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Hi, I'm planning on converting my standalone garage into a home office this summer. The roof is currently uninsulated – it's sheets of corrugated metal screwed straight into the roof joists. Given I can't get anything between the metal and the joists I wanted to know how best to insulate the roof for sound and heat. The inside of the metal sheets can be prone to condensation so I was worried about just using blocks of Celotex. Should I use some kind of spray foam? Any tips welcome. Thanks!

Hi Jonathanwbrown
Suggest you strip the metal sheets, insulated over and between joists, fit a breathable DPM and refit the sheets.
Regvards S

If that is to be a home office than you will need proper insulation. Summer heat and metal roofs really dont do well together and if we assume that you will be using some sort of a HVAC system then you will want a properly insulated roof in order to waste less electricity and lower bills.

Considering all of those things, you should redo the entire roof construction. Remove the metal sheets and place the insulation properly between the joists.

But the most important question is from which material are the joists and what are their dimensions? Specifically the height? This matters because of the insulation and the ceiling construction you will choose. You will probably have a drywall ceiling construction so you should have the joists available for the gypsum boards to be screwed to them which is why their height matters.

Celotex will not give you a great acoustic performance. So in this case I would use Rockwool RWA45 or similar. Although RWA45 isn’t as thermally efficient as Celotex/PIR, it still has a good thermal performance and excellent acoustic performance with the added advantage of being breathable. To help combat condensation, a VCL is recommended to the warm side of the insulation sealed correctly with associated tapes. (Double & single side)
Spray foam is also an option but more expensive.