Insulated plasterboard or plasterboard with separate insulation board

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OrangeJuice09

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Hi,

An extension with a flat roof has been built and BC wants insulated plasterboard installed on top of a vcl for the ceiling.

The plasterer suggested rather than using insulated plasterboard 35mm ( https://www.british-gypsum.com/prod...oc-thermaline-plus-35mm#technical-description ) to use a separate 25mm insulation PIR with a separate 9.5mm plasterboard. He said there is less waste as the insulation boards don’t need to be cut to meet the joists, but only the plasterboard.

I think there are some advantages as the PIR has foil on both sides and would act as another vcl and perhaps it would be easier to install. Are there any drawbacks with this solution ?
 
That sounds sensible, and will be cheaper.

Only downside is a bit more labour, but other savings should make up for that
 
The plasterboard will be screwed through the insulation board (PIR) into the joist. There maybe slight gaps between the two boards. Can this be an issue for mould ?
 
The insulation boards should have expanding foam sprayed between the gaps, then aluminium tape over the joints.
 
You might find using Wedi washers useful, I did, stops screw heads being pulled through the PIR.
 
OrangeJuice09 said:
The plasterboard will be screwed through the insulation board (PIR) into the joist. There maybe slight gaps between the two boards. Can this be an issue for mould ?
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Sorry, wasn't clear. I didn't mean a gap between the edges (which can be foamed/taped), but rather the surfaces of the two boards.
 
OrangeJuice09 said:
Sorry, wasn't clear. I didn't mean a gap between the edges (which can be foamed/taped), but rather the surfaces of the two boards.
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That shouldn't be an issue, as they will be filled with plaster.

Even if they are foil backed boards, there will still be 2 layers of foil in the insulation stopping water vapour
 
Do you also have insulation between the flat roof joists?
How is the roof void ventilated?
 
@jeds Insulation has been added between the joists and sealed with foam. The roof void is not ventilated.
 
OrangeJuice09 said:
@jeds Insulation has been added between the joists and sealed with foam. The roof void is not ventilated.
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Bit of a problem there. A flat roof is normally a cold roof or a warm roof. (there are others but no need to complicate it here) A warm roof has insulation above the deck, keeping the roof deck warm. Being warm, it does not suffer from condensation and therefor does not need to be ventilated.

A cold roof (what you are building) has insulation between and below the roof joists. This means the underside of the roof deck is cold. Being cold it will suffer from condensation unless it is ventilated.

Regarding insulation; the insulation you linked to is polystyrene, which would not comply with regulation standard. (I'm surprised your building inspector recommended it?) In order to achieve the equivalent of PIR it will need to be double the thickness - i.e. 80mm.

Standard insulation to achieve old regs would have been 100mm PIR foam between joists and 40 or 50mm PIR continuous layer beneath joists. New regs would be more than that. Are you old or new regs?

Also, with cost of heating these days, most people are increasing the insulation to the maximum.
 
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