Vapour control in flat roof

shayd

Hi all
I'm insulating an existing flat roof, taking the ceiling down, putting in 100mm fibreglass(7" joists, so 70mm gap left for ventilation), then using 50mm insulated plasterboard to slab ceiling. I was advised to use 500gauge polythene tacked to underside of joists, to act as vapour control, to stop insulation getting damp. But then was advised by someone else to use majpell vapour control, as it allows air through but not humidity. the majpell stuff is over 200 euro though so not sure what to do...
any advice appreciated,

shay

BryanF

Shay, IMO the ordinary VB taped & sealed will do. BUT this vapour control layer should really be inside any insulation.. It's all about avoiding a 'due point' ( condensation forming) within the insulation & structural zone.

shayd

thanks bryan, got a recommendation for some breathable felt that's not too expensive. going to go with that

BryanF

So tell us your build up from roof covering to plasterboard?

shayd

Roof build-up as follows : torch-on felt,
plywood,
170mm joists so leaving 50mm gap for ventilation,
rafterloc 100mm, or maybe 2 x 60mm, depending on price,
breathable felt vapour barrier,
37.5mm insulated plasterboard.

An alternative to the rafterloc would be isover moy metac glass wool insulation, but would need 150mm to get a similar u value to the rafterloc, leaving only 20mm for ventilation. The extra cost of the rafterloc is probably worth it, in the long run.

I have another flat roof to insulate, but it only has 4" joists, and ceiling height doesn't allow for much thickness of insulated plasterboard, so getting advice next week about external insulation, or even replacing it, depending on relative costs.

shay

BryanF

Felt goes on the outside, vapour control layer on the inside of a roof build-up. They are two different things.

shayd

ok, thanks for the tip

jimmytom

Hi All,

I have recently a flat roof constructed as part of new build, however i am not sure as to whether a vapour control layer has been installed under the insulation, most likely not. Can i do anything to ensure there is no future problems with condensation etc. I have yet to slab under the roof joists. Also would the introduction of insulation between the roof joists as well as the insulation which has already been installed above the roof deck cause any issue?

Any info much appreciated.

Regards

Jimmytom