Vapour barrier on underside of rafters.

Kerrigooney

Hi all. I`ve just spent an hour trawling through threads looking for some advice and I`m really sorry if this has been asked loads of times.

I`m converting an attic space in my house and intend to put 100mm insulation between the rafters followed by insulated plasterboard on the underside of the rafters...my question is...do I need to put a vapour barrier on the underside of the rafters before fitting the insulated plasterboard?

Thank you.

Kerrigooney

Anybody? Please?

DOCARCH

If the insulation you intend to use (especially the thermal lining board) is foil faced/backed then generally you do not need a seperate vapour barrier.

Follow manufacturers advice/instructions with regard to installation of the thermal lining board particularly in connection with taping and filling of joints to complete the vapour barrier.

Personally, I would not use thermal lining board, I would use Xmm (whatever is appropriate) of PIR foil faced insulation to underside of rafters (and insulation between rafters) - temporarily fix in place - seal/tape all joints of the insulation with an aluminium foil tape - then apply/fix plasterboard to underside. This is a better method for ensuring completeness of vapour barried.

wayoutwest

YES

Kerrigooney

DOCARCH wrote: »

If the insulation you intend to use (especially the thermal lining board) is foil faced/backed then generally you do not need a seperate vapour barrier.

Follow manufacturers advice/instructions with regard to installation of the thermal lining board particularly in connection with taping and filling of joints to complete the vapour barrier.

Personally, I would not use thermal lining board, I would use Xmm (whatever is appropriate) of PIR foil faced insulation to underside of rafters (and insulation between rafters) - temporarily fix in place - seal/tape all joints of the insulation with an aluminium foil tape - then apply/fix plasterboard to underside. This is a better method for ensuring completeness of vapour barried.

Thanks for the reply DOCARCH. No,the insulated plasterboard I`m using isn`t foil backed so do I need a barrier in that case?

Kerrigooney

wayoutwest wrote: »

YES

Thank you wayoutwest. Would you mind recommending one for me?

boatbuilder

Just use plastic, polythene

DOCARCH

Kerrigooney wrote: »

Thanks for the reply DOCARCH. No,the insulated plasterboard I`m using isn`t foil backed so do I need a barrier in that case?

Yes, you will. I was wrong to assume! What insulation are you using bewteen the rafters?

wayoutwest

DOCHARCH - Are you mean't to have an airgap between the insulation[IN the rafters], and the underside of roofing felt. How does the air flow in and out [via slate vents etc] of the roof when doing an attic conversion? Thanks wayoutwest.

Kerrigooney

wayoutwest wrote: »

DOCHARCH - Are you mean't to have an airgap between the insulation[IN the rafters], and the underside of roofing felt. How does the air flow in and out [via slate vents etc] of the roof when doing an attic conversion? Thanks wayoutwest.

I`ve got 6in rafters and I`m putting 4in of insulation between the rafters so I`ve got a 2in air gap vented at the eaves.

DOCARCH

wayoutwest wrote: »

DOCHARCH - Are you mean't to have an airgap between the insulation[IN the rafters], and the underside of roofing felt. How does the air flow in and out [via slate vents etc] of the roof when doing an attic conversion? Thanks wayoutwest.

Yes, normally 50mm (even with breathable roofing felt I would suggest this gap be maintained - some will say this gap can be reduced to 25mm with breathable roofing felt).

In this situation, i.e. when converting an attic, or insulating along the line of the roof/rafter, you should be providing the equivalent of 25mm continuous ventilation at eaves and the equivalent of 5mm at ridge for airflow.

The OP's query is in relation to something slightly different in that if you have a habitable/heated space that air will contain a much higher level of moisture so you do not want that moist air travelling through your insulation and the moisture condensing within the insulation or the roof space, thus, you need to provide a vapour barrier/control layer. My preference is to have this vapour barrier/control layer on the warm side of the insulation (rather than within the layers of insulation).

wayoutwest

DOCARCH - would fitting rigid insulation between the rafters[as well as below]be better than, say,Rockwool -in that the airgap would be maintained, eg:no slumping [or restrictions caused when fittng]

Kerrigooney

Thanks very much lads...I think I have everything I need to know now. Fair play to ye:D

DOCARCH

wayoutwest wrote: »

DOCARCH - would fitting rigid insulation between the rafters[as well as below]be better than, say,Rockwool -in that the airgap would be maintained, eg:no slumping [or restrictions caused when fittng]

I would suggest yes.