Green Building Forum - Best practice for under-floor insulation? Thu, 21 May 2026 08:04:14 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227675#Comment_227675 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227675#Comment_227675 Mon, 27 Jul 2015 07:37:07 +0100 Nick Parsons
''So having read all this I'd like to know what ways there are to well insulate a suspended timber floor without the risk of incurring problems. Historically it seems to be a building element that relied upon draughts and high heat loss to keep it dry and rot free. Is it even compatible with modern energy efficiency standards?''

I wrote:

I don't think there is a way without running the *risk* of problems. I have been involved with hundreds of under-floor insulation jobs, of which probably less than a handful have had any problems (to my knowledge - I acknowledge that I do not visit the ones I was involved with 30 years ago!)

Tips to maximise the chance of problem-free and successful include:

- Always draught-proof the perimeters before insulating.

- Ensure there is adequate *cross*-ventilation.

- Ensure that ventilation entering at joist level is ducted so that it enters the void *below* the final insulation layer.


I have found that quilt between joists with breathable membrane below - all joints and perimeters taped - seems to work well. I have sometimes had issues where I have used Pu to stuff the gap between the last joist and the cold cellar wall - where quilt wold 'suck' - with condensation on the Pu and being transferred to the joist. With regret (as I don't like un-insulated bits) I have sometimes removed the infill at this point.

*I have started a new thread so that this does not hi-jack the poly bead thread*

'Best practice for under-floor insulation?']]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227678#Comment_227678 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227678#Comment_227678 Mon, 27 Jul 2015 08:18:38 +0100 Ed Davies Posted By: Nick ParsonsI have found that quilt between joists with breathable membrane below - all joints and perimeters taped - seems to work well.VCL above the insulation/joists or not?]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227690#Comment_227690 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227690#Comment_227690 Mon, 27 Jul 2015 11:34:41 +0100 djh Posted By: Nick ParsonsI have sometimes had issues where I have used Pu to stuff the gap between the last joist and the cold cellar wall - where quilt wold 'suck' - with condensation on the Pu and being transferred to the joist. With regret (as I don't like un-insulated bits) I have sometimes removed the infill at this point.
Maybe something like leca or silvapor (if it's obtainable again yet) would fill that gap if you put something underneath to hold it in place?]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227692#Comment_227692 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227692#Comment_227692 Mon, 27 Jul 2015 11:47:30 +0100 tony
Insulating like this is OK for DIY but gets expensive as professional job.

I like simple well insulated, safe and easy]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227724#Comment_227724 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227724#Comment_227724 Mon, 27 Jul 2015 18:22:37 +0100 Nick Parsons I have found that quilt between joists with breathable membrane below - all joints and perimeters taped - seems to work well.

Ed Davies wrote:

VCL above the insulation/joists or not?

Yes when boards have to be lifted. No when insulation from cellar or crawl space.]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227749#Comment_227749 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227749#Comment_227749 Tue, 28 Jul 2015 09:46:27 +0100 haplessDiyer Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227755#Comment_227755 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227755#Comment_227755 Tue, 28 Jul 2015 11:38:38 +0100 djh Posted By: haplessDiyerBut is there an argument for not using a VCL so any moisture can breathe back into the room. This seems to often be advocated for wall insulation but less so for floors.
Because the outside surface of walls can easily get quite warm when the sun shines on them, driving moisture inwards. That doesn't happen for the underside of floors!]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227807#Comment_227807 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227807#Comment_227807 Wed, 29 Jul 2015 12:41:41 +0100 haplessDiyer Posted By: djh
Posted By: haplessDiyerBut is there an argument for not using a VCL so any moisture can breathe back into the room. This seems to often be advocated for wall insulation but less so for floors.

Because the outside surface of walls can easily get quite warm when the sun shines on them, driving moisture inwards. That doesn't happen for the underside of floors!

ah, thanks, that helps explain it. Also I guess walls are exposed to rain that has to dry out]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227861#Comment_227861 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227861#Comment_227861 Thu, 30 Jul 2015 11:02:34 +0100 EasyBuilder
The accepted way of insulating suspended timber floors seems to be by placing insulation of one type or another between the joists. However, given sufficient insulation and the appropriate atmospheric conditions this places the dew point on or around the bottom face of the insulation, and somewhere inside the joists. Sure there are plenty of people who have done this and so far found no problems. It may be that they have not added enough insulation to create these conditions, or that the damp does not persist for long enough for rot.

Thinking about ventilation under the floor, given the required air bricks and sufficient wind all should be fine. However in winter when it's coldest is often a time of high pressure and no wind. With an uninsulated floor the escaping heat will evaporate condensation and vapor pressure will take it out through the air bricks without needing any wind. But with a sufficiently insulated floor it seems to me that the condensation could just keep building up. Put another way, traditional underfloor ventilation is driven by both wind and by heat loss, and taking away the heat loss will at times inhibit the effective ventilation.

