cutting celotex thinner ok?

[markdadude]

Hi there, I have a cold roof in my new flat roof extension. I have a load of 100mm celotex and the joists they are fitted between are 150mm deep. What I have done is cut a slice of celotex to a thickness of 50mm, put that in first and then put a full thickness piece underneath it. Its messy work and im not done yet i would just like to check if you guys think this is ok. Obviously it would be better to have 150mm celotex in the first place. The thinner piece on top cant go into the vetilation void above because there are long battons running perpendicular.

also should I put a membrane over before putting the plasterboard up, and if so what type? thanks

 

[Deluks]

Wouldn't it be easier to just buy some 50mm celotex?

You should insulate between and below the joists (unless your roof deck is insulated?)

Get some more celotex, about 25 - 30mm thick and completely cover the underside of the joists, then tape all the joints with aluminium foil tape, then you can plasterboard. (don't forget to get the longer PB screws)


You can also get plasterboard with a foam insulation backing to make the job easier, but as you can't tape the joints then you should put some medium gauge polythene sheet all over first, you can buy this from screwfix or wickes branded as 'vapour barrier' (cos that's what it is!)

 

[markdadude]

hi again deluks - thankyou. the ceiling is already quite low because there was a window installed between the extension ceiling and existing house ceiling. I was not aware that i MUST place insulation below the joists. is there a way out for those with low ceilings?

celotex is very expensive so would rather not buy more, also 50mm celotex would allow zero tolerance and i can forsee the celotex not finishing flush with joists in every spot because that's the kind of thing that happens to me (the stuff i have is more like 105mm thick - oh yeah and i overbought and i have the stuff to burn).

the roof dieck is not insulated there is about an 80mm sloping to 40mm well vented air space achieved with counter battens over the joists, onto which the ply deck sits.

surely the vapour barrier would be broken by the electic cables for ceiling lights?

edit - hmm I might get some 27mm insulated plasterboard then

 

[Deluks]

edit - hmm I might get some 27mm insulated plasterboard then

Very wise.

You can make a small hole for poking the wires through, be sure to tape up around the wire with pvc tape. Not using downlighters are you? (hope not)

 

[markdadude]

hehe no Im not using flush mounted spotlights. they were my original choice but after much reading i came to the conclusion that they are simply not feasable in a single storey extension unless you have the ceiling height for a large void underneath the joist or want to freeze in winter - correct?. I will go with a track light, 2 wall light and under cab lighting (kitchen extension)


oh btw i have a surplus of the heavy duty membrane i used for screeding. its ok too right? probably trickier to fit tho. can i staple to joists, how fussy do we need to be with holes

 

[Deluks]

It's ok to use heavier, just more awkward to fix. No holes, tape em up with pvc or silver tape. Ditto the overlaps.