DIY Doctor UK DIY Forums

I live in a "Cornish house" The House used to be PC concrete on the ground level . This was changed to Cavity( brick & thermalite) in the '80s. The 2nd floor is tiled with an air cap that runs all around the house.(mansard roof) I have learned the top is insulated with polystyrene & plasterboard on one side, but the other side is just Plasterboard!!!. Can I overboard with thermal plasterboard or do I have to rip all of the plasterboards off ( I hope not) to achieve some type of thermal insulation for the house?

Hi susierose,
The issue is whether condensation will form in the original plasterboard if you leave it in place. You could minimise this by fitting a damp proof membrane (DPM). The construction would then be: old plasterboard; expanded polystyrene; DPM; new plasterboard.
Regards S

Thank you for the reply, it has helped me a lot. I did wonder about the condensation due to the cold air travelling inside and up to the loft area.

Hi susierose,
You need the air movement in the gap all round you house to remove condensation.
Regards S

Why is PIR board not useful instance, it has the foil protection to help with condensation.

Hi susierose,
Foil faced PIR board will work but it will cost you £10 extra per 1.2m x 2.4m sheet.
Regards S

Thank you for your expertise. I will follow your first reply to try and get some form of thermal insulation using the products you have stated. Thank you very much.