Air Sealing's Top Hits

Jul 27, 2016 4:59:20 PM By Stacy Fitzgerald-Redd
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Energy Efficient homes are in high demand among consumers and are a business imperative for builders who want to remain competitive. Air sealing is critical to achieving energy efficiency.  But, when it comes to air leaks within a home, all points of entry are not created equal. Big holes in a building enclosure are easy to avoid, identify and fix, but smaller holes have proven to be a big problem.  For any building to meet ever more stringent energy efficiency targets, all gaps and openings in a building enclosure should be air sealed.  However, with so many possible air leakage points, and limited time/money to devote to air sealing, builders need to focus on getting the most bang for their air sealing buck.

Air Sealing “Top Hits”

Prioritizing the air leaks in a home means focusing on those that have the greatest potential to reduce air leakage measured using a blower door test. This test is required for homes built under the 2012 and 2015 IECC. Research conducted by a major insulation manufacturer to quantify the leakage characteristics of various joints and openings in a residential structure and prioritize them in terms of the amount of air leakage per unit cost to seal it, identified the most important points of air leakage in a house. The study entailed more than 1,000 individual leakage measurements spanning a 12-month period and incorporated two forms of testing: an examination of individual components and wall/ceiling assemblies in a laboratory using a 50 Pascal pressure difference test across the enclosure to mimic blower door conditions for a residential building.

The testing identified the following as the top 5 priorities when air sealing a home:Air_Sealing_Priorities.jpg

  1. Top Plate-to-Attic: This joint is  shown to leak at a range of 0.29 to 1.6 ACH50[1], depending on how well the drywall is sealed to the interior finishes.
  2. Recessed Lights: Combined air leakage through all paths averaged 9.1 CFM50 per light, which would amount to upwards of 0.2 ACH50 depending on the number of lights penetrating the ceiling into the unconditioned space.
  3. Duct Boots: For cases where the HVAC duct work is located in the attic, there will likely be duct boots that penetrate the ceiling drywall to supply or return air to/from the living space. The leakage at the interface between the duct boot and the drywall contributes 7.7 CFM50 per boot, which could amount to upwards of 0.2 ACH50, depending on the number of boots present.
  4. Band/Rim Joist: This joint had a leakage rate that averaged 0.86 CFM 50 per foot of band joist (incudes leakage from both upper and lower joists), which results in approximately 0.4 ACH50 for the whole-house).
  5. Garage-to-House Common Wall: This joint was shown to leak an average amount of 0.6 CFM50 per foot of joint, which results in approximately 0.1 to 0.3 ACH50 for the whole-house result.

Sealant Solutions and Limitations

Builders have multiple air sealing options at their disposal for air sealing a home– among them traditional caulk, sprayed caulk systems, tapes, gaskets, spray polyurethane foam (SPF insulation) and canned foam.  All of these products have multiple appropriate uses, as well as limitations for these top 5 leakage points. Consider SPF insulation. It would have uses for rim/band joists, garage-to-house common walls and maybe duct boots too. However, it is not appropriate for the top or bottom plates. For recessed lighting, there are limitations in how it can be applied, both for safety reasons as well as long-term accessibility to the lighting fixtures.

Where’s my Silver Bullet?

As much as we’d all like one, there is no silver bullet for constructing a tight home. It is best achieved through a combination of People, Products and Practices, our “3Ps” of air sealing. Products are important, but not the whole story. Terry Nordbye, a builder and frequent JLC contributor on air sealing, talks about how every job needs an ASS, an “Air Sealing Specialist”, which is generally a role he performs personally. This doesn’t mean he does all the work, rather he ensures it gets done, with each crew member fulfilling his portion of the responsibility. 

Understanding the top leakage points can allow you to focus your 3Ps on those areas that deserve the most attention based on their contribution to air leakage. Understanding these points makes clear that air sealing demands its own focus, and that it can be combined with any insulation options to deliver the home performance level you are after.

To learn more about air barriers and air sealing, visit our web site for installation preparation guidance.  

[1] ACH is air changes per hour at 50 pascal pressure difference. CFM50 is cubic feet per minute of flow at 50 pascal pressure difference. The CFM figure’s contribution to overall ACH leakage is based on both actual leakage and footage of leakage.



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