Pitched roof insulation for shallow rafters

[yuumei]

Hi all,

 

I'm looking to insulate our garage to make it a bit nicer to work in. However our rafters are only 70mm deep. Is that unusual? Seems shallow compared to what most people seem to have.

 

From what I have read we need a 50mm air gap since it will be a cold roof. Is my best bet to extend the rafters to make them deeper, then install deeper insulation? Or the alternative is 25mm of the expensive phenolic insulation stuff, then add some more on top.

 

The other issue is the diagonal supports (? in the image). How have others dealt with insulating over these?

 

Any comments much appreciated.

 

Thanks

[oliwoodings]

Do you actually need the space inside the roof area, or could you just board out a ceiling underneath the trusses (9mm osb or shuttering ply would do the trick on the cheap) and then fill on top with loft insulation? Would probably work out cheaper than PIR and you could get more insulation in. You could build in a loft hatch for access etc.

Edited December 18, 2023 by oliwoodings

[yuumei]

15 hours ago, oliwoodings said:

Do you actually need the space inside the roof area, or could you just board out a ceiling underneath the trusses (9mm osb or shuttering ply would do the trick on the cheap) and then fill on top with loft insulation? Would probably work out cheaper than PIR and you could get more insulation in. You could build in a loft hatch for access etc.

Yes, wondered about that, we would like to use it as storage. But didn't think about a loft hatch that might work. Thanks for the idea

[oliwoodings]

4 hours ago, yuumei said:

Yes, wondered about that, we would like to use it as storage. But didn't think about a loft hatch that might work. Thanks for the idea

Just be careful with the amount of weight you put up there, those are pre-manufactured trusses which will not be engineered to support significant permanent loading downwards on the bottom chord.

Edited December 19, 2023 by oliwoodings