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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorHoveTom
    • CommentTimeNov 30th 2019
    I’m presently pulling up my floor boards to insulate my suspended timber floor. I'll then install wet ufh.

    My floor joists are old and small compared to newer ones at around 100mm, so I thought of using internal angle bead (the stuff you put on the corner of plaster board before you skim it) to support the rigid Kingspan type insulation instead of wooden battens.

    This means I can use 70mm of insulation rather than just 50mm with a wooden batten and still leave enough gap for ufh and a dry screed.

    The internal bead is galvanised, seems very strong when screwed in, cheap and smaller than a traditional batten.

    Is there any reason I haven’t considered why this is a bad idea?
    • CommentTimeNov 30th 2019
    Sounds like a good idea to me, but I'm no expert.
    • CommentAuthorPetlyn
    • CommentTimeNov 30th 2019
    Are you proposing to replace your floorboards with a cement screed containing the ufh and the heated screed supported in between your 100mm joists and polyurethane boards supported by the angle?

    Our first reaction would be that the flexibility of the 4" joists would lead to rapid breakdown of the screed and so presume this is not your intention and you will retain your floorboards above the heating pipes/insulation.

    If the space to the subsoil is not great, how about filling your entire cavity with recycled glass beads - they are fire proof, indestructible and being pourable will not require close cutting to fill the joist spaces. The beads will surround the wet ufh pipes and joists flush to the underside of the floorboards?
    Sounds neat! but even 70mm PIR isn't a huge amount, have you considered running more PIR at right angles underneath the joists while you're at it? Maybe supported with thick battens (or angle beads!) also at right angles to the joists to take the weight of the screed?
    • CommentAuthortony
    • CommentTimeDec 1st 2019
    Do you mean external angle bead?

    I would forget the screed use more insulation, eps sheets for me. Or fully fill void with beads or foamglass
    • CommentAuthorHoveTom
    • CommentTimeDec 1st 2019
    Hi all and thanks for your thoughts so far.

    I will be replacing the old floor boards with large 18mm tongue and groove chipboard flooring sheets which will be glued and screwed. When I said 'screed’ I mean the traditional dry sand/cement mix which will be about 25mm thick on top of the ufh pipe and beneath the chipboard.

    I did mean internal angle bead. The external seems larger but not as strong or stiff.

    I don’t have a huge gap underneath the joists. About 450mm but it varies a bit. I don’t think building control in my area (Brighton) will allow me to fill that void any more with PIR going in the other direction.

    When you say glass beads do you mean Lytag? Does that still allow the area to be ventilated?

    Should I also tape the PIR to the joists? I’m fitting the PIR as carefully as a I can but there are inevitable cracks. I have around 100 sq m of floor to do and that is a lot of tape along both sides of each joist and it isn’t cheap at around £30 for 30m from the green building store. I know it will help but how much? If 5% it’s really not worth it for the tape cost as it’s a renovation and not an airtight new build. If it’s 30% then I’ll do it... difficult to answer I know but curious if anyone knows the answer. Same applies to putting a scratch layer of plaster down over the bare brick between each joist..
    • CommentAuthorPetlyn
    • CommentTimeDec 1st 2019 edited
    We understand Lytag to be treated waste from power stations. We are suggesting expanded recycled glass which is a regular round smallish bead (4-8mm) - see photo below.

    Our building control told us we could either have a suspended floor which would need to be ventilated or a solid substrate so that no underfloor void existed. This latter solid floor could be achieved with insulation but with no void, ventilation would not be required.

    If your building control wish to retain ventilation between the subfloor and the floor itself, then the purpose of underfloor insulation is entirely negated if this air is allowed through any of the insulation.

    If you have 450mm under the floor then beads could be spread to make a level surface with the ufh pipes installed in their plastic trays on top of the beads, leaving a shallow air space above for ventilation of the underside of the floor. Instead of ventilating to the outside loosing the effect of the insulation, because the floor has now been brought into the warm space, ventilation could be provided through small grilles in convenient places through the floor into the warm space above?
      Glass granules 2.jpg
    Tom, whereabouts is your airtight layer? If it's the t&g then no need to tape the PIR for airtightness.

    Taping the PIR to the joists won't create a vapour barrier as the joists are somewhat vapour permeable.

    You might want to roughly seal the PIR to the joists so the sand/cement doesn't get lost down any big gaps. As a diyer I cannot cut PIR accurately enough to be tight fit (unless I do endless repeat trimming, creating lots of PIR dust) so I deliberately cut it 5mm small (using a knife not a saw) and fill the gap with squirty foam.

    There needs to be contact (no gap) between the ufh/screed and the floorboards.
    • CommentAuthorriboid
    • CommentTimeJan 2nd 2020

    Tom, I am about to do ufh heating too but i am doing electric because I have a suspended timber floor. Several companies I approached said that my floor joists (120mm deep) were too shallow to support any form of cement screed and may cause cracking. So, I have opted for electric but due to the good insulation levels and air tightness (hopefully) then I do not really require it in all of the downstairs rooms anwyay - just doing the UFH in the tiled areas of the bathrooms/toilets etc. Incidentally, I will be getting under my joits sprayed with open cell PIR foam and the insulating on top of the floorboards with 50mm PIR to get my u values as low as possible
    Hope that helps?

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