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?

 

[mydigitalhome]

Need to make sure your light fitting can function with that installation method. Plus cable type due to being buried in insulation. Have fun

 

[Pretty Mouth]

Need to make sure your light fitting can function with that installation method. Plus cable type due to being buried in insulation. Have fun

I hear you fella. But any suggestions on making good afterwards? Easy when there's no insulation stuck to the plasterboard, but not so straightforward with the insulation there.

 

[OnlQQker]

Nothing much will hold it firmly. All I would do in this situation is screw bits of baton up behind the cut out sections and screw them back in afterwards.
Alternatively you could try fixing them in with expanding foam and then taping the joints with

https://www.ebay.co.uk/i/3729762632...7779&msclkid=1607135f59431391a31b4ccffd05cfc4

Finish with caulking tool & easyfill. Easyfill is easy to sand and comes up perfectly flat.

 

[Pretty Mouth]

Nothing much will hold it firmly. All I would do in this situation is screw bits of baton up behind the cut out sections and screw them back in afterwards.
Alternatively you could try fixing them in with expanding foam and then taping the joints with

https://www.ebay.co.uk/i/3729762632...7779&msclkid=1607135f59431391a31b4ccffd05cfc4

Finish with caulking tool & easyfill. Easyfill is easy to sand and comes up perfectly flat.

Thanks mate. Thinking about it, if I initially remove a larger piece, enough so that it spans 2 rafters, then I have something to screw to when I replace it afterwards. I'll use some gorilla glue too, where the cut insulation joins. It's polyurethane, similar to expanding foam (but without the expanding properties obviously), and I suspect it will adhere well to the insulation.

 

[Diytim]

Screw a piece of battening across the hole you have made with plasterboard screws either side and countersunk. Then cut a piece of plasterboard the correct size to fill the hole you have made and screw that to the battern. Finally skim over joint with plaster or filler as appropriate.

 

[Mike Johnson]

Just a Heads up, be careful when using any type of glue with board type insulation, some of the glues will react with the insulation and melt it, try a test piece first.

 

[Pretty Mouth]

Just a Heads up, be careful when using any type of glue with board type insulation, some of the glues will react with the insulation and melt it, try a test piece first.

Thanks. I've seen this happen with expanded polystyrene and certain solvent based glues, but not with PIR insulation. Have been given the job so will do a test and report back.

 

[Simon47]

I would suggest you advise the customer that it's a bad idea.
If I read it correctly, you are going to cut a big hole in the vapour barrier that'll be in the sandwich between the plaster and insulation layers, and take a lump of insulation out of their roof to create a cold spot. So they'll have a spot that's going to let moisture through into the insulation (so over time, waterlog it and stop it insulating), and that will be cold (due to hole in insulation, and moisture in surrounding insulation) so as to cause condensation and mould.
And of course, when they do start to get cold damp mouldy patches in their ceiling - do you think they will say "silly me for having that done", or blame you ?

 

[Pretty Mouth]

I would suggest you advise the customer that it's a bad idea.
If I read it correctly, you are going to cut a big hole in the vapour barrier that'll be in the sandwich between the plaster and insulation layers, and take a lump of insulation out of their roof to create a cold spot. So they'll have a spot that's going to let moisture through into the insulation (so over time, waterlog it and stop it insulating), and that will be cold (due to hole in insulation, and moisture in surrounding insulation) so as to cause condensation and mould.
And of course, when they do start to get cold damp mouldy patches in their ceiling - do you think they will say "silly me for having that done", or blame you ?

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.

 

[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.