Shortly after I moved into my 1848 house in coastal Maine (zone 6), we found a room that had been sealed off. Thinking it was an attic space, I initially ignored an access hatch but then found that there was a full size room above our kitchen measuring roughly 19’x16′ with the roof coming right down to the floor at pretty much a 45°. The original staircase had been hidden behind a custom door sized pantry cabinet and the access panel had been cut through the stairs perpendicularly. It turns out the room was for the scullery maid and housed the original ice chest for the house (which was suspended from the roof rafters with large iron bars). I would like to make the room into a small home theater but I’m not sure how best to approach insulating the ceiling/roof.

After much reading on best practices, I have devised the following approach and welcome critiques: leaving an air gap of approximately 1.5″ between insulation and roof deck, I install two layers of 2″ polyiso rigid board between the rafters and then a third layer of continuous polysio above the rafters giving me an approximate R-39.

The roof ridge is vented but there are no soffit vents. I cannot install soffit vents due to the exterior architecture of the house. To solve for the lack of soffit vents, I plan to cut in a small gable vent at the exterior end of the knee wall space to act as the supply. These spaces would be at the bottom end of each roof. The polyiso would only come down past the top of the knee walls on either side to allow the unconditioned air in that space to vent up to the roof ridge. The knee walls would be insulated in a similar way to the ceiling/roof described above. I’d like to avoid a hot roof situation and also help prevent ice damming from insufficient insulating up there.

Another alternative would be to install shingle vents and insulate all the way down the roof deck thus making the knee walls conditioned space. However, I believe this would require a new roof due to the age of the current roof.

Is there a best or better way forward with this project? My aim is to achieve maximum R-value with the least impact to ceiling height while also not creating a moisture/condensate issue. I also would like to avoid the use of spray foam due to the permanence of the product.

Pictures of the space can be found here: https://imgur.com/a/4ntvv2x