Double insulation

[Albumen]

I'm about to do a downstairs toilet ceiling. The roof above it is a flat roof.

Plan - not bother trying to stuff mineral wool in between the rafters because a)it's a pain, b)there's a nice ventilation space there without it.

Instead. fix Celotex/Kingspan/whatever PIR type board I can get up to the underside of the joist. Then underneath that make a frame for the plasterer to fix his plaster boards to. It's a really small room (1650x650). But to do it without any joints I'll have to buy a full 8x4 sheet of PIR.

Questions:

1. Does that sound like a sound structure? For moisture control I'll either pin some DPM up just above the plasterboards. Or ask him to use the board that's got moisture control built it.

2. If I have to buy a full sheet which is way more than I need. Is there any problem with buying have the thickness I want, and doubling it?

Cheers, Mike

 

[geraldthehamster]

I'm about to do a downstairs toilet ceiling. The roof above it is a flat roof.

Plan - not bother trying to stuff mineral wool in between the rafters because a)it's a pain, b)there's a nice ventilation space there without it.

Instead. fix Celotex/Kingspan/whatever PIR type board I can get up to the underside of the joist. Then underneath that make a frame for the plasterer to fix his plaster boards to. It's a really small room (1650x650). But to do it without any joints I'll have to buy a full 8x4 sheet of PIR.

Questions:

1. Does that sound like a sound structure? For moisture control I'll either pin some DPM up just above the plasterboards. Or ask him to use the board that's got moisture control built it.

2. If I have to buy a full sheet which is way more than I need. Is there any problem with buying have the thickness I want, and doubling it?

Cheers, Mike

This is what I would do (and indeed have done):

1. Cut 50mm Celotex and fit between the rafters. Cut to approx. size, fix in place temporarily with nails or screws then fill round the edges with expanding foam, which will make it air tight and fix it firmly.

2. Screw 50mm Celotex under the whole thing, up into the rafters, using 75mm plasterboard screws. Use foil-backed Celotex, which provides a vapour barrier.

3. Screw plasterboard under the lot, using 100mm plasterboard screws.

4. Get it skimmed.

As an alternative to 2, and 3, you could use insulated plasterboard, that has a layer of insulation bonded to it.


Cheers
Richard