Additional insulation under the joists is harder to install, but pushes the dew point out and down. It would seem to remove most of the problem, but the dew point then may be in the wall plates of the sleeper walls and in the parts of the joists beside the outside walls. Filling the space with poly beads prevents any ventilation at all, and there is probably still a dew point somewhere in a space with timber in it. But with sufficient perimeter wall insulation the dew point should be away from the timber, making this a viable solution.]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227989#Comment_227989 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227989#Comment_227989 Sat, 01 Aug 2015 08:01:56 +0100 EasyBuilder
Rather than keep bodging this old design how about a more radical approach. Chuck out the old structure, insulate over the ground with a polystyrene slab, and install an access floor as used in offices. I guess the air vents would have to go too. Would this work?]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227996#Comment_227996 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=227996#Comment_227996 Sat, 01 Aug 2015 12:26:16 +0100 Mike George
1. Carefully take up and number existing floor boards - hitting nails through boards into joists carefully with a nail punch if necessary (Brads will come out with the boards usually). Cutting some boards on joists will probably be necessary
2. Remove all dividing stub wall and block up any external vents. Ensure however that these are not providing any through ventilation to any other timber floors in the house - If they are this method is a non- starter.
3. Cut out *enough* (full length) joists in the center of the room and keep aside. This to allow installation of insulation. Clean out and remove all timber from where the joists were pocketed into the masonry and brick up holes.
4. Flatten surface beneath joist with stone dust. (awkward and laborious)
5. Lay a DPM on top of the stone dust. Allowing enough to reach up and beyond finish floor level (eventually). Turn up and underneath joists at ends.
6. Install rigid insulation on top of DPM up to 100mm below underside of joists. Layers of 50mm are easier to use and can be overlapped. Foam as you go any gaps.
7. Flood the area between insulation and underside of joists with concrete leaving a small gap - say 5 -10mm. Finish is unimportant.
8. When the concrete is cured, reinstate the previously removed floor joists and pack with plastic window packers and fix (screw) joists to concrete at intervals. This was easy with the floor I did as the joists were the old 4x2's.
9. Cut all joists as far away from the walls they are pocketed into as possible leaving a CLEAR space between the new joist ends and the existing masonry.Clean out and remove all timber from where the joists were pocketed into the masonry and brick up holes.
10. Pull previously folded DPM up and beyond new joist ends.
11. Reinstate floor boards.


With hind sight it may have been easier to completely remove all of the joists and reinstate them after laying a new sub floor - but there were complications in that they were adjacent in places to Edwardian tiled hearths which would probably have become damaged...]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228016#Comment_228016 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228016#Comment_228016 Sat, 01 Aug 2015 21:17:05 +0100 djh Posted By: EasyBuilderChuck out the old structure, insulate over the ground with a polystyrene slab, and install an access floor as used in offices.
The polystyrene isn't a capillary break, so you'd need a DPM as well. The access floor isn't structurally self-supporting, so you'd need a structural floor of some kind underneath it. So a DPM with a concrete slab is the usual solution. The additional cost of the access floor is typically difficult to justify in a domestic situation, or indeed in most offices, hence their use is restricted to computer rooms and premium/specialized offices.]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228230#Comment_228230 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228230#Comment_228230 Wed, 05 Aug 2015 09:21:11 +0100 EasyBuilder Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228234#Comment_228234 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228234#Comment_228234 Wed, 05 Aug 2015 10:07:17 +0100 djh
I've probably forgotten something that would make this a non-starter :devil:]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228423#Comment_228423 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228423#Comment_228423 Sat, 08 Aug 2015 19:25:33 +0100 EasyBuilder Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228425#Comment_228425 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228425#Comment_228425 Sat, 08 Aug 2015 22:03:52 +0100 djh Posted By: EasyBuilderNo need for DPM
You still need a DPM unless the chosen insulation provides a capillary break, which most don't.]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228430#Comment_228430 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=228430#Comment_228430 Sun, 09 Aug 2015 11:57:37 +0100 EasyBuilder Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229763#Comment_229763 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229763#Comment_229763 Thu, 17 Sep 2015 19:03:35 +0100 RILEYP
Simon]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229802#Comment_229802 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229802#Comment_229802 Sat, 19 Sep 2015 16:57:03 +0100 RILEYP I determined that: 1) can access from above only; 2) will do parge coat; 3) use 100mm knauf earthwool flexible slab between the joists; 4) VPL beneath the insulation; 5) VCL above the insulation and 6) direct air from airbrick below the joists and 7) continue with rads for heating. Have chosen the 100mm earthwool over expanded board insulation because of fire-resistance and vapour permeability BUT this is not a lot of insulation - I recall seeing 200mm of kingspan under new build underfloor heating! Hence the question at the beginning.]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229804#Comment_229804 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229804#Comment_229804 Sat, 19 Sep 2015 17:11:47 +0100 tony Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229806#Comment_229806 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229806#Comment_229806 Sat, 19 Sep 2015 17:22:47 +0100 djh
Are you planning to take up the whole floor? You will need to in order to push mineral wool batts below and in between the joists. But presumably you are, in order to be able to fit the VCL, sorry for stupid question.]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229811#Comment_229811 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229811#Comment_229811 Sat, 19 Sep 2015 21:46:28 +0100 RILEYP Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229812#Comment_229812 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229812#Comment_229812 Sat, 19 Sep 2015 21:47:14 +0100 RILEYP Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229828#Comment_229828 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=229828#Comment_229828 Sun, 20 Sep 2015 21:25:08 +0100 djh Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230491#Comment_230491 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230491#Comment_230491 Fri, 09 Oct 2015 19:04:34 +0100 KalyaniPanse Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230493#Comment_230493 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230493#Comment_230493 Fri, 09 Oct 2015 19:12:02 +0100 Nick Parsons
As regards the VCL, I would only use as a VCL something which says it *is* a VCL! I doubt there will be a problem with trapping the floorboards between 2 impermeable layers, if the room's heated, but I agree it doesn't 'feel' right.]]> Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230495#Comment_230495 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230495#Comment_230495 Fri, 09 Oct 2015 19:18:02 +0100 KalyaniPanse Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230501#Comment_230501 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230501#Comment_230501 Fri, 09 Oct 2015 21:59:55 +0100 Ed Davies Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230503#Comment_230503 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230503#Comment_230503 Fri, 09 Oct 2015 22:44:59 +0100 Nick Parsons Best practice for under-floor insulation? https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230514#Comment_230514 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=13547&Focus=230514#Comment_230514 Sat, 10 Oct 2015 12:01:51 +0100 KalyaniPanse