The Challenges and Opportunities of Exterior Air Sealing
Ask building professionals for the best method of air sealing and you’re likely to get a wide variety of answers. Most builders, designers, and consultants have selected materials and developed methods that work for them and meet their budgets. That being said, there are three fundamental steps based on building science than can lead to superior, cost effective, air sealing.
First, identify the building’s thermal boundary. For many situations and climates, a good location for the boundary is the exterior structural sheathing. This layer of plywood or OSB on a wood-framed wall already covers the entire exterior surface of the building. Second, use easily applied sealing materials that will adhere well to the surfaces to which they are applied and will not shrink, crack, or otherwise deteriorate over time. Third, test the effectiveness of the air sealing by using a blower door test to measure leakage and to check for leaks either with a smoke stick or by feeling for drafts. Many experienced builders prefer to run the blower door test while they are air sealing, a process known as blower door directed air sealing.
While new technologies, such as Aerobarrier, have been grabbing attention in the construction industry, it’s good to keep in mind the tried-and-true methods of exterior air sealing.
When following the exterior air sealing approach, it’s rather simple to create a continuous air barrier by sealing the joints where the sheathing panels meet. Over the years, several methods and materials have emerged that apply the three steps of sealing the exterior sheathing. Here are some of the strengths and weaknesses of each.
Traditionally, adhesive has been used primarily to prevent floor squeaks that might develop in the future. Applying dabs of adhesive to the top flange of floor joists keeps the structural subfloor from squeaking. Since construction adhesive is already on site during the framing stage, it can be used to seal joints of the subfloor and wall sheathing. On the floor, running a continuous bead of adhesive along all four edges of the subfloor accomplishes the seal. For walls, a continuous bead along the outer face of top and bottom plates and along each stud where a sheathing joint will occur gives a good seal. Before standing up the wall, another bead, or better two beads, can be placed under the bottom plate where the wall will stand.
Proponents of this system point out that it uses inexpensive materials already on hand at that stage of construction and that framers seem to accept it without too much objection. Blower door tests of homes using this method, conducted several years after construction, support its durability. The big advantage of this approach is low cost and convenience.
However, not everyone is a fan of using construction adhesive in this way. After all, it’s not intended to be a sealant and it becomes rigid when cured. Some have questioned whether it may deteriorate over a longer time span, though there is no data available to prove or disprove that theory. In any case, there are other options.
There was a time that tape was considered a poor sealing material. The gray cloth tape that seems to stick to everything but it’s intended surface has been eclipsed by a new generation of highly effective air sealing tapes. One advantage of tape is that it can be applied after framing is complete, so it doesn’t slow that process down. Products such as SIGA Wigluv and 3M 8067 have aggressive adhesive that will stick to most surfaces, including wood, and are flexible enough to conform to three dimensional shapes, such as window openings and protruding pipes. Since the tape attaches to the outside of the sheathing, a siding contractor is the logical trade to install it.
These tape products have been used for highly airtight homes for several years and so far, there have been no reports of shrinkage, detaching, deteriorating, or other modes of failure. Some builders praise exterior tape for its ease of installation and durability. However the tape can be expensive, especially when buying enough to cover the entire house. To cut costs, a four inch wide roll can be split lengthwise to double the coverage while still providing an excellent seal.
In the past, self-adhered membranes have been used for water-proofing below grade walls and roof sheathing. In recent years similar products have been developed for use on above grade walls for both air sealing and weather barriers. Generally referred to as “peel and stick”, these products include SIGA Majvest and Henry Blueskin. While the cost of the material is rather high, it offers an almost bomb proof weather barrier in addition to being an excellent air barrier. Most products will even seal the holes made by siding nails.
Fluid Applied Membranes
Other wall products have made a similar evolution from below grade waterproofing to above grade air sealing/weather protection. Fluid applied membranes will adhere to a variety of surfaces, including wood, masonry, rigid insulation, and gypsum. They can be quickly applied with a roller, brush, or sprayer and will seal the penetrations of siding nails. While the product may be more expensive than housewrap, the speed of installation and the high levels of air sealing achieved have made it a good value for mostsome builders. They include ProClima Visconn, WR Meadows Airshield, Tremco ExoAir 230, among others.
Among the proprietary products gaining momentum is a system that integrates structural sheathing, air sealing, and weather barrier functions. The ZIP System is easily spotted by the strong green color of the weather barrier layer adhered to the outer surface of the sheathing. Combined with special tape for covering the joints and flashing openings, the ZIP System provides a basic exterior air barrier. Like the systems mentioned above, some builders are convinced that higher material cost is balanced by less labor and a faster build.
Structural Insulated Panels
Structural Insulated Panels (SIPS) have been around for decades and provide a simplified platform for advanced air sealing. Like the exterior sheathing of a conventional wood framed building, SIPS panels must be carefully sealed at every joint. SIPS manufacturers provide detailed information on the materials and methods to achieve the tightest air sealing targets.
While SIPS panels make air sealing easier, careful attention still needs to be paid to air sealing panel seams and around doors and windows. You can learn more about some of the challenges of and techniques for air sealing with SIPS.
Remember the Basics
While advances have been made in air sealing products, none of these options is a silver bullet. In all cases, there are many basic air infiltration measures that must occur to reach the optimal level of air leakage. A good guide to air sealing basics is the ENERGY STAR Raters Guide. And remember that good air sealing requires excellent whole house ventilation using an HRV or ERV.
There have never been so many options for air sealing buildings. Each option offers advantages and disadvantages. As with most construction strategies there is a cost trade off between labor and materials. The less expensive materials may require more installation time to achieve consistently excellent results. Conversely, more expensive materials may take less time and support consistency. Whichever method you choose, achieving success requires double checking the quality of the end result. Successful builders will find the combination that best suits their energy efficiency targets, the accessibility of materials, their workforce skills, and their budgets.
This article was republished with permission and originally on ZeroEnergyProject.org