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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
    • CommentTime7 days ago
    Thanks Nick.
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