Plaster board faced insulation - making good?

[Pretty Mouth]

Hi all. I'm after some practical advice please.

Got a job to quote for, fitting a couple of gimbal downlights in a sloping ceiling in an attic room. The construction appears to be PIR (celotex or similar) insulation between rafters, with plasterboard faced PIR insulation over the rafters, skimmed over. I'm expecting to have to cut out a slab of p-board and insulation at each downlight position, remove enough insulation behind to make room for the downlight, then fix the slab back in place, and make good.

How would I fix such a slab soundly back in place, ready for filling and sanding?

I was thinking when cutting it out, to angle the cuts inwards so the sides are tapered. Then to fix it, use a generous amount of polyurethane glue eg. gorilla glue on the sides and back, push it back in place, and hold it there with battens while the glue sets.

But perhaps you guys and girls know a better way of doing it?

 

[Rockingit]

Hi Simon. I'm not particularly worried about these things happening.

It won't cause moisture to build up in the insulation or roof void. For that to happen you need 2 things: A defective or non existent vapour barrier on the warm side of the insulation, and a vapour barrier/total lack of ventilation on the cold side. I have only ever come across such a situation once, a barn conversion where the builder had used DPM as the roofing underfelt, and left no ventilation at all. The small loft above the vaulted ceiling was damp due to the condensation that would form up there. Any sort of ventilation, or breathable membrane, as should be in place such a roof structure, will prevent that from happening.

It will, however create a cold spot on the ceiling, and so there is a small possibility of mould forming there. I think the risk is low - it tends to happen in moist areas of poorly ventilated houses, which this isn't. A solution is to use insulation coverable downlights, and backfill any void created with rockwool, eliminating the cold spot.

My first thoughts aren't about the repair job afterwards, it's wondering if there's physically enough depth available to fit them - I'd drill a very careful pilot hole first and see how far in a rod goes - as often I've seen builders just jam in celotex to whatever the joist depth is for maximum thermal values and then board over it. Also gives you a potential headache with thermal values for your cables, too.

 

[Pretty Mouth]

Update:

The PIR insulation was only between the rafters, not across the face of them, and wasn't bonded to the plasterboard. After cutting the circular holes, I was able to reach in and pull chunks out of the insulation until there was enough room for the downlights, and was also able to rod the cables in the void behind. So no making good required, an easy job.