Editor s note: Introduction Wind 32 Interface October 2003

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1 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, Introduction This paper reviews the design considerations of air barriers, including the air pressures on buildings, the fundamentals of controlling those pressures, air barrier properties, air and vapor barriers, and the required properties of air barriers. Specific designs will be reviewed, and warm-side air and vapor barriers vs. coldside air barrier systems compared. The complexities of the airtight drywall approach, or ADA, will be discussed. Finally, the paper will review roof air barrier concepts. 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. Figure 1 Wind The annual average wind pressure on buildings is of significance in performing energy or moisture-related air leakage calculations for buildings. When averaged out over the course of the year, it is about mph ( psf) (10-14 Pa) in most locations 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 considerably, creating especially strong negative pressure at the corners and less strong negative pressure on the rest of the building 32 Interface October 2003

2 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 walls and roof (Figures 2 and 3), (Hutcheon and Handegord, 1983). Stack Pressure Stack pressure is caused by a difference 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 top, as seen in Figure 4. The reverse occurs in cooling climates with air conditioning indoors. Figure 4 Fan Pressure Fan pressure is caused by HVAC system pressurization, usually 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 infiltration (and with it pollution) and disruption of the HVAC system design pressures relationships. Figure 5 shows each of these pressures separately and a combined 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 regu- Figure 5 October 2003 Interface 33

3 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: Slab area Gross Walls Windows 5600 Net walls 5600 Total envelope: walls, slab and roof Office Building Air leakage of an average building 5.9 L/s/M 75 Pa Pa Total envelope leakage Pa Window 0.1 cfm/sf % Opaque assembly air leakage 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 recently Massachusetts, take a more comprehensive and conceptual approach, namely requiring an air barrier system in the building envelope. A continuous air barrier system is the combination of interconnected materials, flexible sealed joints, and components 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 permeability not to exceed cfm/sf at 0.3" wg (1.57 psf) [0.02 L/s.m 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, window frames, door frames, foundations, floors over crawlspaces, 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 6 34 Interface October 2003

4 To control stack pressure 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 criteria for exterior doors, or the doors must be gasketed (Figure 7). Functional penetrations through the Figure 7 envelope that are normally 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 buildings 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 combustion. 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 supported 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, potentially 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 0.006" *Polyethylene 0.315" Plywood 0.060" Roofing membrane 0.63" waferboard 0.106" Modified asphalt torched -on 0.5" Exterior gypsum 0.001" *Aluminum foil 0.433" waferboard 0.060" Sheet asphalt peel and stick 0.5" Particle board 0.374" Plywood *Non-perforated spun-bonded polyolefin 1" Extruded polystyrene 0.5" Interior gypsum board 1" Foil-backed urethane 0.5" Cement board 0.5" Foil-backed gypsum board 0.3" wg L/(s/ 2 75 Pa *Membranes must withstand air pressures in both directions without displacement or damage. If not fully adhered, they must be sandwiched between two board materials. Table 1 October 2003 Interface 35

5 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 structural integrity, excerpted from the reference details published by the Commonwealth of Massachusetts at 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 ceiling. Electric back-boxes must be also used to connect to the drywall, 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 8 turing under high wind loads. It is also difficult to seam to itself or other materials (Figure 9). Fastener holes through polyethylene also compromise its air-tightness and can stretch (Shaw, 1985). Materials that do not qualify as air barrier 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 formulated to qualify as air barrier materials. Some of these, as well as specifications and technical help, may be found at 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 predominantly warm, humid side (high vapor pressure side), it can control diffusion and Figure 9 36 Interface October 2003

6 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. October 2003 Interface 37

7 Figure 10-C Figure 10-E Figure 10-D Figure 10-F 38 Interface October 2003

8 Figure 10-G Other liquid-applied elastomeric air barriers products, reinforced with fiberglass mesh. Modified asphalt peel-and-stick with surface properly primed. In short, if the above can be avoided, the building will be more durable for a longer period of time. Figure 10-H Roof Air Barriers The roof membrane can be considered an air barrier since it is designed to withstand wind loads if it is fully adhered or hot- or October 2003 Interface 39

9 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 airtighten those penetrations at the targeted air barrier layer. Figure 11 Figure 10-I 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 containing air without displacement. In those cases, another air barrier 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 underlayment 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 airtight and flexible connection. It is also important to ensure compatibility of the materials being joined together. Conclusion An air barrier system is an essential component 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 environment 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 building envelope also control concentrated condensation 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 negative consequences of ignoring building air-tightness. Bibliography Anis, W., The Impact of Air-Tightness on System Design, ASHRAE Journal, ASHRAE Handbook of Fundamentals, Bosack, E.J. and E.F.P. Burnett, The Use of Housewrap in Walls: Installation Performance and Implications, PHRC, Dalgliesh, W.A., and D.W. Boyd, Wind on Buildings, CBD 28, NRC, Dalgliesh, W.A. and W.R. Schriever, Wind Pressure on Buildings, CBD 34, NRC, 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, Garden, G. K., Control of Air Leakage is Important, CBD 72, NRC, Hutcheon, N. and G.O.P. Handegord, Building Science for a Cold Climate, National Research Council of Canada, Lstiburek, J., Builder s Field Guides, Building Science Corp., Westford, MA., Lux, M.E. and W.C. Brown, Air Leakage Control, NRC, Massachusetts Energy Code for Commercial Buildings, 780 CMR, Chapter 13, Persily, A.K., Envelope Design Guidelines for Federal Office Buildings: Thermal Integrity and Airtightness, NISTIR 4821 US Department of Commerce Proskiw, G. and B. Phillips, Air Leakage Characteristics, Test Methods and Specifications for Large Buildings, Prepared for Canada Mortgage and Housing Corporation, Interface October 2003

10 Quirouette, R., The Difference Between an Air Barrier and a Vapor Barrier, NRC, Shaw, C.Y., Air Leakage Tests on Polyethylene Membrane Installed in a Wood Frame Wall, NRC, Wilson, A.G., Air Leakage in Buildings, CBD 23, NRC, Wilson, A.G. and G.T. Tamura, Stack Effect in Buildings, CBD 104, ABOUT THE AUTHOR 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 troubleshooting perspective, with an eye towards high performance, sustainable 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. October 2003 Interface 41