Insulation the underside of a existing pitched school roof.

[paulandfrodo]

I've got an existing roof structure (see attached photo). The build up is:

Slate
Batten
Roofing Felt
Slate Rafters (100mm)
Perlins holding BOTH Slate Rafters and Ceiling Rafters (50mm - 70mm)
Ceiling Rafters (100mm)

All of the rafters are essentially cut with only one good edge. This edge is either supporting the Felt/Batten/Slate or the Plasterboard. As such, we cannot put PIR 'Between' the rafters.

I need to insulate this ceiling up to 0.18 - which is essentially 120mm of PIR. BC will give me some flexibility as the rest of the place is over insulated. I am pretty sure they will let me drop to 75mm.

My options are :-

1. Blown insulation (breathable open cell product) quite expensive
2. Rockwool/ Glassrock I really don't think we have enough space
3. PIR insulation on the underside of the ceiling rafters held up with battens and helifix bars

My guy feeling is go for the PIR & Helix & Batten then plasterboard/skim ceiling. We will loose some height, but the area it's being done in is tall enough, so should not be noticable.

Should I consider a hybrid of 2 & 3 and shove a load of rock wool up before putting the PIR up ?

 

[paulandfrodo]

After having a bit of a read and search, I was surprised to fine screwfix do some cheap 150mm long screws.

http://www.screwfix.com/p/goldscrew-woodscrews-double-self-countersunk-6-x-150mm-50-pack/13602

£3.99 for 150mm x 50 - I thought that was pretty good value.

I'm going to ignore the helifix hell fixings and try these. I will do a little experiment and on the underside of a ceiling fix some PIR with batten. I will then see if the batten will then hold my weight. If it does with two screws, it's going to hold the weight of 9.5mm plasterboard and a 3mm skim. I will of course use more screws

I'm just waiting to hear back from EcoTherm & Celotex to see if I do a double layer of PIR, with a 25mm air gap (batten) I get a better insulation rating. Let me explain...

Asked them which has the better insulation rating :

A. 100mm PIR
B. 50mm PIR 25mm air gap (batten) 50mm PIR

Or if they are both the same, then total waste of time.