AIA Continuing Education: optimizing moisture protection and air barrier systems
Buildings Teams have long sought to keep water out of buildings, to prevent corrosion, rot, and other damage and to keep building occupants comfortable and dry. Yet only recently has air infiltration also been seen as a threat—this time, to efficient operations and return on investment. At long last, the building codes are also finally beginning to reflect what building scientists have argued all along: that air barriers are an essential component of building enclosure design.
Some states—namely Massachusetts, Wisconsin, and Michigan—were early adopters, requiring air barriers in residential and commercial construction. A number of others, notably Rhode Island, Georgia, Minnesota, and Florida, have recently called for air barriers in buildings through their state building codes and energy codes, according to the Air Barrier Association of America (www.airbarrier.org).
In addition, two standards developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (www.ashrae.org) also require air barriers: ASHRAE 90.1—Energy Standard for Buildings Except Low-Rise Residential Buildings and ASHRAE 189.1—Standard for the Design of High-Performance, Green Buildings. The barriers must pass tests developed by ASTM International (www.astm.org). The standards group created ASTM E 2178 for air-barrier materials and ASTM E 2357 and E 1677 for assemblies intended to block air infiltration.
And there’s more. Soon, with the 2012 release of the International Code Council’s International Building Code (www.iccsafe.org), the International Energy Conservation Code (IECC), and the International Green Construction Code (IGCC), a new air-barrier requirement will be directly referenced in the documents used to guide more than 95% of U.S. construction.
Clearly, with air barriers migrating from optional to mandated, Building Teams, specifiers, and construction professionals must have a strong grasp of how the technology works—and how best to apply these products for optimal results.
UNDERSTANDING AIR AND MOISTURE CONTROL
While it’s well understood that air leakage compromises heating and cooling efficiencies, it’s important to realize that air transports moisture and water vapor, too. “Condensation is primarily caused by air movement into or through building assemblies,” says Wagdy Anis, FAIA, LEED AP, a principal of Wiss, Janney, Elstner Associates (www.wje.com), Boston, and a leading force in the Building Enclosure Council movement (www.bec-national.org). “Air movement is due to either convection looping into building assemblies, entraining water vapor to a surface that is colder than the dew point within the assembly, or from infiltration and exfiltration due to air pressure differentials cause by wind, stack effect, or HVAC pressurization,” he notes. In non-Wagdyan terms, air and moisture control is a very complex, intricate issue.
After reading this article, you should be able to:
- Understand issues of air and moisture permeance and penetration in building façades of various kinds to enhance occupant health and welfare.
- Describe materials and systems for employing air barriers, vapor barriers, and other moisture-control systems for building enclosures.
- Resolve potential design and constructability conflicts between cladding elements and moisture/air control systems.
- Compare products for controlling air and moisture infiltration for improved indoor environmental quality.
For starters, the Air Barrier Association of America provides a definition for air barriers by rating materials based upon their air permeance; this indicates the amount of air that is able to migrate through the material itself, whether it’s rigid, flexible, or fluid-applied. Tested in accordance with ASTM E2178, the air permeance must be equal to or less than 0.02 liters of air per second per square meter of space at 75 pascals of pressure (in U.S. units, 0.004 cfm/sf @ 1.57 psf) in order to qualify as a recognized air barrier.
Working with a good-quality air barrier, Building Teams can be assured that only trace amounts of air will migrate through the material itself. However, it is the building enclosure’s interfaces, transitions, and penetrations that prove the most difficult to detail and seal. “Don’t make the assumption that with enough caulk you can keep the water out,” cautions Mark Finneral, AIA, a staff architect with Shepley Bulfinch (www.shepleybulfinch.com), Boston.
“While sealants are a good way to limit air and water penetration through a cavity wall’s outermost plane, these products don’t last as long as sheet membranes. So they should not be used as the sole water-resistive air barrier in concealed, hard-to-reach locations behind cladding or at transitions,” advises Jason S. Der Ananian, PE, a senior staff member in building technology with Boston-based Simpson Gumpertz & Heger (www.sgh.com).
