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    • CommentAuthorandyman99
    • CommentTimeFeb 11th 2018 edited
    We are currently awaiting planning approval on an extension to our 60’s bungalow, so don’t yet have a direct line to building inspectors. Our lounge is mostly a suspended wooden floor but a small part of it includes an extension which has a solid uninsulated screeded floor.

    We’ve lifted the floorboards ready to insulate the suspended floor. It seems in good shape, there’s about 180mm space below the joists and they stop short of the cavity. The joists are 4” at 18” centres, difficult not to mix measurements here.
    I plan to use 170mm rock wool between the joists with a “sagging” dpm below and another above. Then cover with 18mm OSB3, the floorboards were poor to start with and now mostly unusable.

    I’ve removed a little bit of the screed of the solid part of the floor, the area is approx 4m x 3m. The dpm sits about 4” down, approx 3 1/2” below the top of the joists of the rest of the floor. I’m not sure on the best solution for insulating this part to meet building regs? I’m hoping it can be done without digging down below the existing dpm. Any thoughts welcome.
      IMG_2376 copy.jpg
    • CommentAuthorandyman99
    • CommentTimeFeb 11th 2018
    Work in progress on the suspended floor
    • CommentTimeFeb 11th 2018
    I'm a bit concerned about the idea of a DPM both above and below the insulation. Normally it's a goal to have only one vapour tight layer, to prevent water getting trapped.

    What happens if you spill water on the floor (or wine!)? And if the DPM on top is not perfect, perhaps some years later, what happens then?
    I use breathable membrane below and VCL above. Yes, I share djh's concern re 2 x impermeable membranes. Feels like asking for trouble.
    • CommentAuthorandyman99
    • CommentTimeFeb 11th 2018
    Sorry wrong terminology, I should have said vapour barrier instead of DPM, its 125 micron polythene sheeting?
    I assumed you meant VCL (and indeed when I use a VCL on top I use DPM as I am less likely to put my boot through it - a useful tip!), but - and I hate to make you undo stuff - if you go ahead as you have done, when the inevitable hole appears and lets in moisture, you are more-or-less containing it in a plastic bag. If it were me - however frustrating it is - I'd undo the VCL 'hammocks', replace them with breathable membrane 'hammocks' - print side down - and then VCL (or DPM 'cause of my clumsy feet) and loads of welts/tape above.

    • CommentAuthorandyman99
    • CommentTimeFeb 11th 2018
    Thanks! Best to find out now, as I have only just started the hammocks today.

    Would this be more suitable?


    How do you join the 1m widths`/
    • CommentTimeFeb 11th 2018
    Posted By: andyman99Would this be more suitable?

    No, you're looking for 'breathable' or 'vapour permeable' rather than 'impermeable'.

    Maybe something from https://www.roofingsuperstore.co.uk/browse/pitched-roofing/breathable.html or similar.

    You don't actually need a membrane; many people use a mesh or netting of some kind. All it's doing is holding up the insulation. Neither do you need to join the widths, except to stop the insulation falling through. The only reasons for taping breathable membranes are (a) when they are exposed to wind, to avoid 'wind-washing' through the insulation or wind lifting the membrane up and (b) when they are also serving as the air barrier. Neither applies in your case.
    • CommentAuthortony
    • CommentTimeFeb 12th 2018
    Strawberry netting under, poly over the top please
    • CommentAuthorandyman99
    • CommentTimeFeb 12th 2018
    Thanks everyone, any thoughts on best way to approach the solid floor part?
    I use the breathable membrane underneath as a secondary air-tightness layer - 'belt and braces' - having already sealed all the perimeters. Where I have the luxury of access from below I either tape the membrane to the walls, if the substrate is suitable, or sealant and 'trapping batten' where possible. I take the view that since one has to use *something* to hold the insulation in, the extra to use membrane (and I do use A/T tape) is worth it. In some exposed areas with good cross-ventilation under-floor there can be more wind than trickle.
    Sorry, Andyman, we must have been typing at the same time. My post was of course a response to Tony's.

    However, re your solid floor, you need to calculate the perimeter/area ratio to find the 'base-case' U value.

