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Not sure if "basement roof" is the right term for this...

My houses concrete foundation has a small extension, which sticks roughly 2-3 feet out from the rest of the foundation for a space of about 4-5 feet. This results in roughly 10 sq ft of additional basement space beyond what the floors above have. There is a small "roof" on top of this space; it is framed with wood and covered in shingles.

Currently these is no insulation in this space at all, and this "roof" is a source of heat loss (anecdotally, and based on thermal imaging). It seems the right thing to do would be to insulate here to mitigate this... but how? It's a sloped wooden structure covered in shingles that keeps rain and snow out of the interior, but it is not at the top of the house and doesn't have a soffit/vent. Is it really a roof?

Does there need to be a moisture/vapor barrier (and which way should it face)? Would foam board or rockwool/fiberglass be more appropriate? Do I need any kind of baffle to allow air flow? (The top of this "roof" meets the bottom of the siding on the house)

I'm in Massachusetts; winters are typically between 0-30F (occasionally a bit colder, especially with wind chill) and summers are typically 70-90F (occasionally but rarely a bit over 100).

The "roof" bit is about 5-6 feet above the ground directly below it and along most of this side of the house, but is about level with the ground just a couple feet away at the corner and around most of the rest of the house.

Rough sketch of the shape

Exterior

Interior

Underneath

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  • What's in that space -- bulkhead door/basement steps/basement door? Is this space you actually have to heat, or could you install a door in line with the rest of the foundation and seal it off? Photos might help a lot.
    – keshlam
    Apr 10, 2023 at 3:12
  • @keshlam It's a flat concrete floor with three concrete walls and a wood structure on top. Blocking the whole thing off sounds like it'd be as big a project as just insulating it (or bigger). Will try to get a photo or two tomorrow
    – yoozer8
    Apr 10, 2023 at 3:26
  • Blocking it off might be less surface area to lose heat, and thus more effective than trying to heat it. I'm seriously considering building an insulated panel to go over my own basement door during the winter to isolate it and the steps from the space I'm actively using during those months.
    – keshlam
    Apr 10, 2023 at 3:31
  • Blocking would be more surface area (width the is the same, but depth is less than foundation height), and currently the space is used for some storage. And wouldn't blocking it off still involve the moisture/vapor question? (and would it be a different/more complicated question since the blocked off area would include both concrete and wood/"roof"?)
    – yoozer8
    Apr 10, 2023 at 3:36
  • Need to see details
    – keshlam
    Apr 10, 2023 at 4:09

1 Answer 1

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There are two options: vented and unvented.

I did mine unvented and used flash and batt. I used closed cell spray foam insulation 2" thick. I then did roxul. I have a separate air barrier and vapor barrier so my air barrier is the closed cell foam. I used a vapor barrier paint on my drywall as my vapor barrier.

You could do this as a vented assembly. One way is a snorkel vent where you have air allowed to come in via a soffit vent and the air is allowed out up an exterior wall and out an upper vent. This creates a "snorkel" like vent. Once you create this vented assembly you can insulate as if it was the floor of the attic as long as you keep the air path clear ( baffle / drilling holes ) then you can put batt insulation and poly and then drywall ( as if it were an attic floor ).

I don't really like the approach of allowing cold air to flow through your insulated exterior assemblies and it makes air sealing quite difficult.

The flash and batt disadvantage is that you can't really tell if you shingles start to leak as the closed cell foam will prevent water from leaking into your house but hold it against your roof sheathing which will accelerate the sheathing decay. My roof assembly in this case was a torch on membrane that served as the floor of a covered porch that is sheltered from all but really severe weather.

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  • OP would be advised to follow advice of Joe Lstiburek, buildingscience.com on moisture control and insulation. Why risk rotting out the structure to achieve optimum insulation? Apr 10, 2023 at 15:31
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    Lstiburek is good - perfect wall assembly is what most modern wall assemblies and ab/vb separation is based on. Low risk of rot if you take care of maintenance. I'd swap out the shingles for standing seam metal roof which should be good forever then almost zero chance and you get optimal ab/vp/insulation details. Apr 10, 2023 at 22:46

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