I have a dormer build (badly in the 70's) with 3x3" timbers:
When I have the roof retiled (inc dormer faces with hung tiles), I got
50mm celotex put between plus a breathable membrane to BCO's satisfaction. Now I want to add some rigidity to the structure. And another 50mm insulation which was always the plan.---------
My initial idea was to plate across with 18mm ply, glued and screwed, then celotex, then plasterboard. But I think interstitial condensation could be a problem.
My second idea is to add 2x3" timbers as in picture 3, screwed through with plastic DPC between new and old timbers. Fill in between with 50mm celotex.
Then plate that with ply glued and screwed as in picture 4 (obviously cutout around the windows - please excuse crappy drawing).
This will give 100% vapour barrier in the middle and the ply should stiffen the structure to prevent shearing. Shearing would be bad as it will eventually put strain on the windows and there's no diagonal bracing.
Are there any improvements I could make to this plan?
Main item is with plastic DPC between the timbers, that will have little friction so I'm relying on lots of big screws to lock it together.
Cheers
Tim