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Insulating timber suspended floor

SuperHomes Revamp Forums General Discussion Insulating timber suspended floor

This topic contains 25 replies, has 16 voices, and was last updated by  Daniel Nestlerode 5 years, 11 months ago.

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    I am about to embark on insulating under the suspended timber floor of my 1907 home.  I will be doing this from the 1m crawl space under the floor.  The plan is to insulate between the 300mm spaced joists with sheeps wool (there are too many nails, pipes, wires and bits of wood sticking out to get a good fit with celotex/kingspan).  This will I believe get me to about a U value of 0.3-0.4 W/m2K.  I am contemplating putting an additional 50mm of kingspan underneath the joists but am concerned that this may cause a problem with condensation in the gap and eventually rotting of the floor joists.  Has anyone taken this on, got any experience or have professional advice on this.  I won’t do the latter unless I’m 100% sure I’m not causing a problem.  Thanks.



    SuperHomers who have installed floor insulation are pretty numerous – you can use the filter on the database page to see who they are. They’ve used a variety of materials. From what I’ve read sheeps wool has superb qualities for situations where you generate a bit of condensation and want to safely let it wick out through the building fabric without accelerating heat loss or causing rot/mold issues. It sounds counterintuitive to me, therefore, to butt it up against something impermeable.



    I have a related and similar enquirey: I’m about to insulate my wooden floor from the cellar (starting at least) with bits and pieces of left over insulation board with foil facings and a small bit of mineral wool. The Energy Saving Trust document (Practical reburbishment of solid-walled houses, 2006 edition) I have says “Insulation should be tight up against the underside of the floorboards to prevent cold draughts getting through between the two…” (page 8), though elsewhere it says “Cross-ventilation must be maintained below the floor in order to remove moisture and prevent timber rot and mould growth” (page 6). Unfortunately EST advisers are no longer expert enough to answer this apparent contradiction (and suggest I contact an installer!) Can anyone advise, noting that we intend to leave the floorboards bare (though waxed or varnished, though with gaps largely filled), barring perhaps the odd rug, so any spills should dry out from the top side?



    I forgot a bit: as my peices of insulation board are rough cut would it be a good idea to fill gaps with expanding foam? Does this have good insulation qualities?


    Peter Draper

    I would steer clear of using anything that is not breathable. If you use boards etc then you will need to ensure that there are no points where moisture can get into the structure. This is because once it is in it is more difficult for it to get out again. So you will need excellent tapes, excellent adherence to the walls etc. This is really, really difficult to do. So easier just to stay with breathable insulation and potentially a breather membrane.



    It is absolutely vital that warm moist air from inside the building cannot penetrate exterior insulation. If it does, you get interstitial condensation which will not dry out.
    This at best render insulation ineffective or at worst causes unseen rot in the building structure

    So you must use non=permeable insulation or install a vapour barrier.There must be no holes or gaps in it.

    Also important that there are no gaps in the insulation.
    A small hole can defeat the whole project.


    Barry Sapwell

    Insulating your suspended timber floor is covered in Part L of Building Regulations. To do this correctly you must demonstrate to your local building control officer that you can achieve a target U- value of 0.25w/msq or better. If you do a DIY job or a professional job it is essential you get Building Regs approval as your mortgage provider or the local authority can insist on its removal otherwise. If you don’t get Building Regs approval it will be impossible for you to declare that your floor is insulated when the time comes to sell your house as you will need to provide the buyers solicitor with a certificate issued by the Local Authority. Additionally, the insulation work you have done should not be reflected on the EPC without the certificate. I install suspended timber floor insulation for a living and have found my local authority very helpful. Knauf Insulation have a technical department that are second to none.(check out their website, its full of useful information).
    You need to work out the “Perimeter/Area ratio” of the ground floor to be able to evidence what the projected U-value will be.
    In my own home I have added additional airbricks to improve cross-ventilation in the area under the floor. I have significantly increased the insulation around any water carrying pipes (central heating) that now fall outside of the thermal envelope (as these pipes are technically outside the house).
    I also pulled down a lathe and plaster ceiling in my cellar in order to insulate it. Building Regs require that a ceiling should be reinstated so that 30 minute fire protection is present. I have put up plasterboard and covered this with a plaster skim.
    On a retrofit installation it easier to use a product like Knauf LoftRoll 44 rather than cutting rigid foam boards.
    I hope all this helps.


