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Vapor barrier over unheated spaces

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Got another question about the vapor barrier in our car port/workshop. I'm putting in R28 insulation and was thinking of putting a layer of GPS over that and then covering with a soffit material. 2 x 10 I-Joists.

Really not sure if this should be covered in poly with acoustic sealer or not?

There seems to be a few different opinions on how this should be done including the insulation. Any advise would be helpful.



9) Cantilevered floors and floors over unheated spaces or over the exterior shall be
constructed airtight by one of the following methods or a combination thereof:
a) sealing all joints and junctions between the structural components, or
b) covering the structural components with an air barrier material and sealing it to the
adjacent air barrier material.


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Hello Michael. The reason behind a vapor barrier is to prevent warm air which holds moisture from entering the joist cavities where moisture would be trapped. So I believe a vapor barrier is needed in this case. But the ceiling is also an exterior surface, so house wrap would be called for rather than poly.

Maybe the building inspector could clarify that for you, as it depends on his interpretation of the code, sorry about that.

Spray foam insulation would block moist air better, but it sounds like you are using batts stuffed between the joists.

Those joists are vulnerable to mold damage because the OSB absorbs water. I have seen 5 year old I-joists with that creeping fungus growing in the OSB. Ideally the joist cavities would be ventilated to allow any warm moist air to escape.

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In our climate, you don't want a vapor barrier on the unconditioned side of the insulation. 

The OSB flooring is your air barrier. Caulk the perimeter of every sheet and seal all penetrations. Then just stuff the joist cavities with insulation - any kind - and install a durable soffit under it. Personally, I'd use paperless drywall such as DensArmor Plus.  

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Right, batts also allow for future inspection of those cavities, such as in the event of a flood or roof leak. Spray foam would trap any water leaks from above, which would be trouble.

The new standards call for sealing of the vapor barrier on the interior side of the studs to the bottom plate with caulk. From your quote, option "a" calls for the subfloor seams to be caulked, so that should satisfy the requirement, but best to confirm this with the municipal building inspector. He may want to see tape or have some new product in mind.

An example of how builders comply with these new standards for energy efficiency and such. To get an occupancy permit for a new home here, the bathroom fan must be wired to operate 24/7. This is to expel gases from all the new materials. That doesn't mean you can't install a fan switch, but you do that after you get the permit. Cheers. It is looking good.


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I like Jim's suggestions. He's right, in our climate you don't want it airtight on the underside - just pretty tight so any vapor diffusion that finds it's way into that space can dissipate. The only improvement I might try to make with that is to use two-inch-thick EPS placed against the underside of the floor before adding unfaced batting. Just trim it carefully so it's a light press fit, add the batting and then the covering. Don't seal the perimeter of the foam, you want to allow diffusion to continue to move naturally toward the colder underside of the space and escape.




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On 6/22/2022 at 5:09 PM, Jim Katen said:

Spray foam is good and has some advantages, but it's messy, expensive, very tricky to install, and it shrinks over time. Personally, I'd stuff the cavities full of R-30 unfaced fiberglass batts. 

Shrinks by about how much, over how long a time period?

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It varies with the type of foam and the mix. Other factors probably come into play as well. I've seen gaps between the foam and the rafters up to about 3/16" on 10-year-old foam. 

Of course, the old UFFI tended to turn into dust over time. 

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