Flat Roof – Torch Down + ISO Board Underneath for Rain Noise Mitigation? – GreenBuildingAdvisor

We’re about to put a roof on our home addition which has a flat roof in California. Our roof was approved with min r-30.

Underneath the base plywood rough roof structure is 2×6 rafters which we will either put iso board or spray foam, I’m thinking just boards to mitigate any off gassing risks.

On top though, we are thinking of doing torch down, but can we put ISO boards instead of a vapor barrier to increase the r value even more? This is all to make sure there’s as little rain noise as possible. This seems like an inappropriate use of ISO boards unless we use the sticker versions of the roof materials, but still is uncommon, so we likely will just do full torch down. I went to home depot yesterday and was tapping on the roof materials and it does seem like rain noise won’t be too much of an issue.

Underneath the roof for the ceiling, can we do a combination of ISO + rockwool? Or is spray foam entirely better?

The roof will look like this:

cap sheet

modified bitumen

base sheet

plywood (roof structure)

6″ sprayfoam/iso board

3″ – 0″ air gap because of slope (what do we put here)

drywall interior ceiling

room