I have just done a similar thing with Diffutherm wood-fibre boards between joists (other boards are available, cheaper), screwed up to the floorboards, meshed and lime plastered. Nothing like as good as Aerogel, or even Pu/PIR, but significantly better than nothing.

Hi Andy,

You said you were looking for a thickness of around 40mm. I used 40mm Diffutherm reveal board, which is not T&G. It's about 600 x 1200, and strangely, if I remember rightly, the 20mm version is 600 x 1100.

I doubt you could paper it, though!

For a not dissimilar job, I used ordinary plasterboard carefully cut to shape. I then laid it on the bench and stuck Styrofoam to the back with spray adhesive, then screw fixed. You can choose the Styrofoam thickness to suit.
Careful "Foam gun,- fine nozzle" foam-filling around the edges and then the choice of finish is up to you, - plaster, standard or heavy duty or even themo-lining paper, and then emulsion finish.

Square edged woodfibre boards are readily available. You'd most likely have to prime paint it first, then use a good heavy duty, ( best is ready mixed ), wallpaper adhesive. Getting a nice tight fit between the joists will be the hard bit.

Can't manage to start a new topic so have had to hijack and old one so apologies about that.


The son of a good friend is currently extending the back of his house. As part of this the ceilings will be insulated. His architect's plans have a rolled insulation between that ceiling joists with solid sheet insulation (Kingspan style) under the joists. Then 2"x2" used to create a service void to which plaster board will be attached.

What would be the best way to attach the plasterboard, 2x2, and sheet insulation to the joists with minimal thermal bridging?

The extension sounds like a single story with a flat or minimal slope roof.

I would fix the insulation to the joists with screws which could be removed once the 2x2s are in place. I would also foam between the insulation boards. Fix the 2x2s through the insulation to the joists with good sized screws spaced so as to match the PB board size (to minimise cutting) (I would also use EPS rather than Kingspan)

You don't mention a VCL - this will be needed if it is a flat roof. To maintain the integrity of the VCL it should be placed between the board insulation and the 2x2s. If you are hoping the Kingspan will act as the VCL then very close attention need to be paid to the joints. I don't trust sticky tapes to stand the test of time.

Put cabling in the service void before the PB goes up. Easy if it is a DIY job, otherwise trades people need to be time managed - which they don't like if they are not the same firm.

I wouldn't worry about the cold bridge of the screws holding the 2x2s through the insulation

Posted By: WillInAberdeenConsider filling the service void with quilt insulation, after the cabling has been done. Then you won't need to use such thick rigid insulation boards, and then the screws can be shorter. Empty service voids are a conduit for drafts and mice and noises ime.

Which will need any power cables to be de-rated. If the cables are put on the underside of the insulation then the de-rating required is less severe. (Insulation on one side only).
In either case this results in the loss of a service void the advantage of which is the ease future changes. Although I take the point about

Posted By: WillInAberdeenEmpty service voids are a conduit for drafts and mice and noises

And then again how often are changes to wiring needed?

Posted By: WillInAberdeenIt's no problem derating lighting circuits (the ones in ceilings) - you can run a lot of lights with 1A these days!

Correct
But it is when power circuits are run across the ceiling or worse when the upstairs electric shower cable uses the service void as a convenient route that problems arise.

as mentioned above, use temporary nails/screws to hold the rigid insul, then remove once battens fixed in place. Also, foam the edges ad joints of the rigid boards.

Not sure why a 2"x2" is spec'd. Unless there's a very particular need for a large cavity, use 25mm thick timber. I tend to use 100x25 rather than 50x25, as it gives a much better chance of securely catching the plasterboard with p/brd screws. With 50mm batten, often only 45mm, shared between two boards, that's 22mm each. You don't need to be out by much on the batten spacing to not have adequate edge cover for the screws.
Even in walls, 25mm spacing is usually adequate for cabling, as a fast fix back box is 35mm (25mm plus plasterboard gives 37-40mm).

U valuve calcs including the 25mm unventilated air cavity with a low emisivity surface (foil), will give a lift to the total Uvalue, and help mitigate there not being insulation in that small cavity, which you want to keep clear for cabling in the future.

Presumably the arch will provide condensation calcs if it's a flat roof, unless it's very well ventilated above the insulation, as it would otherwise be a cold roof, which is likely to trap moisture, even with high quality VCL membranes. It's a really important detail, not just perfectionism.

Thanks for the replies.

I don't know why 2"x2" was is specified for the ceiling void. I will ask. Ditto with the walls if that is the case.

I believe the sheet insulation is to be foil backed and this will create the vapour barrier.

It is a single storey extension and my understanding is that it is a cold roof which has been stipulated by building control as the existing roof is a cold roof.

Just a reminder that cables in service cavities need to be derated, whether or not there's insulation in the cavity. This has changed quite a bit in the current edition of wiring regs so may be different from previous experiences.

The cable rating is based on it being 'clipped direct' to a wall, so cool air can freely rise up past it to carry away the heat. (Ref Method C in the regs)

If instead it is run in a service cavity across the ceiling then that air cannot circulate freely, and heat cannot rise away because of the insulation above, so the only heat sink is through the plasterboard down into the room (Ref Method 101).

So it can carry less current - see eg

https://www.elandcables.com/media/3hoka0mu/table-4d5.pdf

Is why power circuits are preferably run in walls/floors rather than ceilings. Doesn't matter for lighting, as the currents are very low these days.


Cold flat roofs are banned in Scottish building standards.

45x45 battens would need less frequent fixings than 22mm, which could be a good saving, those long screws are £1 each.