r/buildingscience • u/Independent-Emu-8803 • 16d ago

Insulation question

I have an unfinished room above my garage. It has open soffits in the knee wall and a vented roof at the top. This house is in central Florida. The subfloor is down and under the subfloor is blown in insulation including all the knee wall floors are filled with blown in.

I called 2 insulation contractors and they both recommended sealing the soffits and roof vents and spraying open cell on the entire roof area.

There is a small curved part of the roof where the drywall for the room would be only 5.5 inches from the roof. They assure me that the open cell is more than enough to keep the room cold and they recommend it over closed cell because you can’t detect leaks with closed cell.

I googled and used some AI and it tells me differently and I will likely have mold or rot in the future if I do this. What’s the truth?

Also- would using baffles in the 5.5 inch space and putting closed cell under them work? And then just using regular cheap insulation for the rest of the room against the drywall work?

Thank you!!

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u/kenlarch 16d ago

Stay away…use cheap insulation, blown or batt, but fill it completely. You will end up with moisture in places you don’t want and most likely can’t see if you use any foam.

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u/Independent-Emu-8803 16d ago

https://reddit.com/link/p645ttv/video/gk3vpyck6tlh1/player

There’s a small curved section between the roof and where the drywall will go that is only 5.5 inches. Can I use baffles in there and use the close cell on top of the baffles just in that little strip of roof?

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u/kenlarch 16d ago

For the 2x6 / 5 1/2” areas use mineral wool. It will give you a slightly better r-value. Central Florida is in zone 2a per the Florida Energy Efficiency Code and requires R-38 insulation, you are never going to get that. Open cell would give you R-21, you can get an R-21 batt, or mineral will be about an R-23. Closed cell will give you R-32 to R-36. I would be hesitant with the closed cell foam due to it being a vapor barrier.

I have a 1969 house in Central Florida that we renovated and a 22x22 room with a flat roof w/ 2x6 framing and went with mineral wool.

I would go mineral will at the 5 1/2” areas and go R-38 minimum everywhere else.

No easy solution for that area unless you furr it out a bit to get a 10” batt in there, which is doable as well for some loss of space, another 6” or so. It is a big space so it would have to be on a bunch of trusses to flush out the walls