DIY Doctor

Insulating Loft Room in an old 1865 House (Sorry Long post)

Postby wateringbury » Thu Jan 02, 2020 4:20 pm

Hi all.
1st post so be gentle.
Our 1865 town house has 4 stories with the top being a loft room ( It was built this way for servants quarters i think) Although its only a spare room Its blooming cold in the winter and baking in the summer so i wanted to improve the insulation.

The room has 2 dwarf walls (140cm high) leading into the eaves for storage.
The rafters (100mm) in the main part of the room are exposed. In-between the rafters there is a felt membrane (not sure if its breathable or not but it looks in good condition),1 layer of rockwool then some 18mm baton to which plasterboard is fixed. this leaves 30mm of the rafters depth.
There is no insulation in the rafters of the storage area but the floor of that area is insulated.
the tiles are slate.
My original plan was to staple some insulation (rockwool or foil backed wool) to the existing plasterboard then cover the rafters with insulated plasterboard.
But I am worried about air gaps and condensation.

Bearing in mind the insulated plasterboard would sit on the rafters and then there would be 30mm rockwool, 9mm plasterboard, 60mm gap with one layer or rockwool before the roof membrane and there is 1.5m of roof each end with no insulation do i need to worry??

I know the logical thing is to rip all the existing plasterboard out and use celotex in-between and on the rafters but if i could get ok results with my plan it would save loads of time and money.

Any help and advice is much appreciated.
Thanks in advance
wateringbury
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