PIR + Plasterboard?

[CrawfW]

Evening,

Can i fix PIR insulation to rafters, then drill through it to secure plasterboard?

If i use 75mm PIR, that seems like using long screws for plaster board - can they be standard ones, rather than plasterboard ones? Perhaps with washer's to stop the heads pulling through the board?

I was hoping to fit large pieces of the insulation to the bottom of the joists, rather than messing around cutting it to fit between them.

I was thinking of boarding a ceiling with the combined Plasterboard+PIR products, but both of the (Big Name) Builder's Merchants i went to to get some were astonished. One proposed a full pallet, the other swore that such a thing didn't exist - despite me pointing to it in their catalogue......

Advice appreciated!

Crawf

 

[cjard]

The reason why plasterboard/plaster requires black phosphated screws is because it's alkaline and will eat away at normal screws, particularly if the surface gets damp. Rust patches coming through your nicely painted ceiling isn't a good look. Damp isn't necessairly a leak; condensation from human activity such as washing, kettle, cooking and clothes drying are the major sources of internal moisture. You wouldn't put washers under screw heads for PB, as it would leave the screw standing proud and seriously **** the plasterer off. PB screws are best driven with a tool that sets the head just below the surface of the paper, without breaking the board, such as these: https://www.amazon.co.uk/Silverline...0_5?ie=UTF8&psc=1&refRID=HQ9V30QDBY1Y1158S4VJ

Plasterboard screws come in lengths up to 100mm from screwfix, and you should aim to get 20 to 30 mm of thread into your rafter, which leaves you looking at pb+PIR that is up to 70mm thick in total. Screw spacing should be 150mm, at least 10mm from the board edge. if your rafters are 600mm apart, use 15mm plasterboard, 12.5mm maxes out at 400mm unless you're happy with a bit of sagging in future. Don't use 9mm board unless your rafters are really close together.

Personally, I'd fit PIR between and below, and if 75mm is your goal then 50mm between and 25 below would work out well. If this work is notifiable (it sounds like you're converting a loft, so it's quite likely you'll need building control involved) to building control you might find at 75mm isn't enough so check the latest regs for what you're doing.. You should definitely use gun foam to seal the board in between the rafters, so cut the board at least 5mm smaller all round. If you have access to the space above the rafters (maybe not unless this is a new build or the roof is off) do the foaming top down after the ceiling is in as the guns work better that way up. If you don't have access then you can support the board either with cheap screws wound a few mm into the rafter and sticking out, or even a few lines of string stapled perpendicular to rafters. If you're really good with a saw you can cut the PIR on an angle so the top surface is wider than the bottom (wide enough to be a friction fit between the rafters) and then foam fill the triangular gap that remains.

You may find that it's cheaper to buy sheet 25 mm and normal plasterboard, which is why some BM don't do PIR backed PB; it's too expensive and only useful in applications whre it is being glued to a surface. Your mechanical fixing route will work just fine for separate sheets, but you can also glue them together with a small amount of expanding foam(squashed flat immediately after application, it doesn't expand, just becomes PUR glue) for less than the cost of a bonded sheet