One important rule of thumb for vulnerable joints is to seal and flash these points with two layers of protection. According to Judd Peterson, AIA, president of Judd Allen Group (www.juddallen.com), Edina, Minn., the inner seal remains intact while the outer seal takes most of the weather abuse, as long as there is a weeping mechanism in between the two seals. The goal is to create a continuous barrier throughout the entire plane, while allowing for differential structural movement.
“If you can’t trace an unbroken line continuously across all assemblies and details, then the detail isn’t correct,” says David Altenhofen, AIA, an enclosure specialist with The Façade Group, Philadelphia.
For example, it’s not unusual for the material serving as the air and water control layer to change across an intersection. Similarly, different materials must be carefully joined together. This sometimes requires the introduction of an additional product to address compatibility concerns and accommodate movement at the joint, explains Altenhofen. Along with Anis, Altenhofen was one of the initiators of the AIA and National Institute of Building Sciences (www.nibs.org) effort to create the national and local Building Enclosure Councils; he is co-chair of the BEC national committee and a board member of the NIBS Building Energy and Thermal Envelope Council, or BETEC.
HOW TO ACHIEVE CONTINUITY OF PROTECTION
One useful tool for addressing continuity of air and vapor protection is a spray-applied product, such as polyurethane foam or a liquid barrier layer. For example, at the roof-wall interface, structural members often penetrate the air and moisture barrier, thereby creating discontinuities.
“The common response is to spray a lot of polyurethane foam up there,” states Phil Kabza, FCSI, CCS, AIA, who heads SpecGuy, Charlotte, N.C. “This actually works pretty well most of the time, but some careful detailing and planning ahead can make the air/moisture and thermal insulation performance at this critical joint perform much better.”
In addition, Building Teams and their product specifiers should verify compatibility and bonding of all seal materials with the sheet and liquid-applied membranes, by checking with the manufacturers and performing tests in the field, says Judd Allen Group’s Peterson, who chairs Minnesota’s BEC chapter.
Another important strategy for the roof parapets is to install continuous metal copings with drip edges and watertight joints. “Providing a backup waterproofing beneath metal copings further reduces the risk of water leakage into occupied spaces, and eases reliance on the metal flashing to exclude all water from penetrating through transverse joints in the metal,” adds Simpson Gumpertz & Heger’s Der Ananian, who specializes in building enclosure design and building science with the Waltham, Mass.-based engineering firm.
Alternatively, Peterson favors continuous strips of EPDM membrane under the sheet metal roof copings as subflashing over the wood blocking, lapped over the roofing base flashings and sealed against backflows and puddles. When waterproofing the foundation walls, Peterson recommends thick, viscous materials that bond and flex over any hairline cracking which may develop in the masonry and concrete walls. He suggests further that the waterproofing be protected from backfilling by water-resistant protection board, as well as applying a drainage mat to the exterior side of the waterproofing to reduce hydrostatic pressures against the waterproofing and drain the water away.
For punched window and door openings, Der Ananian advises that sill and head flashings be carefully detailed and installed. For the sills, the specification should include an upturned leg at the interior and end dams at the jambs, so that any water seeping through the window or door frame will be immediately collected and drained to the exterior. The SGH engineer instructs that head flashings be installed continuously above window heads to direct any water running down the cavity to the outside of the wall assembly.
While careful detailing and sealing will offer much resistance against air and moisture leakage, condensation remains a tricky opponent to conquer. Shepley Bulfinch’s Jonathan Baron, AIA, LEED AP, recommends hygrothermal and thermal modeling to identify the moisture and heat diffusion properties of various materials, and to evaluate temperature levels at different points within the enclosure. If, for example, the software identifies different cold spots, then strategies such as placing insulation outside of the backup wall construction and carefully detailing the structural elements that penetrate the insulation can be very effective.
A rule of thumb here: Maintain surface temperatures on the inboard water-resistive barrier above the dew point of interior air. This will minimize the risk of concealed condensation within the building envelope.
An even more aggressive approach: 1) locate all the insulation in the drained cavity to the exterior of the air/water/vapor control layer, 2) use an inverted roof-membrane assembly, and 3) insulate on the outside of the waterproofing at the foundations, in addition to installing a vapor retarder above continuous rigid insulation below the slab-on-grade. “This way, every surface exposed to interior conditions stays well above the dew point and is therefore nice and dry,” says Altenhofen. “Any condensation that may occur is within a ‘wet’ zone and is harmless.”
