Insulating timberframe walls - please help

[dmor]

I've just moved into a 1970's house with dormer windows. I crawled into the loft space that surrounds the dormer windows to find that there was no insulation on the plasterboard walls. They're simply backed with foil. What should I use to insulate the walls with? The timber frame is 70mm deep. There are lots of small sections, so there's going to be plenty of cutting. What should I use to attach the insulation to the plasterboard?

Thanks

Dave

[sm_jamieson]

Are you *sure* the foil isn't one side on some celotex/kingspan ? The usual solution is celotex or kingspan as thick as you like (say

50mm) inside the timber frame sections, friction fit, and some more (e.g. 25mm) over the studs to prevent cold bridging if you have the space (you can glue it or put a board over and screw through). This extra sheet is usually put on the inside, but this should be OK, since you have foil-backed plasterboard, so you have a vapour barrier. If you have no space for the sheet over the studs, you will have to live with the cold bridging, or take off the plasterboard, put the sheet on the inside of the studs and re-board etc. Or ... you could blow in paper fragment type insulation, and long as an air gap is kept around the roof timbers. Simon.

[dmor]

Hi Simon

Thanks for the reply. I'm pretty certain that the foil backed plasterboard is simply nailed onto the timber frame. That's why it's so cold in the rooms!

Should I completely fill the 70mm gap with Kingspan if possible or leave a gap? Do I need the second layer of Kingspan or can I board straight onto the timberframe? What should I use to board the outside of the timberframe, plywood?

Dave

[sm_jamieson]

You need to leave 50mm gap for ventilation around the roof timbers, but without knowing the exact arrangement and spacing of beams cannot advise on this. There will be a diminishing returns situation with the amount of insulation between the timbers fighting the cold bridging, but not sure how much is worth it. If you do not use a second layer over the frame, the cold bridging will limit the practical insulation value you can get. However, I'm sure things would improve a lot for your cold room without the second layer. The best solution would be to vent the back of the timber frame as the rest of the loft, and have the second layer on the inside, but that would mean replastering etc. Simon.

[dmor]

Hi Simon

I'll make sure that I leave a 50mm gap around the timbers, but that shouldn't be too much of a problem as most of the timberframe is basically inside the roofspace.

I shall fill the gap with 50+mm of Kingspan and cover with a 25mm layer, then board over (with 3mm plywood?) as suggested. It sounds like the second layer and boarding is important.

Replastering is not an option unfortunately.

Thanks very much for your help. Looks like I've got a weekend in the roofspace ;-)

Dave