    If you are *really* (implausibly?) lucky the DPM will be sat on something really really flat, allowing you to do a floating floor (from the bottom, DPM, PU or PIR, 18mm T & G OSB, final floor finish. However as you only have approx 3.5 inches, you can probably use a max of 87.5mm (75 + 12mm) PIR/PU even if you have no final 'decorative' floor.

    As I said, whether that will give you a compliant U value (0.25W/m2K or better) depends on your 'base case'.

    For example, if your P/A ratio is 0.6, the base-case U value will be around 0.83. 1/0.83 = 1.205 (that's the R value). 1/0.25 (your target U value) = 4, so your insulation needs to give you an R value of 4 minus 1.205, or 2.795. About 65mm of Pu/PIR would give you that.

    If, however, your base-case U is worse, or the sub-base is not level, this won't work.
    • CommentAuthortony
    • CommentTimeFeb 12th 2018
    Re Solid floor, how about converting it to a floating floor, insulation on the dpm, t&g sheets on that?
    • CommentAuthorandyman99
    • CommentTimeFeb 12th 2018
    Nick/Tony, yes if I can convert it to a floating floor that would be perfect for me, happy to convert to wood if possible. I will dig some more screed up once I've got the suspended floor finished. Do you need some battens/joists within that make up? Assume it needs some tethering somewhere or is the weight of it alone enough to keep it from moving?
    "Do you need some battens/joists within that make up?"

    No. I have done 3 floating floors recently with just 18mm OSB, and it's fine.

    ''Assume it needs some tethering somewhere''

    Not if it's good and flat.

    '' or is the weight of it alone enough to keep it from moving?''

    • CommentAuthorandyman99
    • CommentTimeFeb 13th 2018
    Thanks Nick.
    • CommentAuthorTimSmall
    • CommentTimeFeb 21st 2018
    There are a few foam glass aggregates which you might be able to use for levelling. e.g. Glapor, Geocell, Technopor.

    I prefer to get the DPM up higher if possible - on top of insulation, or part way through it, otherwise you have potential downward vapour drive problems which can even effect insulations like EPS over time, and if it floods for some reason it'll take ages to dry out (or never will).
    • CommentAuthordelprado
    • CommentTimeFeb 22nd 2018
    Tim do you do that even with geocell? I am about to use geocell to 350mm depth
    • CommentAuthorTimSmall
    • CommentTimeFeb 22nd 2018
    With foam glass granuals you wouldn't have the risk of the closed cell (I assume geocell is mostly closed cell?) voids getting filled with water, but the voids between the granuals could definitely end up wet, diminishing insulation properties quite a lot.

    DPM on top or less than half way down (if DPM damage could be an issue with it on the top of the insulation) won't have these problems.

    Water vapour will diffuse from high pressure to low pressure.

    room at 20C, 60% humidity - vapour pressure 1.4 kPA
    room at 20C, 50% humidity - vapour pressure 1.2 kPA
    ground at 6C, 100% humidity - vapour pressure 0.935 kPA
    ground at 10C, 100% humidity - vapour pressure 1.2 kPA



    There's usually a weak downwards vapour drive. Vapour barrier should probably be on or towards the top.

    In the event of a liquid water leak, the drying time could be years, I think it's best to allow any liquid water to be able to drain out - which is probably a stronger argument for a higher DPM.

    You're also likely to get microbe growth in the humid bits (google image search for "lowest isopleth for mould"), with or without a water leak, and since the DPM is also probably your airtight later, probably best to keep those microbes outside the building envelope.
    • CommentTimeFeb 22nd 2018
    I can't speak for Geocell, and I can't find appropriate data, but I have used Technopor and they appear to look similar.

    You don't need a DPM with Technopor. Technopor forms a capillary break, so it is its own damp proof layer.

    Ah, see http://www.geocell-schaumglas.eu/en/products/foam-glass-gravel/properties/ - "closed cell – nil capillary action".

    The airtight layer is much more likely to be the slab on top. Using a plastic membrane in the ground and expecting it to be and to stay airtight would be pretty crazy IMHO. Using a DPM anywhere other than sandwiched between two flat surfaces seems like a pretty good way to ensure holes.