Essentially, if there is a significant differential between the vapor pressure outside and inside the building, the vapor will try to equalize, which is one instance where condensation is likely to occur. Buildings in colder climates are particularly vulnerable to this phenomenon. Sometimes the problem can be addressed mechanically by introducing drier air ventilation to reduce the humidity level inside the building. But in cases where this is not sufficient, the wall’s insulation should be increased, while eliminating or isolating any thermal bridges or ineffective window and door framing, says Peterson. “If the condensation is occurring within the insulation layers, within the wall construction, it is my opinion that the vapor barrier must be perfected at the interior warm side of the insulation,” he says.
However, the question of where to locate the vapor retarder depends very much on climate. Generally speaking, in cold climates, the vapor barrier is placed on the interior; in hot and humid climates, on the exterior. Where things get tricky, however, is when the building is located in a mixed climate with seasonal changes that can invert the vapor drive direction. “If condensation occurs within the insulation during the reverse drive, this moisture will have to be allowed to pass through the opposite side of the wall, through permeable weather barriers, so that it does not become trapped within the construction and cause damage,” cautions Peterson.
BUILDINGS THAT MANAGE WATER
While effective air and vapor barrier installation goes a long way toward sealing the building envelope, moisture still has to go somewhere. Therefore, it must be carefully managed.
“The most important rule to live by in addressing water management is to apply pressure-equalized rainscreen design principles,” explains James Oglesby AIA, an associate with Shepley Bulfinch. “That means taking an approach that provides two lines of defense, using exterior cladding as a primary weather barrier and separated by a vented air space from the drainage plane on the backup wall.”
Peterson also likes these rainscreen assemblies because the backup wall is often a flat plane, which makes it easy to install a continuous weather barrier. “Because this continuous, weeping plane is interior of the insulation, it can also do triple duty as the impermeable vapor barrier layer and the continuous air barrier as well,” he adds.
However, to enable the cavity wall system to operate effectively, the amount of water entering that space needs to be as limited as possible. In order to accomplish this, Altenhofen recommends designing a robust and durable air/water/vapor control layer and then covering it with continuous rigid or semi-rigid insulation. Next, it is crucial to detail the framing that will support the rainscreen with as few thermal breaks through the insulation as possible. Finally, says Altenhofen, select and detail the rainscreen to deflect as much water as possible. “The air/water/vapor control layer will not be perfect, which is why we started using rainscreens,” he says. “No barrier is ever perfect.”
Further to this point, Der Ananian recommends continuous through-wall flashing, preferably sheet metal, at regular intervals along the floor lines and wall bases to direct water out of the cavity at regularly spaced weep holes. Also, by installing the horizontal part of the flashing at a slight outward slope, the structure will enhance drainage and help prevent ponding.
Adding technical detail to the through-wall installation, Peterson explains that end-dams are required to direct water to the exterior at corners and at cavity interruptions. For brick masonry walls, many enclosure experts suggest using continuous 100% cotton rope wicks running along the bottoms of each brick cavity, on the through-wall flashings, and then turning and weeping out to the exterior every 16 inches on center.
In addition, to give this brick cavity a true rainscreen-type function, Peterson recommends putting vents in the vertical head joints, alternating with the cotton rope wick locations, and at the top and bottom of the cavity. He notes that temperature and pressure differentials develop daily in these wall cavities and “ventilation is the most effective method of clearing the cavities of unwanted water.”
SELECTING AIR AND VAPOR BARRIERS
When selecting barrier products for air and vapor protection, experts counsel that there’s never a one-size-fits-all approach. Each project must be evaluated based upon climate, construction conditions, and even computer analysis such as hygrothermal modeling, which evaluates how moisture and heat will diffuse through different materials.
“We look at published test results for air permeance, water vapor permeance, durability, structural support, and other related properties,” relates Shepley Bulfinch’s Michael Harrison. “It’s also important to consider the availability of accessory material for different systems. Once we’ve done the research and weighed our options, we use integrated exterior wall mockups to test the chosen air barrier assemblies in their proposed configurations.”
