Insulated Plasterboard

[Delboy61]

Hi
Am building a single storey extension and the builder has left the plastering to me as at one point we knew a good plasterer.... His wife has produced twins so out of action for a bit. However, the Architect has specified insulated plasterboard for the ceiling (warm roof construction). The builder says he would use 25mm Celotex and then plasterboard. I could do the former leaving the plasterboard to our lasterer.

Do I cut the Celotex and fit in the joists with battens or- sheet the whole underside with 25mm or 20mm Celotex ?? That surely makes finding the joists to screw the plasterboard tricky ??

Thoughts welcome
Delboy

 

[cross thread]

Mark the joist position on the wall,

 

[nickjb]

If it is a warm roof construction why do you need insulated plasterboard? All the functional insulation will be above the deck and normally you just plasterboard onto the joists otherwise there is a risk of condensation onto the joists.

 

[Notch7]

the Architect has specified insulated plasterboard for the ceiling (warm roof construction

That makes no sense.

If it’s a warm roof the insulation must be directly below the roof decking boards and firrings and roof joists below the insulation.

Maybe these days you can add insulation on the warm side to help get the u value down….but it doesn’t sound like a good detail

 

[Delboy61]

Thanks Guys for your replies
Good points

 

[robinbanks]

but it doesn’t sound like a good detail

Not sure why not?

surely makes finding the joists to screw the plasterboard tricky ??

Mark the joist position on the wall,

And then a laser level whose line is touching both marks will line out the entire joist for you

You should buy boards to match your joist spacings - if your joists are 400 buy 1200 boards. You can then cut yourself a stick 400 long, and using two hands and a pencil hold the pencil against the end of the stick, and your other hand guides the other end of the stick along the plasterboard edge so you can quickly draw a line at 400 the entire length of the board. Now when lifted into place and lined up with the laser or a board edge, you have lines for the other joists (you can do it with a tape measure suitably locked too, but I tend to find a bit of wood quicker and easier)