idiot roof question

[Alexphd1]

I will put my hands up and be honest, i do not have a clue about the make up of the roof. the architect and engineer have asked what do I want. Its a pitched  slate/metal roof with a room in loft . We are in Scotland so sarking will be used. Its a ridge beam design with a target u-value 0.1 with emphasis on air tightness. architect has suggested  rockwool between and space blanket (?) on top of rafter under sarking which i am not keen on. I was more thinking solid insulation (spec unknown) again on top with rockwall between but how this works with sarking etc i dont know. I have already bought the roofshield membrane. any advice/opinions welcome. cheers. 

Edited July 21, 2016 by Alexphd1

[Ferdinand]

Rockwool probably good and cheap, if you have enough thickness.

Space blanket (=multifoil) is best left to spacemen and mountain rescue people, as being very expensive for the insulation it actually provides.

There should be lots about this already here, even though we have just started. Try a search for multifoil.

Ferdinand

[PeterW]

You can get a pre-insulated rigid sarking board which has PUR insulation already bonded to it - its designed for the situation you describe. What's unclear is why you would do it that way, and not go with sarking > rockwool > PIR insulation from outside to inside as that removes the thermal bridge and also puts the least permeable insulation on the inside as the vapour control membrane.

 

[Alexphd1]

Peter w. So the pir would be between the rafter and plaster board? Where what would the best for air barrier for tightness? 

[PeterW]

That's correct - this probably helps a bit more with the make up of the roof.

The VCL provides the air tight layer in this model [from http://www.greenspec.co.uk/building-design/unventilated-roof-insulation ]

 

Edited July 21, 2016 by PeterW
Added source URL

[ToughButterCup]

Idiot roof question.

There but for a syllable....  We like all sorts of questions

Sorry ...couldn't resist it.

Ian

[Crofter]

My roof design is exactly as PeterW shows (although with metal sheeting rather than tiles).

You could use foil-faced PIR and tape it to create the VCL, but I'm just going to use plastic.