    I'd also be interested to know the detail for the claim about problems with EPS due to vapour drive.
    • CommentAuthorTimSmall
    • CommentTimeFeb 22nd 2018 edited
    I remember reading a North American report which had some samples of EPS which had accumulated over 30% water content by volume over a few decades, but I can't seem to find it now. Here is a report showing long term absorption in XPS in a situation with high vapour drive:


    Whether the DPM is the airtightness layer or not depends on what you put above it, and how that's connected to the walls... I've used DPM sandwiched in the insulation layer as airtightness in the past, but I understand that this isn't necessarily the case.

    I have run wufi simulations for EPS in floors with DPMs at different locations. With the DPM at the bottom, it showed a gradual water content accumulation in the EPS.

    Same thing with non-inverted green roofs with shading.
    • CommentTimeFeb 22nd 2018
    Posted By: TimSmallHere is a report showing long term absorption in XPS in a situation with high vapour drive:

    Yes, exactly. There are known problems with XPS underground, not EPS as far as I know.

    I have run wufi simulations for EPS in floors with DPMs at different locations. With the DPM at the bottom, it showed a gradual water content accumulation in the EPS.

    That's what I would expect. I certainly wouldn't put a DPM on the cold side at the bottom of a construction, myself.

    Same thing with non-inverted green roofs with shading.

    Not quite sure what situation you're describing or where you expect condensation?
    • CommentAuthorandyman99
    • CommentTimeFeb 22nd 2018 edited
    Just started the destructive work today, initially tried my SDS drill with chisel, not man enough so hired a Kango at lunchtime. The picture represents a couple of hours work and is about 25% of the screed by surface area. A few blisters starting to form so enough for 1st attempt. Screed breaks up to a more sandy layer, but (mostly) not to a DPM. This is still some way above the oversight level of the suspended floor part. At the edges I can see there is polythene sheeting, but not had time to explore where it goes yet. The sandy layer is very flat so far. Thanks for ideas, will digest once I have a better idea of what depth there is to play with.
    Weird thing with attaching pictures. First attempt shows it rotated 90 degrees (but OK on local preview). Rotate the file locally by 360 degrees, save it and re-add it here, then it's ok? Only mention it as its happened before.
    • CommentTimeFeb 22nd 2018
    Posted By: andyman99Weird thing with attaching pictures. First attempt shows it rotated 90 degrees (but OK on local preview). Rotate the file locally by 360 degrees, save it and re-add it here, then it's ok? Only mention it as its happened before.

    I don't really know what I'm talking about, but pictures often include some extra metadata, called EXIF, that includes an indication of the picture orientation. This is sometimes different to the apparent layout of pixels in the file. Lots of room for misinterpretation by software, and lots of different software to make the misinterpretation. So I've never bothered to try understand it exactly.
    • CommentAuthorTimSmall
    • CommentTimeFeb 23rd 2018 edited
    responding to djh:

    EPS, XPS and PIR are all prone to long term water accumulation where you have permanent high vapour drive. I researched XPS vs. EPS to death a few years ago, and came to the conclusion that the below ground performance is similar (so I use EPS generally - because it is cheaper). There are one-off experiments which show EPS out performing XPS below ground, and there are also experiments showing the opposite. Differences in formulation by the manufacturer are one explanation, and the overall greater vapour permeability of the EPS are another. In some situations this increased permeability is an advantage. In others it's a disadvantage.

    I raised this because one of the suggestions prior to this in the thread was "from the bottom, DPM, PU or PIR, 18mm T & G OSB, final floor finish".

    This seems to be quite a common detail (I've seen it recommended by manufacturers). I've also seen manufacturer details showing DPMs both above and below the PIR. I don't use either.

    I've previously used, and am happy with (from bottom up):

    sand blinding, EPS, DPM, concrete slab.
    sand blinding, EPS, PIR, DPM, concrete slab.
    sand blinding, EPS, DPM, EPS, T+G chipboard, thin screed (with UFH), MS-polymer adhesive, T+G oak.

