Platerboard and/ot polythene membrane for roof

[SENCO]

Hello - the pitch of the roof forms part of the upstairs room in the barn we have, i.e. there is no ceiling. I have put 75mm Rockwool between the joists and then stapled "tri-iso" ( the insultion matl. contructed of ali foil/multi layer insulation/ali foil) to the joists to form a blanket of this material on the inside of the roof . In accordance with good practice (eg - data sheet from Yorkshire Building Supplies which shows how to use their "superquilt" ( www.ybsinsulation.com/PDFs/superquilt.pdf) I have then put battoning onto which I have started to fasten the plasterboard. I have now been told that it's advisable to use foil backed p/board or produce a polythene moisture barrier between the p/board and the "tri iso".

Given that the "tri-iso" must be a moisture barrier in itself is it necessary to use foil backed p/board in the roof ? -

If I didn't use foil backed p/board, and  constructed a polythene membrane behind the p/board,  what thickness or gauge should the  polythene be please and do all joints have to be taped etc?

Thanks for your help

[thunderhorse]

[quote user="SENCO"]

Hello - the pitch of the roof forms part of the upstairs room in the barn we have, i.e. there is no ceiling. I have put 75mm Rockwool between the joists and then stapled "tri-iso" ( the insultion matl. contructed of ali foil/multi layer insulation/ali foil) to the joists to form a blanket of this material on the inside of the roof . In accordance with good practice (eg - data sheet from Yorkshire Building Supplies which shows how to use their "superquilt" ( www.ybsinsulation.com/PDFs/superquilt.pdf) I have then put battoning onto which I have started to fasten the plasterboard. I have now been told that it's advisable to use foil backed p/board or produce a polythene moisture barrier between the p/board and the "tri iso".

Given that the "tri-iso" must be a moisture barrier in itself is it necessary to use foil backed p/board in the roof ? -

If I didn't use foil backed p/board, and  constructed a polythene membrane behind the p/board,  what thickness or gauge should the  polythene be please and do all joints have to be taped etc?

Thanks for your help

[/quote]

Hi, SENCO. Why are you wanting a moisture barrier? Is it to prevent ingress of rain? If you mean a vapour barrier, then the rockwool itself is so backed. Had the rockwool been thick enough, then the expensive foil stuff would have been totally unnecessary. Ordinary BA13 plasterboard should suffice hung from rails.

Generally speaking, stone-built houses and barns need to breathe.

Hope that helps a bit.

[SENCO]

hello - thanks for your reply. Concerning the polythene membrane - yes - it's a vapur barrier and i am just copying what iis on ybs website - http://ybsinsulation.com/superquilt14.htm - I am just worried about moist air from inside meeting the cold from outside and condensing onto the roof timbers and creating a problem later. The builder put a layer of some kind of reinforced polythene  underneath the tiles (a bit like roof felting used to be used here) and as i said I have now put Rockwool between the joists and stapled this tri-iso to the joists, then put 25mm cross battoning on to create the the required gap between the p/board and the tri-iso to get the full insulation effect. You seem to think that ordinary p/board will do and that I do not need to create a vapour barrier with polythene or  use foil backed board - is this what you suggest? - Again - thanks for your reply