In the 1963 film “The Great Escape”, we were rooting for McQueen, Bronson, Attenborough and crew to safely escape the Nazi prison camp. When it comes to air fleeing our homes, our hope is exactly the opposite. Codes and customers are pushing for more energy efficient homes, and that means tighter envelopes. Tighter homes have significant impacts on both moisture management and air quality, but at a more basic level, they are just plain hard to build. A tight home is a result of successfully combining People, Practices and Products (our 3 Ps of air sealing) to deliver the desired result. Executing requires diligence from multiple subs, at many stages of a build, typically employing multiple products, both air barrier materials and sealing accessories. This complexity begs the question: how do I prioritize my focus (and my money) to attack the biggest problems first?
Look down below
The actual prison break on which “The Great Escape” was based involved the creation of a 335 foot tunnel that would allow 200 POWs to escape. For air, a major leakage point is the connection between the bottom plate and the foundation. In a 2014 report, DOE calculated the contribution of the top and bottom plate connections to the overall air leakage limit of the 2012 IECC (they used a 3 ACH50 limit for a typical home in Climate Zones 3-8). They found these two connections accounted for 50% of the overall leakage allowable. DOE further commented that it is common for these joints to be left unsealed. This perspective was reinforced by Matt Risinger in one of his recent videos. In it, he mentions that use of sill sealer is very common between the bottom plate and foundation. However, contrary to what many believe, the product provides limited air sealing benefit.
Plate sealing and wall assembly tightness
Matt provides some “good-better-best” suggestions for sealing the bottom plate-foundation gap. Specifically, use of a standard elastomeric caulk as well as some interior and exterior tape options. While many different approaches can work based on performance targets and cost limitations, it is important to recognize how critical the plate joints are. The DOE report examined eight specific wall air barrier assemblies. When a wall using spray foam insulation was examined, DOE noted that “In order to maintain continuity in the air barrier assembly, the wall to foundation and the wall to roof joints were caulked.” They also stated that measured leakage “would have been much larger if only the foam had been applied.” The comment that it would have been “much larger” is due to the significant contribution of the plates to overall leakage and the inability of cavity foam to address these leakage pathways.
Teamwork and focus
600 men were involved in the preparation for the real Great Escape. What they did to make it happen is pretty staggering[1]. The DOE report reinforces the importance of teamwork for air sealing too, stating that regardless of the product chosen, people and practices are integral to success, noting that:
“The general contractor needs to be educated on the various air barrier types and manufacturer’s instructions to ensure proper installation and sequencing. It is very likely that various subcontractors (e.g., framers, drywall installers, electricians and plumbers) will be collaborating on the installation of the air barrier system and on maintaining its integrity during construction. Therefore, the general contractor should coordinate all of these trades accordingly.”
This is good advice certainly, but easier said than done. To make this aspirational guidance implementable a builder must identify and prioritize high volume air leakage pathways. Telling EVERY tradesman that EVERY leakage point is equally critical is not a recipe for success. Focusing on the top leakage points, the products that can address them and the trades involved in doing so is a more realistic way of delivering the performance outcomes you’re after.
Interested in learning more about the science of air flow? Check out our earlier blog post on this topic.
[1] According to Wikipedia: “Following the escape, the Germans took an inventory of the camp and found out just how extensive the operation had been. Four thousand bed boards had gone missing, as well as the complete disappearance of 90 double bunk beds, 635 mattresses, 192 bed covers, 161 pillow cases, 52 20-man tables, 10 single tables, 34 chairs, 76 benches, 1,212 bed bolsters, 1,370 beading battens, 1219 knives, 478 spoons, 582 forks, 69 lamps, 246 water cans, 30 shovels, 300 m (1,000 ft) of electric wire, 180 m (600 ft) of rope, and 3424 towels. 1,700 blankets had been used, along with more than 1,400 Klim cans.
