Green Building Forum - Insulating a suspended timber floor Thu, 11 Jun 2026 14:29:48 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278852#Comment_278852 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278852#Comment_278852 Sat, 15 Feb 2020 00:37:50 +0000 djh Posted By: bardoHello all, I've decided to go with Knauf omni fit glass wool. Though still looking at how to best support the insulation. Just using a membrane or mesh doesn't do it for me and I also see the potential issues with boarding up with OSB as was my original plan. There is a polypropylene product on the market which has holes punched through for ventilation and liquid release. This looks good however the price is way too high. And besides if I used this approach I would also want to cover with a glued or taped breather membrane on top to prevent wind. Has anyone on this forum come up with or heard of an alternative solution? I can see that a strong synthetic mateiral in this situation has merit.
Sorry but it's late and although I've reread the thread I seem to have lost the plot. What was the problem with OSB? And why is plastic now the right answer, especially when its main benefit of water/airtightness is compromised by holes?

We're talking about supporting insulation in a void under the ground floor, yes?]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278853#Comment_278853 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278853#Comment_278853 Sat, 15 Feb 2020 11:14:57 +0000 WillInAberdeen Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278854#Comment_278854 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278854#Comment_278854 Sat, 15 Feb 2020 11:33:41 +0000 bardo Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278859#Comment_278859 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278859#Comment_278859 Sat, 15 Feb 2020 12:59:22 +0000 bardo Posted By: djhSorry but it's late and although I've reread the thread I seem to have lost the plot. What was the problem with OSB? And why is plastic now the right answer, especially when its main benefit of water/airtightness is compromised by holes?

We're talking about supporting insulation in a void under the ground floor, yes?

So I was all set to take the 9mm OSB route however there was concern expressed on this forum about the lack of breathability and permeability. I moved on to looking at Omni-fit glass wool and their tech person does not recommend OSB. I was exploring the use of coroplast and making holes in it and then glueing a layer of breather membrane on the inside of the tray covering the holes. More work yes, though this would create both a sturdier structural support which would also prevent wind coming in and allowing water a way out.]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278861#Comment_278861 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278861#Comment_278861 Sat, 15 Feb 2020 13:10:47 +0000 bardo Posted By: GreenPaddyHi Bardo,

cut from an earlier message above...
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What about running a foil faced breathable membrane under the JJI's, running perp to the JJI's?

Start by fixing a timber 25x50 batten to the external block underbuild, at JJI underside height. Staple (and glue) the VPM to that batten, to give a seal to the wall. Then, VPM to first JJI, passing under it, bring the VPM up the vert face of the lower flange and onto the top side of that flange, fold back on itself, staple through the double thickness of VPM onto the top side of the flange, then go back down side of flange, and across to next JJI, passing under it, before repeating.

The foil should face the solum! That will give a well supported continuous layer, at full JJI depth, not losing the depth of the lower flange. You could actually let the foil VPM droop below the JJI's, by say 100mm, and even put 400mm of the cheap loft roll 44 (Uvalue for that arrangement = 0.12W/mK).

That's deff how I'd do it, and not just on paper, have done that on a previous project. Remember to fluff up the wool insulation as you install it, so it's got lots of air en-trained in it, and not squashed, as it comes out of the roll.
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The foil faced vapour permeable membrane should be around £2/m2, but see what your local merchant has. You don't NEED to use foil faced, but it's a bit tougher, and enhances the Uvalue a little. Also, there is the possibility of getting some condensation on the outer surface, when the strange weather occurrence of very cold for several days, then much milder damp weather, so the foil face gives a slightly more water repellent surface.

You could just get the cheapest house wrap VPM, at 50p/m2. I don't use netting, as I feel a continuous layer is better, not least in reducing wind washing, but it's not wrong.

I would lay a vapour check on the top of the JJI's, before fixing the flooring, to improve air tightness, but don't ask the question on here, as you'll get lots of different equally well argued reasons. Make the floor air tight in what ever way you see fit (PU glue on every board edge etc) is a must. You've a breathable house, so continue that through the floor, into the void, and show that on your drawings so the BSO can confirm he's happy with that approach.

Hi, looking again at the option you mentioned. Not so easy to overlap membrane in some areas of house by stud walls though I guess I could over come that with fresh battens to start and stop. I've looked at some foilbacked breather membrane however I wonder how they would deal with liquid spillage. If I go down the glass wool route then I need to ensure the material can get adequate air and dry out if needed.]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278863#Comment_278863 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278863#Comment_278863 Sat, 15 Feb 2020 15:31:16 +0000 djh Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278864#Comment_278864 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278864#Comment_278864 Sat, 15 Feb 2020 18:55:34 +0000 bardo Posted By: djhNote that breathable membranes are VAPOUR permeable, but are WATERPROOF. Keeping liquid water out of a building is their purpose in life.

