I'm in the process of loft insulating, not to meet building regs on heat loss, but trying to get an occasional play space that can be heated. I am going to put 75mm celotex under the rafters which are topped by a non breathable membrane and tile roof..I will probably finish with plasterboard one day. I have bought the celotex so I am not buying plasterboard topped foam in case anyone advises me to.
My questions:
Is there a nice easily applied cheap gooey adhesive for sealing the joints between the insulation boards?
There are 4 large purlins across my loft space: Do I insulate over them, I do not really want to add 150 mm to their dimensions. Would it be reasonable to seal them with a plastic membrane making them part of the cold space and plasterboard over them., I'm worried about warm damp air getting into the timber and condensing part way through. If I did the latter, they would act as a cold bridge but they shouldn't become damp with rising warm air and the heat loss through them should be small.
I see a similar problem with the floor joists below . Insulation will come down the inside of the roof to the inside leaf of the cavity wall but will seal in the floor joists so that the joist will be part in cold space and part in warm with the potential for warm air to cause condensation in the cold timber. There are also the occasional vertical timbers with top in cold and bottom in hot space.What do the experts do with both these situations.
Dave