    Rafael Delimata

    Hi all
    My name is Rafael and I am director of London Insulation. We specialize in all types of insulation and air-tightness to passive house level.

    I would recommend to use spray foam insulation called Icynene. It is an open cell breathable insulation that can be sprayed from under the floor directly on timber, pipework and cables (providing protection for junction boxes and plumbing valves).
    None of the fiber insulation would provide good air-tightness without additional membranes, which are hard work to install. Rigid insulation cut between joist would be even worst. Rigid board, even if cut perfectly, would not shrink, move and expand like timber joist. Small gaps would allow for air circulation between warm and cold side of the board lowering it designed performance. To many people concentrate on U-values and thickness of insulation. Air-tightness and workmanship are more important that most of people think.

    Space below suspended timber floor is designed to be cross ventilated. If you just use fiber insulation it’s efficiency would drop by 40% during windy day. Cold air would travel through fibers and remove heat in the same way, that wind cools your body, if you are walking in windy day, wearing thick wool jumper. Additional thin (vary poor U-value) wind protection jacket would sort out the problem and would act as air tightness layer. If we spent couple extra £ and buy Goretex jacket it would also be breathable and air tight.


    Jon Ferguson

    I’m doing something similar to a 103 y/o house. As Rafael says, you need a vapour and air barrier, to keep the air penetration down. The problem with Icynene however, is expense. It looks to me to be the most expensive way to insulate. So much so that many go with a hybrid. Using foam to create an air barrier and glass or rock wool to fill up the cavity. I’m planning something similar where I’m using rockwool between the joists and a vapour barrier on top of the floor boards, completed by another flooring layer such as hardwood. That does mean you miss out on using your original floor boards which is a shame. However, as HaroldA says, you don’t want to create a situation where you invite interstitial moisture from the warm areas above the floor. I’ve used rockwool between the joists because when you spend days crawling around down there.. cutting solid boards just is not feasible.. and would result in poor leaks. Also, even though there is loss for wind penetration I think this is overstated for many homes, and rockwool will also dry out allowing joists to dry as well.. Mine is surrounded by other houses and seems relatively calm. I did think of adding air bricks since these old buildings have fewer, however, after spending a couple days on my back down there I found the dirt quite dry, and therefore do not think this is a problem in my case.


    Rafael Delimata

    Dear Jon.

    It is very important to maintain good ventilation under the suspended floor. From previous projects we have learn that after insulation and air-tightness measures are applied, the area under the floor gets damper and without proper ventilation can cause serious damage. There are two reasons for this: One, before you install insulation, heating from your house would help to dry the space under the floor. Two, old floors are droughty, significantly improving ventilation between your house and space below the floor. Even if the space below the floor looks dry, this condition can change and good cross ventilation is essential.

    From my experience as we have completed around hundred floors. Costs are as follow:
    Icynene – under the suspended floor installed cost £25/m2 and it would take a one day to complete and provide you with insulation and airtightness. Foam can also be injected further away, were you are not able to reach (access is a real issue on most of our projects). You get 25 year warranty and guaranteed results.
    Equivalent performance using fiberglass insulation cost:
    £3.00 for insulation
    £2.00 for fixings
    £2.00 airtightness membrane (below insulation)
    £2.00 tapes and silicone for airtightness

    Total for materials £9/m2

    The most difficult to assess is labour. You can do work yourself but it is the most difficult and unpleasant section of the house to insulate (personal experience). Achieving good airtightness without removing floor boards is not possible. What is more, you have to deal with all services and electrics. At the end of the day you are still not sure if work done, would achieve desired results.

    My summary:
    If you removing floor boards and insulating from the top you can use membranes and flexible insulation to achieve excellent standard of insulation.
    If you are trying to insulate from under the floor, open cell spray is recommended.


