How to stick celotex to plasterboard

[Hl.house]

Dear all, this forum is a treasure for me. Saving a fortune on many works.
I have a good amount of plasterboard and 100m celotex left over. Can I use some adhesive to stick celotex to a plasterboard and screw(insulation + PB) them under the rafters of one of the sloping ceilings that I have.
1) If yes, which adhesive? Insta stik, siroflex, dot&dab adhesive, any others?
2) What screws do I use? 100mm cletotex + plasterboard + 25mm?
3) Which type of screws? Plastic, phospate coated? Intention is to avoid thermal bridging. Or, is thermal bridging only an issue when used on masonry and not when screwed to timber? Complete novice here.

There is already 100mm earthwool insulation in between the rafters. As the span is small the rafter depth is less(140mm total). Please advice.

 

[Hl.house]

Anyone please ...

 

[tomfe]

Your fixings will be enough but for ease of installation any grip fill or PVA/pu will do it. It will only be as strong as the foil face though.

 

[^woody^]

Double sided tape.

 

[Hl.house]

Thanks tomfe and woody!!!
Will 150mm screws be good and do they need to be any specific type or normal dry wall screws are OK?

 

[chappers]

150 mm drywalls, no need to glue the board to the insulation use enough screws to hold the insulation up then 150mm drywalls through the lot, use plenty.

 

[cjard]

Use black phosphated screws intended for drywall. Normal screws ma corrode due to the plasterboard being alkaline.

If you're screwing then you don't really need to glue the celotex and PB together, but gun foam works well.. when fired at a distance it's like silly string.. put a spaghetti mess of it on the pub, wait a couple of minutes for it to go tacky then drop the plasterboard on it and weight it down

 

[Hl.house]

150 mm drywalls, no need to glue the board to the insulation use enough screws to hold the insulation up then 150mm drywalls through the lot, use plenty.

Thanks chappers. I did not quite get how I will hold the celotex board under the ceiling joists? Do I screw up the celotex boards to the joists first(just enough screws to hold) and then screw plasterboard on top with usual 15 -20cm between two screws?

 

[Hl.house]

Use black phosphated screws intended for drywall. Normal screws ma corrode due to the plasterboard being alkaline.

If you're screwing then you don't really need to glue the celotex and PB together, but gun foam works well.. when fired at a distance it's like silly string.. put a spaghetti mess of it on the pub, wait a couple of minutes for it to go tacky then drop the plasterboard on it and weight it down

Thanks. Yes, I have used the black phosphate coated drywall screws for normal plasterboards. Can they be used while penetrating through the insulated board? I read somewhere that you have to use plastic screws to avoid cold/thermal bridging. Is that true?

 

[^woody^]

read somewhere that you have to use plastic screws to avoid cold/thermal bridging. Is that true

No, not in your case. 100mm of insulation, and relatively warm timber will not permit a thermal bridge

 

[Hl.house]

No, not in your case. 100mm of insulation, and relatively warm timber will not permit a thermal bridge

Thanks woody! Is there an insulation thickness threshold for plastic screw requirement?

 

[^woody^]

Thanks woody! Is there an insulation thickness threshold for plastic screw requirement?

No, its all really down to science. The screw needs to be in contact with a colder surface (or colder air) for there to be a chance of thermal bridging in the first place. That wont be the case in your situation.

 

[oldbutnotdead]

Gripfill (the solvent free version) does the job you're after quite well.
If you're screwing the celotex to the joists you may need some penny washers (celotex is quite strong under a distributed load but rubbish under a point load), especially if the joists/rafters are old and not particularly straight

 

[Hl.house]

Gripfill (the solvent free version) does the job you're after quite well.
If you're screwing the celotex to the joists you may need some penny washers (celotex is quite strong under a distributed load but rubbish under a point load), especially if the joists/rafters are old and not particularly straight

Thanks. We have put a couple of drywall screws to hold and the big long sheet seems to be holding up for now. Plaster boarding will happen Today.

 

[^woody^]

Here we go. Just use steel strap banding instead of washers.