Green Building Forum - ceiling insulation Wed, 10 Jun 2026 11:00:38 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269696#Comment_269696 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269696#Comment_269696 Thu, 02 Aug 2018 15:51:38 +0100 andylebalcon Would appreciate input from all.
We are renovating an old farmhouse that predominantly has no insulation or heating.
So far we have insulated between ceiling joists in various rooms where we can take up loft boards and stuff sheepswool between.
Over our dining room however it is difficult to do this.
i attach a detail cross section of ceiling construction.
Above is a very large windy, unventilated loft, which we would like to use in spring and autumn (unheated/uninsulated) for yoga/spare bunkroom and the like.
In the dining room we wish to keep the feel/look of the room with the white soffit between dark chestnut beams.
So I'm looking for a insulating board that can be cut specifically to each gap fixed to underside of deck and then lined with heavy duty lining paper and painted.
At 40mm thick (or so) it will improve a great deal the insulation and we can retain the very nice cherry boards above intact.
I have considered at length taking these up, but after various thoughts and investigations and experience am unlikely to be persuaded otherwise!
Any and all suggestions very welcome
Cheers
Andy
bty I have considered aerogel but after gaining a quote I'm looking for something much cheaper.
-]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269701#Comment_269701 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269701#Comment_269701 Thu, 02 Aug 2018 16:48:58 +0100 Nick Parsons ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269769#Comment_269769 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269769#Comment_269769 Mon, 06 Aug 2018 16:18:47 +0100 andylebalcon Thanks for that suggestion, I've looked into those now and whilst almost ideal they are slightly thicker than I would like and have t&G edges, plus overall size might create a bit of waste.
Despite all this would you say they could be papered over?
Not looking for a fine finish, all surfaces in room quite rustic!
Thanks
Andy]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269770#Comment_269770 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269770#Comment_269770 Mon, 06 Aug 2018 16:50:34 +0100 Nick Parsons
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!]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269831#Comment_269831 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269831#Comment_269831 Sat, 11 Aug 2018 19:02:16 +0100 Rex
My house is a new build, timber frame and insulated with Warmcel. For the ceilings, I stapled the paper scrim, supplied by the Warmcel guys across the joists. Then they came in and blew the insulation into the space. Later, plaster boarded over. Full insulation, loads of sound deadening.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269834#Comment_269834 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269834#Comment_269834 Sun, 12 Aug 2018 10:55:59 +0100 owlman 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.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269883#Comment_269883 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269883#Comment_269883 Wed, 15 Aug 2018 11:05:46 +0100 andylebalcon ideally want a woodfibre board capable of being papered, perhaps it doesn't exist!
I'll keep hunting
Cheers]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269884#Comment_269884 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269884#Comment_269884 Wed, 15 Aug 2018 11:34:00 +0100 owlman ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269887#Comment_269887 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=269887#Comment_269887 Wed, 15 Aug 2018 18:16:38 +0100 Nick Parsons ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307731#Comment_307731 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307731#Comment_307731 Wed, 12 Feb 2025 21:25:44 +0000 Jonti

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?]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307735#Comment_307735 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307735#Comment_307735 Thu, 13 Feb 2025 08:31:38 +0000 sgt_woulds
Woodfibre is generally Fire Reaction Class E; used in a certified system behind a render or plaster it can achieve a B-s1,d0 classification for both internal and external use.

This is a classification of the render system; depending on which system is used the manufacturer can provide the relevant data.

I cannot open or download documents from this site so I don't know what your construction is, but if you have floorboards above, why not lay woodfibre boards on top? These can be found in high enough densities to take light foot traffic and storage. A thin layer of hardwood-faced ply on top could be waxed to create a nice surface for your Yoga sessions.

I've used 5mm ply over woodfibre, (and over xps overfloor UFH boards) without issues for a few years now.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307739#Comment_307739 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307739#Comment_307739 Thu, 13 Feb 2025 09:57:06 +0000 Peter_in_Hungary
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]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307740#Comment_307740 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307740#Comment_307740 Thu, 13 Feb 2025 12:10:05 +0000 WillInAberdeen
There is a limit to how much insulation you can put on the inside of the VCL without condensation (ask the architect) but using wood fibre quilt helps buffer moisture and heat.

Quilt needs some temp support between battens so it doesn't fall out until the pb is put on.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307742#Comment_307742 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307742#Comment_307742 Thu, 13 Feb 2025 19:18:21 +0000 Peter_in_Hungary 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?]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307746#Comment_307746 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307746#Comment_307746 Fri, 14 Feb 2025 11:40:08 +0000 WillInAberdeen ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307747#Comment_307747 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307747#Comment_307747 Fri, 14 Feb 2025 12:25:29 +0000 Peter_in_Hungary 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.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307748#Comment_307748 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307748#Comment_307748 Fri, 14 Feb 2025 15:14:41 +0000 djh Posted By: Peter_in_Hungary
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.Indeed, all the circuits of whatever type run in our first-floor joist void, since that's the only available horizontal space in our house. (no loft and solid slab ground floor) But they don't run within (noise) insulation. Vertical drops are fastened to timber studs.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307749#Comment_307749 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307749#Comment_307749 Fri, 14 Feb 2025 16:18:31 +0000 GreenPaddy
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.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307750#Comment_307750 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307750#Comment_307750 Fri, 14 Feb 2025 19:42:45 +0000 djh Posted By: GreenPaddyPresumably 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.There's a lot we don't know about Jonti's friend's son's roof :bigsmile: but indeed the ventilation above the the insulation is very important, if it is a single storey extension.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307751#Comment_307751 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307751#Comment_307751 Fri, 14 Feb 2025 20:25:13 +0000 Jonti
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.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307752#Comment_307752 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307752#Comment_307752 Sat, 15 Feb 2025 01:44:20 +0000 djh Posted By: JontiI believe the sheet insulation is to be foil backed and this will create the vapour barrier.As long as the foil is pristine and undamaged ... and the joints are taped with proper (expensive!) airtightness/VCL tape.

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.
If it was me, I'd double check the ventilation arrangements under the top deck. It is very important.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307753#Comment_307753 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307753#Comment_307753 Sat, 15 Feb 2025 23:41:09 +0000 WillInAberdeen
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.]]> ceiling insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307754#Comment_307754 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15826&Focus=307754#Comment_307754 Sun, 16 Feb 2025 09:22:25 +0000 Peter_in_Hungary Posted By: WillInAberdeen45x45 battens would need less frequent fixings than 22mm, which could be a good saving, those long screws are £1 each.
Ouch - e.g. over here 6mm x 140 wood screws are 16p each by the box and 6 x 200 are 36p each.]]>