do i DPM between rafter insulation and plasterboard?

Doglover88

got some Kingspan to fit between my rafters (will be leaving minimum 50mm between insulation boards and the felt/roof to allow for air movement). I heard I should fit a damp proof membrane on top of the insulation before I plasterboard over the top, is this right?

many thanks

thehairybuilder

glenbat ,
I`m sure there is no need for this as your kingspan is a sandwich of foam between 2 layers of foil so moisture will not penetrate. Also there should be no moisture inside your roof . . . . ? If you`re worried you could simply tape the joints.

bcoman

To get the air movement you will need ventilation at the eaves and at the top of the roof to get a flow of air and disperse any water vapour before it condensates in the void, I'm assuming you have the old fashioned bitumen felt. Taping the joints certainly helps to stop moisture getting through, but if you fit between rafters its difficult to seal the sides. The polythene vapour control layer would be the best way. Check with Kingspan technical, they are usually quite helpful. That way if you do get any problems at least you know you have installed in accordance with the manufacturers guidance. Also website https://www.kingspan.com

kat21

just to add you could also install foil backed plaster boards too.
kat21