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How can I airseal if there is no rim joist or sheathing?

cold_feet | Posted in General Questions on

I live in a home built in the mid-1980’s in zone 4A.  It’s an open truss floor frame with no rim joist and no wall sheathing.  From the basement, if I look at where the rim joist would wrap around many homes, all I see is blue rigid foam.  On the other side of the rigid foam is the aluminum siding.  There is no house wrap.  There is no sheathing.  The actual floor on the first floor gets really cold as do the outside walls on the first floor.

For this frame design, do the traditional rim joist air seal and insulation approaches work?  As in, do I treat the existing rigid foam board as if it was a rim joist and cut and glue another rigid foam board between the trusses, fill the gaps with caulk or spray foam, and add batt insulation on top?  Or do I need to do something else? 

We’re planning on replacing the aluminum siding anyways.  Is there something we can do at that time to greatly improve the comfort of our home instead of, or in addition to, my main question?

If anyone can help, I’d really appreciate it.

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  1. Expert Member
    NICK KEENAN | | #1

    The way I was taught to think about it is you have to define the building envelope, which is the line where one side is conditioned and the other side isn't. If you have defined the envelope properly on a plan you should be able to draw it without ever lifting your pen from the paper.

    I don't see why you can't define the existing blue foam as the boundary of the envelope along the rim. The next question I would have is, what happens in the wall and in the basement? You want to tie the envelope together throughout so you have a continuous air and water barrier and consistent insulation thickness.

    When you reside you could add sheathing under the siding. It will increase the thickness of your wall, which means you have to adjust the trim on all the windows and doors. It also redefines where the building envelope is.

  2. CMObuilds | | #2

    You could spray foam it or cut foamboard in every individual space and foam in place, Ive done both with good results.

    Buildings that skip sheathing or eliminate most sheathing except for shear panels and use polyiso or xps foamboard as sheathing Ive found to be some of the leakiest buildings Ive ever tested and generally are difficult to airseal. Usually I've found a lot of leakage along the top plate into the wall cavity that can be difficult to get at. As well as the rims like you have when floor trusses are used.

  3. Expert Member
    BILL WICHERS | | #3

    You can just seal up the “rim foam” the same way you would if you had a rim joist. Spray foam will be easiest and also best with open trusses since it will get into all the goofy shaped spaces and ensure a good seal.

    When you have your siding off, make sure all the gaps between insulation panels are sealed. I’d probably use a canned foam gun (great stuff pro) with the narrow tip attachment to inject a little foam into all the gaps, and then tape the gaps. That’s for a bit of extra insurance since tape doesn’t always stick well to XPS foam. Make sure the top plates are sealed too, and pay attention to all the corners.

    You could add more exterior foam in another layer if you want to as well. Just be sure to decide on what you want to do before the new siding goes up since you can’t easily change it later.


  4. cold_feet | | #4

    Thank you guys so much for the advice. When residing, will there be a major or minor difference between adding sheathing under the siding, adding another layer of rigid foam, or just adding a house wrap? Or is some combination of house wrap and sheathing/second layer of rigid foam the best?

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