Roof insulation detail

lownhard

http://www.quinn-therm.com/roofinsulation/abovebetween-rafters.cfm

What do people think of PIR as sarking board above PIR between the rafters as above when aiming for a warm roof? Any issues with breathability for the rafters or condensation?

What is the purpose of the breatheable membrane on top of the sarking board?

Should there be a 50mm gap on the warm side of the rafters?

Thanks

BryanF

lownhard wrote: »

http://www.quinn-therm.com/roofinsulation/abovebetween-rafters.cfm

1. What do people think of PIR as sarking board above PIR between the rafters as above when aiming for a warm roof?
2. Any issues with breathability for the rafters or condensation?
3.What is the purpose of the breatheable membrane on top of the sarking board?
4. Should there be a 50mm gap on the warm side of the rafters?

Thanks

1. Lesser but cheaper alternative to breathable roof build-up. I'm not keen on rigid insulation between joists as it's rarely fitted snug enough
2. Potentially You need fully sealed VB warm side and breathable felt cold
3. You answered question 2 ( pir board is not a sarking board)
4. double battens are essentially for air movement above the breathable felt to avoid vent space below

From experience you should have details of roof structure (amongst others) for builder at tender stage. Don't leave the specification loose.

lownhard

BryanF wrote: »

1. Lesser but cheaper alternative to breathable roof build-up. I'm not keen on rigid insulation between joists as it's rarely fitted snug enough
2. Potentially You need fully sealed VB warm side and breathable felt cold
3. You answered question 2 ( pir board is not a sarking board)
4. double battens are essentially for air movement above the breathable felt to avoid vent space below

From experience you should have details of roof structure (amongst others) for builder at tender stage. Don't leave the specification loose.

Thanks Bryan

From inside out was thinking:

12.5mm plaster board
50mm service cavity with rockwool
Breathable airtight membrane
225mm rafters full fill with pir
50mm pir over rafter
Breathable felt
Batten
Counter batten
Slate

How does that sound?

Would I better off with metac between the rafters than pir performance wise as you get a more snug fit even though you compromise the uvalue of paper?

Any potential issues with interstitial condensation in the above build up?

Thanks for all the help

BryanF

Id increase pir over rafter and go with mineral wool or cellulose between. I'd increase service to min 100mm note even at that it's tight to fit spot lights. Do take air-tightness seriously. And do note that roofing battens will be as per breathable felt certification.
As regards condensation risk - your certifying arch will assess products and build-up.

lownhard

BryanF wrote: »

Id increase pir over rafter and go with mineral wool or cellulose between. I'd increase service to min 100mm note even at that it's tight to fit spot lights. Do take air-tightness seriously. And do note that roofing battens will be as per breathable felt certification.
As regards condensation risk - your certifying arch will assess products and build-up.

Thanks Bryan.

Very helpful stuff.