Because there are so many products available on the market, Altenhofen judges product choices not only by their performance attributes, but as much by the integrity of the manufacturer’s technical representatives. “I want to know that the manufacturer trains the applicators they are willing to sell to, and that they follow up with the projects at the job site,” he says. “Anyone with a pickup can go to the big-box store and pick up material that complies with the performance attributes, but a good manufacturer controls that process and doesn’t allow the substitution of commodity products.”
In terms of product types, many enclosure scientists prefer thick air barrier membranes over wraps and papers, many also favor spray-applied liquid coatings. The key is to specify and apply the product with enough strength and durability at seams and penetrations, as well as sufficient capability to bridge cracks and fill voids. For the type of construction on each building in question; the applied thickness of spray-applied products ranges from 5-7 mils up to 40 mils for sheathing, and between 10 mils and 20 mils for concrete masonry.
For his part, Altenhofen believes that heavy elastomeric membranes offer good long-term performance. Similarly, Philip W. Kabza, FCSI, CCS, CCCA, AIA, the founding chair of BEC's Charlotte chapter and former chair of the AIA MasterSpec Architectural Review Committee, likes membrane-type air and moisture barriers as he believes they are more resilient.
Kabza says that, while he is somewhat wary of building wraps, he does acknowledge that the products themselves can work well in light-frame construction. He says, however, that he has witnessed careless installation practices, which is where a wrap’s potential falls short. In his opinion, says Kabza, “The more expensive and specialized coatings and membranes get more respect on the job site.”
Another option: self-adhered sheet air barriers, also know as self-adhering sheet membranes (SASMs). Wiss, Janney, Elstner’s Anis, who chairs the NIBS Building Energy and Thermal Envelope Council and is a founding Air Barrier Association of America board member, says he likes these products because “the sheet material is quality controlled in the plant, and the adhesion quality is formulated to resist design air pressures such as wind, stack, and mechanical fan pressures.”
Der Ananian says he, too, likes self-adhering sheet membranes, particularly for their ability to integrate easily with adjacent flashing materials and cladding assemblies. He lists several other benefits of SASMs that he likes: 1) the membrane offers a consistent, uniform thickness, 2) it is less susceptible to ambient moisture during application, 3) it is easy to inspect, and 4) it can bridge small cracks and gaps in the substrate more effectively than fluid-applied products, in his view.
At the same time, Der Ananian and other experts contend that fluid-applied membranes can work very effectively when installed properly, but they warn that successful application depends heavily on skilled field workmanship. Building Teams should spot inspect during the construction administration phase to ensure adherence to manufacturer recommendations. “The quality of a fluid-applied system installation is more sensitive to a variety of field conditions, including weather, equipment, skill of the applicator, substrate condition, and mixing than a sheet-applied system,” says Der Ananian.
Another highly regarded product: spray polyurethane foam (SPF), which is often chosen for sealing hard-to-reach locations. SPF coats continuously and offers a good R-value—about 6.0 or more per inch of insulation. It can also act as a vapor retarder when applied at a thickness of approximately 2½ inches. Der Anian points out, however, that the foam should not be relied upon as the sole air/vapor/thermal and weather-resistive barrier in cavity walls because it lacks redundancy, and any breaches in the spray foam can compromise wall performance.
PAY HEED TO CLADDING TYPE
When working with different cladding types, backup walls, and sheathing, it’s important to carefully consider each material and then approach air and moisture control accordingly. Following are typical structural systems and their key considerations for air and moisture control:
Concrete has its own checklist of points that must be addressed both before and during sealing for air, water, and vapor infiltration. For starters, all fins, oils, ties, and formwork must be removed, and the concrete must be fully dried and cured to enable proper bonding of the membrane materials. In addition, says Peterson, specifiers should select a membrane product that is thick and flexible enough to accommodate future shrinkage and cracking.
Another key point, this from Kabza: “It is difficult to stop water vapor from coming through a concrete wall without forming a visible film on one surface or the other, whether it’s a pigmented elastomeric paint on the exterior or an epoxy vapor barrier on the interior. So, try to avoid this design circumstance.”
