Attic rafter insulation

jwt

Saw this on another site and wondered what people here thought of this as a way of insulating the rafters in an attic.



Basically using a strip of open cell foam to act as the 50mm spacer air gap, using a 12mm panel of polystyrene foam cut 3mm wider than the rafter gap and lodge it in place.

Use fibreglass to fill the remaining void beteen the rafters, apply a vapour barrier and effectively drywall over the vapour barrier with more polystyrene and finish with ordinary drywall lining.

Flexible caulking is used to fill any gaps in the final polystyrene layer.


JWT

RKQ

Quite an economical way to do it.... not sure it complies with the latest u-value requirement.
Note the external ply - standard scottish detail. All looks very neat and cleanly installed.:)

jwt

A bit of a google showed styrofoam having a U-value 1.75 times better than glasswool per mm.

So for some rough calculations (and lots of guesstimating)

After looking at the picture if the pink open cell spacer between the plywood and polystyrene is 50mm it looks like 50mm polystyrene is being used as well????

2 x 8 rafters?

So
50mm air gap for ventilation
50mm polystyrene (= 87.5 mm fibreglass)
100mm fibreglass (= 100 mm fibreglass)
vapour barrier(= 0 mm fibreglass)
50mm polystyrene (= 87.5 mm fibreglass)
ignoring the plasterboard

so equivalent to 275mm of fibreglass?


JWT

RKQ

Personally I'd replace the aeroboard with foil backed kingspan / xtratherm.
Especially on the inner face with plasterboard.:)