Hi everyone,

Our ICF new-build is cracking on and I'm finalising the detail of the roof. The rafters are in with attic trusses. The original plan was to insulate at first floor ceiling level and go for a cold roof, but since we changed the design to attic trusses, I'm leaning towards insulating either between and under the rafters or between and over. Our ICF structure is 50mm insulation - 100mm concrete - 100mm insulation inside to out. The attic is only for storage; we have no plans to move into it, but having spent a month climbing around a fink-trussed loft for the in-laws, I vowed I wouldn't be doing the same in our own place!

The query I had was mainly around air tightness and also the vapour barrier. If I insulate between and above the rafters, taping joints, etc falls to the roofers to do and it becomes more difficult to do anything retrospectively. However, it means that I can join the roof insulation to the outer leaf of the ICF and wrap the whole place up nice and tight.

If we insulate between and under the rafters, it becomes easier for me to check that we've done all we can to promote air tightness and perhaps add more (although I doubt it will be needed) at a later date. However, I can only link the roof insulation to the inner leaf of the ICF and then the concrete core remains essentially outside of the "big coat" I am looking for.

In both cases I would have the vapour barrier on the warm side of the insulation; however, do either of these methods pose a higher condensation risk to the rafters? Obviously, we'd be fitting breather membrane with an air gap between the slates and any insulation using counter-battens if necessary.

To be honest, I am leaning towards the over and between approach, but I'm also concerned about how much longer this might take and I'd like to get it water-tight as soon as possible!

I'd welcome any thoughts / suggestions / recollections!

I agree with Tony. If the pitch of your roof is 45 degrees then you will need twice as much roof insulation to keep the same overall heat loss. The surface area increase by 41%, so you need 41% thicker insulation for the same heat loss, meaning you need twice the volume of insulation.

In addition, its much more difficult to avoid thermal bypass in a trussed rafter warm roof & very difficult to make it airtight. So I would put the insulation at 1st floor ceiling level unless making the attic into a habitable space.

If you only need storage then create a platform above the insulation by hanging joists off the trusses with spacer blocks to the ceiling joists below for longer spans.

David

I used a continuous sheet of poly as a joint air tightness and vapour barrier all joined up to the air tightness barrier in the walls.

You should have an air barrier.