CMU and brick masonry. When working with concrete masonry units (CMU) and brick masonry veneers, the same advice applies as with concrete in terms of removing all excess mortar, cleaning ties and anchors, and making sure the masonry is fully dried prior to membrane application. “The key is to maintain proper flashing techniques as well as maintaining a continuous vapor barrier in the void between the two walls,” advises Lalo Edery, chief operating officer with Tropic Construction Corp., Chicago. Similarly, for spray-applied membranes, the nozzle should be rotated to ensure that all sides of the surfaces are covered.
When working with concrete and masonry backup walls, Judd Allen Group’s Peterson explains, attention must be paid to the expansion joint slot locations in the substrate surfaces so that they can be spanned with a positively adhered sheet membrane.
Steel and other metals. It is important to consider steel’s heat-conducting properties and the fact that extreme temperature variations can melt the asphalt cores of some of the air/vapor/weather barrier membranes. “Where this may be a problem, be sure to use high temperature-resistant sheet membranes that have a butyl-based core, not asphalt,” says Peterson. “Also, use polyether sealants to bond and isolate these membranes from the exterior. These polyether sealants are also resistant to melting from high temperatures, and will not drip from the steel lintels.”
In addition, metal has a tendency to move much more than other building materials; this can potentially lead to failures at fenestration interface points. “Some aluminum frame manufacturers are redesigning their extrusions to allow for bridging components that help ensure the continuity of the air/moisture barrier,” says Kabza. “One or more manufacturers of sealants and air barriers have also devised transition components that assist with this challenge.”
Wood. In terms of old-fashioned wood sheathing, Building Teams need to be aware that wood products have more joints and fasteners, experience moisture shrinkage over time, and have organic vulnerability to moisture, rot, and mold. Consequently, the air/vapor/weather barriers must be able to handle these conditions. “Gypsum on rigid foam wall sheathing and a high-quality vapor barrier work well for wood, making sure that all of the joints are taped,” says Edery.
Caution: The random stapling often used to adhere the sheathing and unsealed staple pinholes have been proven to cause leakage failures through weather barriers. “It is important to align staple fasteners and then use manufacturer recommended sealing tapes and caulking to continuously seal all fastener penetrations in such weather barriers,” states Peterson.
Glass curtain walls. Fortunately, curtain walls of glass and metal offer strong air and moisture protection. However, dealing with transitions, perimeters, and penetrations can be nettlesome. Der Ananian advises that sheet membrane products, such as EPDM, neoprene, or silicone sheet, can serve as good transition materials at the perimeters of curtain walls because they can accommodate differential movement and, he believes, they are more reliable than using sealant alone.
At the same time, the spandrel section can also be difficult to seal, particularly since these surfaces are more vulnerable to condensation. “For this reason, it is important to be sure the perimeter framing of the spandrel is continuously sealed off from interior humidity,” says Peterson. “It is also more important to provide a sealed, weeping path for any liquid condensation or leakage that make occur within the spandrel because it will be sealed off.”
Overall, while glass curtain walls do a good job of air and moisture sealing, their Achilles heel is thermal performance. To help with this, Anis suggests, “Advanced thermal breaks provide higher performance of framing systems and careful selection of glass, gas fill, and spacers all contribute to higher performance.” However, Altenhofen believes that until the industry comes out with alternate framing systems that provide much higher thermal performance, curtain walls will not be able to deliver the R-20 or higher performance that is needed.
PROVING THE CASE FOR BARRIERS
Although it may seem like a lot of work to correctly specify, install, and detail today’s air and moisture barriers, it’s well worth the effort. The national movement for codes to adopt air barriers in 2012 will have a substantial effect on building performance and ROI.
In fact, a recent study by the National Institute of Standards and Technology found that air barrier systems in commercial and industrial buildings reduce air leakage by up to 83%, shrink gas bills by more than 40%, and reduce electrical use by 25%. In addition, says the NIST report, indoor air quality and acoustics are improved, buildings are more sustainable, and moisture problems are significantly reduced.
Is there a reason not to make buildings with better air and moisture control?
No additional reading is required. To earn 1.0 AIA/CES Discovery learning units, review the required reading and take the exam at www.BDCnetwork.com/MoistureProtection.