Best internal insulation strategy: soft or rigid?

Ush1

Just wondering from the experts what would be the preferred insulation strategy at rafter level in an attic. It would be a cold roof scenario.

Soft insulation and an airtight membrane over it or foil backed rigid insulation such as kingspan with tape sealing the joints?

Would you ever use a siga type membrane with rigid insulation?

Cheers.

EDIT: Doesn't seem to be getting much feedback. Mods, any chance this can be moved to construction or a suitable sub forum therein?

Andrew_Doran

I don't have time to type out a considered reply but here is some reading that should give you more info. You typically need a 50mm gap above insulation for ventilation, possibly ridge vents, roof membrane in good condition.

http://www.greenspec.co.uk/ventilated-roof-insulation.php
http://www.google.com/search?q=cold+roof+insulation+site%3Agreenbuildingforum.co.uk&oq=cold+roof+insulation+site%3Agreenbuildingforum.co.uk&sugexp=chrome,mod=15&sourceid=chrome&ie=UTF-8

Ush1

Andrew_Doran wrote: »

I don't have time to type out a considered reply but here is some reading that should give you more info. You typically need a 50mm gap above insulation for ventilation, possibly ridge vents, roof membrane in good condition.

http://www.greenspec.co.uk/ventilated-roof-insulation.php
http://www.google.com/search?q=cold+roof+insulation+site%3Agreenbuildingforum.co.uk&oq=cold+roof+insulation+site%3Agreenbuildingforum.co.uk&sugexp=chrome,mod=15&sourceid=chrome&ie=UTF-8

I'm familiar with warm and cold roof construction, however in that first link it says:

Air tightness


• In most instances, a combined air tightness / vapour control layer (VCL) should be located on the warm side of the insulation.

Now on kingspan and xtratherms documentation they say that taping joints between foil backed insulation slabs forms the VCL and I also notice on sigas webpage that all examples have soft type insulation.

So my question is, which is better u value wise?

Andrew_Doran

It's hard to know without a sketch or even some requirements, like is space below the rafters at a premium? Is the room to be inhabited?

With foil faced insulation board the foil face and tape act as the airtightness and vapour control layer. Generally the foil faced insulation boards have lower thermal conductivity than mineral wool insulation so will give you a better overall U value when you consider the whole buildup.

For fitting these between rafters the method preferred from the second link I sent is to cut the boards slightly below size leaving a small gap all around, hold them loosely in place temporarily with a couple of battens and screws, fill all the gaps with expanding foam. Then remove battens, cut back foam and tape everything up.

The rafters will act as a "cold bridge" in this setup so ideally you would have another layer of rigid boards below them. In that setup you'd only tape the joints between those boards as you only want 1 VCL and 1 airtightness layer.

There are specialised semi-rigid boards designed to be friction fitted between rafters but no doubt those are more expensive and are likely polystyrene not PIR/PU so will have a higher thermal conductivity hence higher overall U value.