Topic: Air Sealing and Ventilation
Air sealing in retrofits should always be thought through carefully. Where is the best place to air seal? Do homeowners understand the relationship between air sealing and ventilation? Listen in as our Industry Experts discuss these important questions.
Joseph Lstiburek Principal, Building Science Corporation
August 28, 2013: One of my favorite quotes from John Dillinger was when he was asked, well, why does he rob banks, and he said “because that’s where the money is.” Why do we air seal attics? Because that’s where the holes are.
Gary Parsons Fellow, Dow Building Solutions, The Dow Chemical Company
August 28, 2013: I guess another way to think about where best to air seal is that it depends. It depends a bit on whether you’re going to pay somebody else to do it or whether you’re going to do it yourself, and if you’re going to try to do some of it yourself, the simple answer is to get people past the activation energy of getting started. So to me, even though attics probably have the best bang for your buck, I’d be happy if somebody bought a can of consumer polyurethane foam and went after all the holes in their band joist, and just places like that, and just got started.
John Straube Principal, Building Science Laboratories / Building Science Consulting Inc.
August 28, 2013: I agree with that – “are you doing this yourself? Is this a labor of love? Or are you paying somebody $37.50 an hour?” I think that does change the answer. It just happens that attics are where the holes are. We can provide guidance for different types of houses, and say “this is where you’re likely to find the holes for this kind of a house”. We could probably find a number of good ways to get people to obsessively fill all the holes in their houses, but I think one of the issues that’s being raised with this question is how do you communicate to people the importance of the changes that they’re making on the ventilation side as they’re doing that? Where are the thresholds? When have they gone too far? Is it like 10 cans of foam and now they’re in trouble because the cans of foam don’t come with a warning that says you also can’t have that gigantic 1200 cfm exhaust on your stove while you have a non-power-vented water heater?
Achilles Karagiozis Global Director of Building Science, Owens Corning
August 28, 2013: Do we give enough information to the typical homeowner or builder to understand the importance of air flow? If for example you perfectly seal the ceiling plane in a residential building, what is the percentage of air flow going through your walls? A leaky ceiling plane acts like a chimney and if you seal it, the leakage from the other envelope parts of the building is substantially reduced. We need to translate the physics of air sealing into simple terms: if you seal that part of the envelope, you reduce the impact of any air flow that might be going through joints, not through the wall, but at the joints and perimeter of the wall. The misconception still exists that air flow goes through the insulation, this is apparently not what happens in real life. We also want people to be able to seal with confidence and seal those areas that are most cost effective first. That is why there are a number of advantages in either using a board and flexible air sealant as an air barrier for interior sealing or using insulative sheathing board with a combination of tape and gasketing strips for exterior air sealing. I kind of get the feeling from these discussions in the industry of where the limits are that there’s some fear of crossing a threshold where you turn on your exhaust fan and all of a sudden your house collapses. We want people to seal with confidence and to understand how their house works, or should work.
Mac Sheldon National Accounts Manager, Demilec USA, LLC
August 28, 2013: Start at the top, go to the crawl, do the rim joist and hit the outlets and switches and call it a day. Seal the attic. Stopping exfiltration due to stack effect stops the coincident infiltration through the crawl, rim joist and walls. That’s your best bang for the buck. Seal the crawl since air and air-borne moisture come from the crawl both directly into the house but also through the plumbing and electric penetrations into and through interior walls. Work on the rim joist. Often we can peel back the carpet on the upper floor, drill the sub-floor and squirt foam into the rim joist space. Put gaskets under outlet and light switch covers, or seal the receptacle or switch box with a pillow of foam shot behind and beside the boxes with “straw foam” (single-component can foam). Forget about the walls unless you’re uncomfortable due to excessive heat or bad drafts when it’s cold and windy out. Filling walls with foam is highly effective but has a terrible ROI. Filling with cellulose is a little better, but neither will return the investment anytime soon. And my sermon to the SPF contractor doing the above retro – do not pull the trigger on this or any other air-sealing job until there’s a ventilation plan in place.