    Paul Adamson

    Hello, I am converting the space above a timber framed cart shed into accommodation. The floor is 150×47 timber joists at 450 centres. Below the floor is an open fronted cart shed (garage). I cannot achieve the required u value just using mineral wool between the joists (using kingspan between the joists is impractical due to the amount of wires, pipes etc). I have about 75mm to play with on the underside of the joists and could put 50mm kingspan across the underside followed by plasterboard. Is this ok – I am concerned about condensation. Should I put a VPM across the top of the joists and under the floor boards (it would obviously be penetrated by numerous nails when fixing the floor boards). Or should I leave the kingspan off and live with the worse u value? By the way I cannot put the kingspan on top of the joists as I’m already struggling with headroom.
    Any help appreciated


    Alan Chesterman

    Hello. I insulated under my wooden boarded floor with Knauff space blanket loft insulation rolls. The rolls are encapsulated in thin polythene film, one side is silvered – meant to belated upwards in lofts. I simply stapled it to the underside of the floorboards from the underfloor space using an electric staple gun. Not a pleasant job, but it seems to work fine. The insulation is compressed in the supplied rolls, when unrolled it expands to about 150mm. Each roll is 530 mm x 5330 mm, about 2.5 m2. I paid £5 a roll, so pretty cheap.

    Harold said that non permeable insulation should be used, or install a vapour barrier, to avoid possible condensation within the insulation layer. In contrast, Peter said that insulation should be permeable. Knauff says that the polythene encapsulation is permeable. I did not install a vapour barrier and there is evidence moisture in the floor or the insulation nor signs of rot. I don’t think much moisture If any is getting through the floorboards. The boards are tongue and grooved and well sealed from draughts, and I guess wood is relatively impermeable. In theory moisture permeation and condensation could be issue, however My floor seems ok in practice. I would also be concerned about installing an impermeable insulation or vapour barrier in a floor just in case water is spilled or the floor flooded. If the underfloor is permeable and well ventilated it will eventually dry out.


    Alan Chesterman

    Sorry I meant to say there is no evidence of any moisture in the insulation, boards or joists in my floor.

    I have related question about the Superhomes website page “how do I insulate a floor”. The diagram at the bottom of the page shows insulation applied on the wall next to the floor joist to prevent thermal bridging, and a “breather membrane” attached to the wall and the underfloor to prevent “infiltration” i.e. draughts passing through gaps in the floor and the space behind the dry lining. My 60’s vintage house has a single solid outer wall, with dry lining attached to battens on the inside of the outer wall. The space behind the dry lining is open to the under floor space at the bottom to the attic space at the top and all the way around the house. The floor and attic voids are well insulated and freely ventilated by air bricks and vents, consequently very cold in winter, and the cold air circulates between the attic and underfloor voids behind the dry lining making the inner walls of the house cold.

    I have always been paranoid about inhibiting ventilation behind the dry lining, in case moisture were to collect there either from condensation or ingress through the outer solid wall, causing damp and or rot. Cold air flow in the dry lining space would be greatly reduced by the breather membrane as shown in the diagram, warming up the inner dry lining walls considerably. The breather membrane is permeable, and would allow some water vapour to escape into the floor void, but the area of membrane stopping the flow is very small, the between the wall and lining only an inch or so wide. Breather membranes are primarily designed to allow vapour permeation across the entire area of walls or roofs. The question is, would the lack of ventilation and small area for vapour to permeate across the barrier cause damp problems? It is all very well making savings in heating costs, but the consequences and costs of damp damaging the house fabric could be dire. Appreciate your thoughts……


    Jean-Marc Dick

    Just wondering if anyone has considered this solution?




    I have seen similar fitted to caravans and portacabins years ago.
    Back then it wasn’t too good.
    It may have improved but I would like to see a finished job, ie get under the floor and see how good a job is was before agreeing to anything.
    Looking for gaps and thin spots, etc. I can see where this could go wrong with a bit of carelessness.

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