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I am in the process of getting an extension designed for the house and part of the new drawings is insulated plasterboard.

As I will be doing most of the internal work myself and have never used insulated plasterboard, I had a few questions.

1. Is it worth it? All the walls will be cavity walls and have PIR based insulation in them. Is it still worth having insulation between the plaster and inner course of blockwork?

2. I have been reading a lot about vapour barriers to prevent the possibility of mould build up where there are thermal variences. Generally speaking we have not needed to consider this in the past with plain plaster.

3. Has anyone done this and if so any advice?

Some feedback I have had generally is that it is not worth it .... but I am quite interested in chasing as low energy consumption (for heating at least) as possible so in my mind I am willing to go further for a bit more energy efficiency.

Thoughts welcome.

It is much better to put 100mm of PIR in the cavity than to put 50mm in the cavity and 50mm inside. This is because you get the thermal mass of the inside course of bricks (high density blocks are best for this even if not required for structural calculations).

I’ve fitted insulated plasterboard as retrofit to improve a super-cold extension built by a previous owner. Went up easy and made a difference to the room. But as I say, not what I’d do if building from scratch.

I have to question the use of plasterboard in many builds / renovations.
We know disposal is a bitch (specialist depository at tips, if at all)
Why not board out with Hardiebacker board or similar? (& insulate with foam board behind it)
What room is it (regardless we humans do produce a lot of moisture), build the problem out of it as much as possible.

We ripped out the naff plasterboard in the bathroom & hardiebacker'ed the lot as well as tanking the floor (despite it not being a wet room, but "water" so logical step.

The rooms being extended are the living room and the kitchen.

I had a quick look at this hardie backer board and it looks like what you would put behind tiles?

I am not sure it is the appropriate thing to put in a living room/living area or the kitchen?

Stinsy wrote: ↑Sat Aug 12, 2023 12:20 pm It is much better to put 100mm of PIR in the cavity than to put 50mm in the cavity and 50mm inside. This is because you get the thermal mass of the inside course of bricks (high density blocks are best for this even if not required for structural calculations).

I’ve fitted insulated plasterboard as retrofit to improve a super-cold extension built by a previous owner. Went up easy and made a difference to the room. But as I say, not what I’d do if building from scratch.

The cavity wall insulation will already be approximately 100mm ... The insulated plasterboard seems to be in addition to the cavity wall insulation.

In terms of your super cold extension, did the insulated plasterboard bring up the efficiency of the room to what you hoped or was it just a stop gap measure?

Thanks

It is more appropriate than plasterboard if moist kitchens are a thing you want to minimise.
We used the thickest variety screwed into the uprights, (insulation behind it obviously) & yes tiled over with big travertine.

Has given excellent sound reducing qualities & added fireproofing, incredibly durable, improved rodent resistance etc.

I'm certain (barring painted exterior colours for fireproofing house sidings & moisture control) that someone with have overlaid it with something more than tiles.

We used it 15 odd years ago, would jump straight on it for the kitchen as added protection & strength over PB & its famous moisture migration throughout the home.

Plasterboard is a cheap solution & accepted because it is everywhere, but I wouldn't use it anywhere unless I really had to.

cojmh wrote: ↑Sat Aug 12, 2023 8:18 pm

Stinsy wrote: ↑Sat Aug 12, 2023 12:20 pm It is much better to put 100mm of PIR in the cavity than to put 50mm in the cavity and 50mm inside. This is because you get the thermal mass of the inside course of bricks (high density blocks are best for this even if not required for structural calculations).

I’ve fitted insulated plasterboard as retrofit to improve a super-cold extension built by a previous owner. Went up easy and made a difference to the room. But as I say, not what I’d do if building from scratch.

The cavity wall insulation will already be approximately 100mm ... The insulated plasterboard seems to be in addition to the cavity wall insulation.

In terms of your super cold extension, did the insulated plasterboard bring up the efficiency of the room to what you hoped or was it just a stop gap measure?

Thanks

Certainly made a noticeable difference. Was worth the time and effort.

Bit late probably, but no the proposed design is stupid. If you want a better U-value make the cavity wider. Isolating the internal leaf from the inside just reduces thermal mass and thermal mass on the inside is a good thing as it gives you steadier temperatures. I have a 200mm cavity in my extension (using the build details from the Green Building Store passivehouse. That has dritherm 32 rockwool in it full-fill (I'm in east Anglia not North Wales coast so that's fine), for a wall U-value of 0.15. You could use PIR instead to save space but it has to be built right not to have loads of gaps, and PIR/PUR is quite a high-carbon material unless you get the new Unlin low-carbon stuff. Also use low-conductivity wall ties.

The other thing I'd say about building an extension is make sure there is a thermal break in the inner leaf at/below floor level to break the thermal bridge of the intneral wall onto the footings. I used foamglass perinsul blocks but there are cheaper options these days.

And finally think about the thermal bridge where it joins the house. Most extensions are just butted up against a brick wall so there is a massive thermal bridge to the inside here. Cut a slot and make the insulation continuous. Again I put foamglass blocks in where the roof joined the wall (my extension was single-storey). I thought this would be expensive but it only took my builder a day with one bloke. Sadly most architects are useless on building physics and thermal efficiency and never think of this stuff.

Hi,

Thanks for all of the suggestions, they make sense.

It is not too late - currently things are still tied up with planning so I can certainly implement many of the suggestions.

I am sure that I will be back on here when we come to actually build it as I am convinced the architects are taking the standard easy way to do stuff and I am not convinced that there are not better ways to do most of this.

Thanks again