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Insulating an overhang under main level


Morgan19's Avatar
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12-08-14, 11:55 AM   #1  
Insulating an overhang under main level

The backside of my house has a small overhang that runs along that entire side, about two feet up off the ground. I can access the space inside the overhang through the basement, up by the ceiling; it extends out maybe two or three feet into the area beneath the deck and main level.

The previous owners stapled some small sections of insulation (marked in pink) between the overhang space and the basement's interior to stop cold air from coming into the basement itself. But it looks like the layers of construction that make up the overhang itself (marked in red) are either very thin wood or not insulated well (or both) because the main-level floor immediately above the overhang gets super-cold in the winter.

Common sense tells me that the cold air outside is penetrating the empty overhang space through the thin wood and, being blocked from going into the basement by what insulation is there, instead goes up through the floorboards.

So my question is, what can I do to stop this from happening? Would stuffing additional insulation into each bay of the overhang space (where there currently isn't any) help at all? The tricky part is that I can only reach into the space from the basement; going around from the outside and adding something to the exterior under the deck isn't an option, for example.

I'm attaching both a diagram and photo of the overhang's interior past the insulation, if it helps: just imagine that same bay between each joist, maybe twenty total.

Thanks!

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Last edited by Morgan19; 12-08-14 at 12:30 PM.
 
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12-08-14, 02:03 PM   #2  
Hi Morgan,
Two good opportunities here, improve comfort and decrease heating costs.
Your objective will be to air seal all leaks and then insulate the outer rim and bottom of that cavity and any sides that are exposed to the outside. The reason for leaving perhaps an inch or two at the top uninsulated is to allow that space to be as warm as the ceiling air in the basement. If you have mouse concerns then cover the opening facing the basement with a1/4" wire screen.

As for materials, I like Roxul for insulation and the caulking can be anything from regular caulking to can foam or even a DIY spray foam kit. Although spraying into a closed cavity might be a blow back issue.

If there are any ducts or plumbing in those cavities they need to be exposed to the inside and insulated from the outside.

Bud

 
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12-11-14, 08:18 AM   #3  
Thank you, Bud! This looks promising to get me going in the right direction. I sheepishly admit half-understanding what you're saying, so if you wouldn't mind some clarification...

Your objective will be to air seal all leaks
Would that just be caulking the interior seams where the planes and corners come together?

and then insulate the outer rim and bottom of that cavity and any sides that are exposed to the outside.
So you're saying I should insulate any interior surfaces whose exterior sides are directly exposed to the outside: which in this case would be the back and bottom of the cavities. That makes sense.

Is there any benefit to insulating the ceiling of the cavity as well (which translates to the underside of the main level's floor), the thought being that if any cold air is sitting in the cavities, it'll help stop it from penetrating up through the floor?

As for materials, I like Roxul for insulation and the caulking can be anything from regular caulking to can foam or even a DIY spray foam kit. Although spraying into a closed cavity might be a blow back issue.
I'm not familiar with Roxul so I did a quick search, and their products all seem to be non-foiled batts. Is the thought that I'd cut each piece to the proper dimensions and attach them (how?) to the cavities' back side and floor? Is it preferable to leave the middle of the cavity empty, or is there more "stuff" that should get put in there to held insulate more?

If there are any ducts or plumbing in those cavities they need to be exposed to the inside and insulated from the outside.
There is one plumbing line (from the kitchen sink above) and a couple air ducts; to the best of my knowledge they all stay within the cavities and don't actually go outside into the exterior. I assume "exposed to the inside" means not bundled up, but what do you mean by "insulated from the outside"? If they're insulated in the cavity, wouldn't that mean they're no longer exposed?

I'll be removing the little batts of insulation the previous owners put up between the cavities and the basement. Once I've done all the work to the cavities' interior, would it make sense to put those batts back in place?

Again, thank you! I'm looking forward to getting started on this soon with all this great info.

 
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12-11-14, 08:31 AM   #4  
Exposed to the inside: subject to the air/environment inside the house
Insulated from the outside: protected from the air/environment outside the house

Don't insulate the ceiling of the space - Bud addressed this here:
The reason for leaving perhaps an inch or two at the top uninsulated is to allow that space to be as warm as the ceiling air in the basement.

 
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12-11-14, 09:27 AM   #5  
A lot of questions so I'll try to be brief.
>Mitch got the top space issue, fill everything below that, plus the back and if either end has an exterior side, fill those cavities completely, unless there are pipes.
>I'll add a link on air sealing and a link on overall home repairs. But basically, you picture frame all sides of those cavities. Air leakage is sneaky and there can be seams hidden behind a framing member.
>Top of cavity? With all of the insulation and air sealing in there, the top of those cavities should follow the interior ceiling temperature. The floor temp above will be cooled by cold air falling off of the exterior walls above and that gap will help the basement air warm those floors from below.
>Friction fit with the Roxul (14.5" cavity, cut pieces to 15") with no need for the foil faced. Only thing I would put across the cavity on the inside might be some 1/4" grid screening to keep rodents from nesting out there.
>For pipes, you want as much insulation as possible on the cold side and zero to limited on the warm side. With ducts it is less of an issue, but a good practice.
>"Once I've done all the work to the cavities' interior, would it make sense to put those batts back in place?" No, you want the top open to the basement air.

Vapor barriers are another long discussion and technically could be addressed with this improvement. Their purpose would be to limit warm (moist) air from reaching a cold surface, those exterior cavity surfaces. The other approach is to cover the exterior surfaces with 1" of pink or blue rigid foam board. the rigid insulation raises the inside surface temp (the inside surface of the rigid itself) above the dew point, thus no condensation. By using Roxul and fitting it tight, the hope is it will restrict the air flow enough to keep it away from that exterior surface. But, to be technically correct, a layer of rigid on the bottom and on the back, sealed in place, would be a better job. A lot of cutting but I have done similar. 1" in your climate should be sufficient.

I didn't do too well being brief, but do appreciate posters who respond as you have.
Bud
http://www.efficiencyvermont.com/ste...ide_062507.pdf
http://www.buildingscience.com/docum...build-renovate

 
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