how to join plasterboard on PIR insulation on sloped corner of dormer??

[Tirglas_7]

hi

rennovating my 80's house and am retrofitting PIR insulation into the top floor dormers. i have 50 mm between the timbers and am overboarding with 100mm then plasterboard on top.

where the internal vertical side of the dormer cheek and the internal sloped surface of the roof meet, i will end up having an overlap of insulation, which will mean there won't be anything to attach a corner bead to as the corner will be made up of PIR insulation.

i have contemplated using 2 100mm timbers along the length in a dog leg affair or cutting a channel in the insulation and inserting a batten of some sort?

is there a simpler way round this or some trick of the trade?

thanks in advance

 

[Doggit]

This is a difficult one to describe, but you need to cut back the insulation on both of the boards, one by the thickness of the insulation, and the other by the thickness of the plasterboard. They then join together and allow you to put the corner bead on.

 

[Tirglas_7]

ok not sure what you are explaining if im honest , but i'm glad there is a method!

so just to confirm this is on separate insulation and plasterboard, not the insulated backed plasterboard right?

how is the bead fixed in place with nothing behind?

 

[Doggit]

Sorry, I thought it was insulated plasterboard; but if it isn't, what's the issue. Are you coming down the vertical cheek, and then under the overhanging slope. The insulation goes on the slope, then goes on the vertical cheek down past the sloping insulation. The question becomes how you're fixing the insulation, and then the plasterboard, as ye, it'll be slightly wobbly if not done securely. The insulation needs to be screwed with washers underneath the screws, otherwise it'll pull off, but once it's secure, you could use adhesive foam to fix the plasterboard to the insulation.

 

[Tirglas_7]

plan was to overboard with the insulation attaching with enough long screws and metal discs to be solid, then attach the plasterboard with more screws through the pb and insulation into the timbers.

is that a bad idea? should i be using the adhesive foam only? or could i use both for extra security?

yes down the cheek with one piece that overhangs the slope by 100mm, then another piece on the sloping part so that both faces have a continuous covering of 100mm.

will the bead attach somehow to a plasterboard corner with only insulation behind though? i assumed it would need some kind of timber or masonry behind to nail the bead to??

this kind of thing for the adhesive ok? https://www.screwfix.com/p/dow-insta-stik-expanding-foam-grab-adhesive-gun-grade-750ml/72793

many thanks for the advice btw

 

[Doggit]

It sounds as though you've actually worked it all out already. And belt and braces wouldn't be a bad idea considering it's got to be self supporting, and I'd almost go as far as putting some adhesive behind the insulation on the wood battens etc first.

I can now see where you're coming from regarding fixing the angle bead, but they don't need screwing in, and they can be glued to the plasterboard with the foam adhesive. If it was a vertical beading, then a lot of plasterers will use the plaster itself underneath the beading to act as an adhesive for it.

 

[Tirglas_7]

brilliant, thats just saved me a load of work i thought i needed to do then!

i'll just stick them on then, would you recommend a specific type/size e.g. the plastic ones in that area? or will the normal galvanised ones do the job?

plan is to do the room in the tapered edge pb and do all the joining and filling myself. then paint directly onto the pb. anything to watch out for using this method? i realise i'm not going to get a pro plasterer's finish however its a kids room and were trying to keep spiralling costs down.

 

[Doggit]

You either want a nailable fixing, or a washer for standard screws. But I don't think you can use a corner bead on tapered edge plasterboard, but it might be possible to then add the adhesive to form the corner. Corner beads are normally for ordinary finish plaster, and I've no experience of what you're trying to achive - sorry.