Thanks for the reminder - I get confused. Earlier in the thread you suggested a permeable board fixed to the underside such as panelvent. I wonder if such a board would provide sufficient ventilation? And what would happen if there was a spillage? Would it ruin the board?]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278866#Comment_278866 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=278866#Comment_278866 Sat, 15 Feb 2020 20:59:05 +0000 djh Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288785#Comment_288785 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288785#Comment_288785 Fri, 09 Apr 2021 16:59:13 +0100 monkeywood Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288788#Comment_288788 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288788#Comment_288788 Sat, 10 Apr 2021 08:46:41 +0100 jms452 Posted By: WillInAberdeenScaffold debris netting?

+1]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288789#Comment_288789 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288789#Comment_288789 Sat, 10 Apr 2021 13:42:06 +0100 tony Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288803#Comment_288803 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288803#Comment_288803 Mon, 12 Apr 2021 08:49:37 +0100 monkeywood Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288807#Comment_288807 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288807#Comment_288807 Mon, 12 Apr 2021 20:30:41 +0100 tony
More is better]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288808#Comment_288808 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288808#Comment_288808 Mon, 12 Apr 2021 20:59:19 +0100 Viking House Posted By: tonyI would go down there and insulate with inorganic quilt between joists from underneath, assumed 100mm then add 100mm eps under that, More is better Agreed]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288817#Comment_288817 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288817#Comment_288817 Tue, 13 Apr 2021 12:41:59 +0100 owlman Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288819#Comment_288819 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288819#Comment_288819 Tue, 13 Apr 2021 14:54:41 +0100 monkeywood
I can only find engineer/design companies rather than builders on the AECB website.

I am unlikely to complete this myself as I'm currently working over my hours for the NHS and completing my masters :shocked:

Owlman - there is a chipboard flooring at the moment with a layer of kingspan beneath. I will carpet it once the insulation is completed. The room doesn't really get any sun so always is a bit chilly.

Thanks in advance for comments/suggestions.]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288822#Comment_288822 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288822#Comment_288822 Tue, 13 Apr 2021 15:30:09 +0100 owlman Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288834#Comment_288834 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288834#Comment_288834 Tue, 13 Apr 2021 22:38:58 +0100 Viking House https://www.mybuilder.com/tradesmen/uk/wales Interview 2 or 3 Tradesmen and get a vibe]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288936#Comment_288936 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288936#Comment_288936 Sat, 17 Apr 2021 00:27:34 +0100 monkeywood Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288937#Comment_288937 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288937#Comment_288937 Sat, 17 Apr 2021 07:46:43 +0100 jms452 Posted By: monkeywoodThere’s a thread about filling the void/max depths and another regarding replacing suspended timbers with concrete - I will go off for a deep dive into those! Thanks all for your time.

If the void is 1.5m deep you are disappearing into an expensive rabbit hole replacing a good suspended floor with a full fill void or solid concrete. Tony's suggestion of quilt + EPS is much faster, cheaper & lower-carbon.

Posted By: monkeywoodI’ve done that a few times and there were lots of concrete pouring suggestions (and lots of disappearing off the face of the earth..) It’s proving difficult to get anyone to turn up to even have a look at the moment!


Sounds to me like you've asked about insulating a floor and got the builders default. I'd suggest making a written spec., working out where all the materials come from and making it easy for the tradesperson to quote using a portal like viking's suggestion.

e.g. Insulate __m2 suspended floor from below - access is by _____, void is ____m deep so fairly accessible. Fit ___mm deep mineral wool between rafters (____mm spacing) from below held in place with debris netting. Then fit two staggered layers of ___mm EPS from below with ___ fittings into the floor joists.]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288940#Comment_288940 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288940#Comment_288940 Sat, 17 Apr 2021 10:44:14 +0100 SimonD Posted By: monkeywoodHi Tony, in the dining room, at its deepest, about 1.25-1.5m gradually getting shallower towards the front of the house where it is about 0.5m at its shallowest.

How much of the floor void is that deep and how much is shallower?

Can you actually access the floor void by other means than through the actual floor?

I think this will have a bearing on both quotes and interest from trademen/women - sorry to be negative. If there's too much of the floor area that's only crawl space, it's going to put them off. Having done this kind of job myself, it's really unpleasant to be crawling around under there. However, it generaally seems like trades are stacked with work ATM.

From a practical perspective, I'd look at installing the insulation from above, if you can.

How deep are your current joists?

And one final thing.I came across this page which I thought interesting. http://www.sofiepelsmakers.com/suspended-timber-ground-floors.html]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288942#Comment_288942 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=288942#Comment_288942 Sat, 17 Apr 2021 11:50:44 +0100 GreenPaddy
Who'd have thought you could write so much about insulating a floor, but all good valid points from my quick skim over. Worth while bookmarking, for future questions on this issue - explains lots of pro's/con's.]]> Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=289192#Comment_289192 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=289192#Comment_289192 Wed, 28 Apr 2021 22:28:40 +0100 monkeywood Insulating a suspended timber floor https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=289300#Comment_289300 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16281&Focus=289300#Comment_289300 Mon, 03 May 2021 07:35:45 +0100 jms452 Posted By: monkeywoodThanks all for your comments, suggestions and useful links. I think I’ve found someone who can help!

Great - let us know how it worked out!]]>