Insulating a basement roof

Question

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.

Answer 1

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.