Posted by: Li Ling Young | September 25, 2012

Air Sealing the Attic

Removing the vermiculite from the attic has this fortuitous side effect: the attic floor is exposed for any work we’d like to do up there.  Generally air sealing the attic floor is a dusty, sometimes itchy affair involving respirators, Tyvek suits, moving a lot of insulation, moving it all back again, missing some spots because you didn’t see them, giving up on some spots because it’s just too much trouble, and in the end maybe insulation that is so disturbed you need to replace it anyway.  Our attic is as empty as a new house.  It’s the best attic either of us has ever worked in.

Why air seal the attic?  You can tell by looking at our attic that air sealing was uncommon for most of the history of wood framed buildings.  Why change now?  Air sealing and insulating are both strategies for creating a controllable environment in the house.  In our climate we’re trying to heat the inside of the house, but in Mumbai they’re trying to cool the inside of the house.  Either way, you want a boundary that clearly and effectively defines the inside, where you’re spending energy to create a comfortable environment, from the outside, where you’re not making any effort to control the temperature or humidity.

Sealed top plate

Seal everything as well as you can. This one-part spray foam reacts with moisture in the air and expands about 10 times in volume. This type of foam sealant is a mainstay of air sealing work, but I have doubts about its longevity.

Between the two, air sealing seems much easier to grasp than insulation.  Yet, when I talk to folks about making their houses weather-proof everyone knows they need insulation but few people accept they should seal up holes.  Let’s start with this proposition: you’re trying to stay comfy on a raw November day in North America.  The kids come in from apple picking: red cheeked, laughing, trailing the dog.  They kick off their boots, but they leave the door wide open.  What do you do?  You air seal the door.  You don’t want the door open.  An open door lets the chilly, outdoor air in and the warm, indoor air out.  Not only does that take some expensive air and scatter it to the backyard, it also makes you uncomfortable.  You close the door.

Now imagine that the air you’ve burned fuel to warm up is leaving the house through hundreds of teeny doors in the top of your house, and chilly, uncomfortable air is coming into the house through hundreds of teeny doors in the bottom of your house.  Close the doors and you will be more comfortable and burn less fuel doing it.

I think the reason folks are reluctant to air seal is that they think the unintentional holes are so small that air couldn’t possible move through there in any quantity they’d care about.  While that might be true if the holes never were under any pressure whatsoever, the nature of trying to control your indoor environment is that you create a pressure difference between the inside of the house and the outside.  When it’s warmer inside, the top of the house is under positive pressure (air is exiting through the teeny doors); when it’s warmer outside, the top of the house is under negative pressure (air is coming in through holes).  It’s physics.  You can’t stop it.  And some of the holes aren’t small, either.  As long as there are holes and the inside of the house is a different temperature from the outside, air will leak into and out of the house… continually.  And I haven’t even talked about wind.  Let me just sum it up this way: holes in your house leak air and make you uncomfortable.

Penetration through a top plate

Some holes in the building aren’t small.

After having the vermiculite removed so I could work in the attic without being exposed to carcinogenic dust, and doing a blower door test so I’d know the baseline condition of the house, it’s time to air seal the attic.  I’m approaching this as though I want my ceiling as tight as the hull of a boat.  I’m going to seal absolutely everything and I’m going to do it in the most thorough way that I can.  My attic is a relatively pleasant place to work but I pretty quickly discover that some important areas are nearly inaccessible.  The low-pitch roof that is so nice for our solar system makes it impossible for me to seal up under the eaves.  I’m absolutely not willing to concede defeat in those areas so I talk Nicholas into exposing those areas through the soffit in the front of the house and through the original, concealed roof deck in the attic of the addition.

Tight access to perimeter wall

Under the lowest part of the roof the top of the wall is almost impossible to seal up.

When our solar installers obligingly remove the soffit for us, guess what?  Vermiculite, that’s what.  Turns out that spot was just as inaccessible to the vermiculite removal folks as it is to me.  So much for not exposing our workers to hazards.

Under eave of roof

The top of the outside walls can be accessed from the outside once the soffit has been removed.

Up in the attic Nicholas takes a sawzall to the board sheathing on the old roof that got covered up by the addition, and removes about 8” of roof deck.  I see a big gap where the original house meets the addition.  Just what I expected, and definitely something I want to seal up.  Guess what else?  Vermiculite, that’s what.

Old roof in attic of addition

The original roof was covered up when the addition was built. The junction between the old and new house needs to be sealed, but the original roof is in the way. You can see the gap between old and new in the debris-filled bay.

It doesn’t take long to find isolated spots of old insulation throughout the attic: some of it in plain-sight places that would have been very easy to clean up.  We avoid it, but I realize that in terms of how much vermiculite I’ve been exposed to, the remediation was a waste.  Nonetheless, without the remediation we wouldn’t be able to do any work in the attic, so seal on!


  1. […] One: Air seal every hole and juncture between different materials.  We used one-part spray foam for most of this seal […]

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