Insulating roof .Space between insulation and felt ???

njkmr · 31 Oct 2009

Hi Guys ,just a quick question .I want to insulate my roof and plasterboard over it .My roof timbers are 4 inch by the look of it and I was told I could just use normal wool type insulation up against the roof felt ,and net over it to keep it in place ,then plaster board over this onto the roof timbers .Not much depth to insulate so the insulation would be a bit compressed against the felt .

Question is ,do I need an air gap between the felt and insulation ??
Some fella from a insulation company who was doing a survey for my house to be cavity wall insulated said it was OK to do this .

Is this right what he says ??

Regards
Rob .

freddiemercurystwin · 31 Oct 2009

Assuming you mean conventional felt and not a breathable membrane you need a 50mm airgap between the insulation and the felt and vents at the eaves and the ridge. In any case 100mm of wool type insulation as you put it will not comply with the regs. If you want to have a roof that complies you'll need something like 50mm rigid insulation between the rafters (to leave you a 50mm airgap) and around 60mm below the rafters with plasterboard beneath that fixed with long screws through the 60mm layer into the rafters.

njkmr · 31 Oct 2009

Thanks freddiemercs ,
I thought he was talking nonsense to be honest .(he said it was OK because the insulation was breathable .!!)
I have the vents at the eaves and was expecting to use the fixed insulation boards set away from the felt to let everything breathe .And air to circulate .

When you say ,then put 60 mm insulation after the boards ,is it OK to squash this down with the plasterboards ,when fixing the plasterboards to the rafters .??

freddiemercurystwin · 31 Oct 2009

No, mineral wool insulation is not designed to be compressed, when I said 50mm rigid insul between and 60mm rigid insul below I was referring to this kind of stuff

either Kingspan or Celotex is good. To use mineral wool you'd need to use at least 200mm as its less effective but as you'd need so much thickness you would need to fix at least 150mm timber battens beneath your existing rafters to end up with 250mm deep rafters rather than the 100mm rafters you have now to have enough room for the mineral wool and 50mm air gap.

Of course if you're doing a bit of a bodge job and are not concerned about complying with any regs then go right ahead. But it will be a colder loft room.

njkmr · 31 Oct 2009

Arghhh ,with you now freddiemercs ,yes I see .
So put 50 mm between the rafters and then put more across the rafters ,which I can then put my plasterboards against and then screw through ,with the long screws ,into the woodwork .

Fantastic . I certainly dont want to do a" Botch Up job " .Thats why I now come onto this forum if I am about to do a job .Get armed with the facts and at least have a reasonable idea of what I should be doing .

Thanks Again Freddiemercs .
Regards
Rob .

freddiemercurystwin · 31 Oct 2009

Have a look here: http://www.insulation.kingspan.com/uk/pdf/tp10.pdf figure 2a applies to your roof.

njkmr · 1 Nov 2009

Thanks again Freddymercurystwin .
Just the info I needed .
Regards
Rob.

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Insulating roof .Space between insulation and felt ???

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