vapor pressure side), it can control diffusion and

Figure 10-A Figure 10-B

Air barriers on the exterior side:

Air barriers that are subject to thermal changes are more dif- The best tapes for non-moving joints are: ficult to keep airtight for the life of the building because of the • Silicone (extruded) bedded in wet silicone. integrity of the jointing tape or sealant over a long period of time. • Wet silicone reinforced with fiberglass mesh.

Figure 10-C

Figure 10-D

Figure 10-E

Figure 10-H Figure 10-G

• Other liquid-applied elastomeric air barriers products, reinforced with fiberglass mesh.

• Modified asphalt peel-and-stick with surface properly primed.

Roof Air Barriers

In short, if the above can be avoided, the building will be more The roof membrane can be considered an air barrier since it is durable for a longer period of time. designed to withstand wind loads if it is fully adhered or hot- or

Figure 10-I

Figure 11

cold-mopped. Mechanically fastened and ballasted roof systems, because they displace and momentarily billow or pump building air into the system, do not perform the required functions of con­ taining air without displacement. In those cases, another air bar­ rier must be selected in the system. Either a peel-and-stick air and vapor barrier on the inboard side of the roof system (interior conditions and weather dependent), or taped gypsum underlay­ ment board beneath the insulation can be used in a system with adhered under-layers of thermal protection board and insulation. Those layers must be designed to withstand maximum wind loads without displacement.

Because of the critical importance of continuity with the wall air barrier, a pre-construction conference on roofing must include a discussion of the connection between the roof air barrier and the wall air barrier, as well as the sequence of making that air­ tight and flexible connection. It is also important to ensure com­ patibility of the materials being joined together.

Penetrations into roof systems, such as ducts, vents, and roof drains, must be dealt with, perhaps by using spray polyurethane foam (or other sealant) or membranes to air-tighten those penetrations at the targeted air barrier layer.

Conclusion

An air barrier system is an essential com­ ponent of the building envelope necessary for controlling air pressure relationships within the building, ensuring intended performance of building HVAC systems, and enjoyment of good indoor air quality and a comfortable environ­ ment by the occupants. HVAC system size can be reduced because of a reduction in the “fudge factor” added to cover infiltration and the unknown, resulting in reduced energy use and demand. Air barrier systems in the build­ ing envelope also control concentrated conden­ sation and the associated mold, corrosion, rot, and premature failure; they improve and promote durability and sustainability. Building codes should require air barriers systems, and building designers and builders should be aware of the nega­ tive consequences of ignoring building air-tightness. ■

Bibliography

Anis, W., “The Impact of Air-Tightness on System Design,” ASHRAE Journal, 2001.

ASHRAE Handbook of Fundamentals, 2001.

Bosack, E.J. and E.F.P. Burnett, The Use of Housewrap in Walls: Installation Performance and Implications, PHRC, 1998.

Dalgliesh, W.A., and D.W. Boyd, Wind on Buildings, CBD 28, NRC, 1962.

Dalgliesh, W.A. and W.R. Schriever, Wind Pressure on Buildings, CBD 34, NRC, 1962.

Emerick, S.J. and A.K. Persily, “Energy Impacts of Infiltration and Ventilation in US Office Buildings Using Multi-zone Airflow Simulation,” A paper delivered at the ASHRAE IAQ and Energy Conference, 1998.

Garden, G. K., Control of Air Leakage is Important, CBD 72, NRC, 1965.

Hutcheon, N. and G.O.P. Handegord, Building Science for a Cold Climate, National Research Council of Canada, 1983. Lstiburek, J., Builder’s Field Guides, Building Science Corp.,

Westford, MA., 2001.

Lux, M.E. and W.C. Brown, Air Leakage Control, NRC, 1986. Massachusetts Energy Code for Commercial Buildings, 780

CMR, Chapter 13, 2001.

Persily, A.K., Envelope Design Guidelines for Federal Office Buildings: Thermal Integrity and Airtightness, NISTIR 4821 US Department of Commerce. 1994.

Proskiw, G. and B. Phillips, Air Leakage Characteristics, Test Methods and Specifications for Large Buildings, Prepared for Canada Mortgage and Housing Corporation, 2001.

Quirouette, R., The Difference

A

A

Between an Air Barrier and a Vapor Barrier, NRC, 1986.

Shaw, C.Y., Air Leakage Tests on Polyethylene Membrane Installed in a Wood Frame Wall, NRC, 1985.

Wilson, A.G., Air Leakage in Buildings, CBD 23, NRC, 1961.

Wilson, A.G. and G.T. Tamura, Stack Effect in Buildings, CBD 104, 1968.

Wagdy Anis’ career as an architect has spanned almost four decades. His professional focus is the integrity and performance of the building envelope from a research, design, and trouble­ shooting perspective, with an eye towards high performance, sus­ tainable building design, and indoor air quality. As a principal of Shepley Bulfinch Richardson and Abbott (SBRA), a national design firm based in Boston, he is head of Technical Resources. In that capacity, Anis is responsible for maintaining the high technical quality of the firm’s projects He also serves as director of sustainability at SBRA and manages the continuing education program. Anis has written many technical papers and is the author of Indoor Air Quality: A Design Guide, published by the Boston Society of Architects.