    When I simulated variations on these setups, *BUT* with the DPM under the insulation in the past, I was surprised to see that the water content of the EPS crept up over time, so wondering where the equilibrium point was, I extended the simulation period, and was again surprised to see that it just kept going up in a reasonably linear way over a period of decades. PIR shows similar behaviour (more slowly with foil faced PIR, but the vapour can get in at the edges).

    With the DPM higher up this water build-up didn't occur in the simulations (50% of the depth or higher was OK for my simulated soil temperature, and indoor climate conditions). I also did a load of simulations when I was planning a green roof, and decided that it was pretty marginal too (this time with PIR insulation showing long term accumulation).

    The experimental findings from the XPS inverted green roof which I linked to above bear out the simulation results as being realistic, I think. I hadn't seen those at the time, but did come across another report (which was either in Canada or Alaska), in which EPS had accumulated significant water in a high vapour drive environment, (something like 30% water content over a period of a decade or so), I can't find that right now, but I'll have another go...

    Here's experimental results showing water accumulation in PIR and PUR under high vapour drive:


    [edited for clarity 24th]
    • CommentTimeFeb 24th 2018
    Thanks for the further explanation. I would be very interested to see the EPS data if you can find it.

    I find it intuitively obvious that water will collect at the bottom of any construction that has a watertight barrier underneath it. I would find it very odd if it didn't.

    I'm also still curious about what you meant by
    Same thing with non-inverted green roofs with shading.
    • CommentAuthortony
    • CommentTimeFeb 24th 2018
    So high up is acting as a vb and dpc.

    I only ever put poly on top of insulation, then concrete.
    • CommentAuthorTimSmall
    • CommentTimeFeb 24th 2018
    Re geen roofs. A typical green roof maybe (bottom downwards):

    "growing medium" i.e. something soil-like
    PIR (or EPS/XPS) (*)
    Deck (OSB/ply)
    Joists (possibly with insulation between)
    Variable vapour resistance "Intelligent" membrane e.g. Intello etc. (optional).

    The long-term sims I did showed water accumulation towards the top of the (*) layer, eventually passing downwards as the U value of the insulation dropped due to water accumulation.

    If you take away the growing medium and vegetation, then no problem - the EPDM is vapour permeable enough to loose some vapour through it, and drive any accumulated moisture (~1% from memory) which might accumulate during winter back down during the summer. With a green roof, then the growing medium could keep the environment immediately above the EPDM cool and humid enough that water build-up occurred (this situation is in-fact very similar to the sorts of build-up that I was talking about previously in this thread, just "upside down"). Higher interior humidity (e.g. the sort you might get in a kitchen etc.) would lead to high water accumulation, as would northward facing or shaded roofs. The "Intello" helped, but just slowed the process down in many of the more tricky scenarios.

    Adding in a tiny amount of ventilation (e.g. a few mm gap between the EPDM and the insulation) fixed the problem, but seemed tricky to arrange.

    I the end, due to a combination of this potential accumulation, and the sort of fire breaks I should put in according to https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/230510/130819_SW3529R_-_Issue_3_-_Green_Roofs_and_Walls_Project_web_version_v3.pdf ...

    I scrapped the green roof idea, and put on solar PV instead.
    • CommentAuthorTimSmall
    • CommentTimeFeb 24th 2018 edited
    Posted By: tonySo high up is acting as a vb and dpc.

    Yes, which is the right thing to do if you have a permanently wet environment on one side of your building element (in this case the ground, which you have to assume is both permanently at 100% humidity, and also cooler than your indoor environment in the UK all year round.

    I only ever put poly on top of insulation, then concrete.

    Yes, I think this is the right way to do it, but is contrary to most PIR board manufacturer instructions e.g.

    "Install a damp proof membrane below the Celotex. This can either be over the top or below the slab. The damp proof membrane must provide continuity with the damp proof course in the surrounding walls".

    Which I think is hygrothermally daft ... even without considering what happens if you have a pipe leak somewhere above it, at some time during the life of the building.
    • CommentAuthortony
    • CommentTimeFeb 24th 2018 edited
    It does not surprise me to find the manufacturers being wrong.

    I had a lot of trouble with building inspectors often pulling the wool over their eyes. Did get one who insisted Dom went under eps and I finished going back to do liquid applied dpc on top of the concrete due to problems found by a surveyor.

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