Thanks for looking at my problem.
I live in a 1938 semi which has solid brick walls. We have had a major problem with mould appearing on the walls in the upstairs bay window. We did have really bad condensation although we have replaced the 20yr old windows with new double glazing last winter which reduced this massively. However whilst the windows were being replaced it was drawn to my attention that there was a empty cavity which was making the bay walls very cold and due to the heat difference causing codensation to occur thus I'm assuming create the mould issue.
Well i'm about to (this weekend) rip the lathe and plaster off the wall and fill the cavity with insulation board and then plasterboard back over the top. The cavity I have is 50mm deep and i was going to be filling this with 50mm Celotex insulation cavity wall board from Wickes, however I've had different people tell me that I need an air gap to avoid damp issues and others say no air gap is needed, so i'm dead confused!
I need help to confirm if i need to keep an air gap between the outside wall and the insulation, if so i'm going to have to get the 25mm, or if i can fill it fully with the intended 50mm insultaion.
Many thanks for your replies in advance!!