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Heating and cooling: Lightweight screed for bathroom

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    • CommentAuthorrevor
    • CommentTimeNov 10th 2022
     
    Anyone done a lightweight screed for wet UFH and tiling over for en suite. I note one can make a "concrete" mix using vermiculite but have not ever tried it. Have previously made on a different bathroom a "biscuit mix" 8:1 sand cement which was bit crumbly. This time it is on a suspended floor. Floor is engineered easy joist with P5 chipboard glued and screwed. Joist manufacturers say it will take a standard screed of 25 mm but won't comment on a 35 mm screed I need to do to match floor levels. Will have a bath, shower and usual sanitaryware, so my floor loading concern. May batten out at 400mm cnt for pipe clips and overlay with 6mm cement board before tiling but open to options. Thanks in advance.
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      CommentAuthordjh
    • CommentTimeNov 10th 2022
     
    Have you considered using grooved dry screed boards, or preformed insulation boards to hold the UFH with a dry screed over?
    • CommentAuthorrevor
    • CommentTimeNov 11th 2022
     
    Posted By: djhHave you considered using grooved dry screed boards, or preformed insulation boards to hold the UFH with a dry screed over?


    Sort of. Have used the preformed insulation boards on another bathroom (but do not think it is as good a job as if it was a screeded floor) but not dry screed boards. Was a bit awkward as well as not particularly flexible in routing pipe around sanitaryware to avoid potential problems with fixings. This is the last of my upstairs ufh install and would like to use up as much of the materials I already have, that is left over. i.e pipe, clips, tile backer board etc without buying extra materials if I can avoid it but at end of day not averse to any solution if needs must.
    • CommentAuthorTimSmall
    • CommentTimeNov 11th 2022
     
    We did this for our en-suite bathroom. The floor joists were designed for low deflection (1 in 600 I think - half the normal max deflection for a domestic floor), and had OSB screwed and glued to battens to maximise depth. OSB was sealed with SBR. We used polycarboxylate super-plasticiser - this reduced water needed by about 40%, which gives a higher thermal conductivity screed (better for UFH), and less risk of shrinkage cracking during the cure. Polythene over the top for 14 days with occasional water misting to get a good cure.

    I wouldn't use vermiculite - you want high thermal conductivity in the screed!

    Cement in the screed was 85% GGBS and 15% OPC (because we had a few bags of GGBS spare - not essential by any means). "typical" screed sand:cement ratio.

    We also used nylon screeding fibres to increase strength.

    The mix was standard to wet consistency (but still used less water than a normal screed mix would, because of the super-plasticiser).

    Most important ingredients are the polycarboxylate and the fibres.

    Depth about 25mm I think (I can probably go and measure it if you're curious). It's been down for 9 years with no problems, and is now tiled over.
      2f-bathroom-ufh.jpg
    • CommentAuthorrevor
    • CommentTimeNov 11th 2022
     
    Thanks Tim. Lots of stuff there I did not consider, yes vermiculite not a good idea not thought that one out, and have never heard of polycarboxylate, every day a learning day. Presume your insulation is under the OSB. Done something similar to you before but battened on top then screeded between battens and 6 mm Hardi backer on top.
    Battens position to take fixings. Would do something similar with this next ensuite but floor span is greater so need to watch the weight, but if your 25 mm works well then, I think will be ok with that. I have 60 mm depth to work with to include the tiles in order to meet the 1st floor finished level so may get some extra insulation in there as well.
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      CommentAuthordjh
    • CommentTimeNov 11th 2022
     
    Just rereading the thread, I would guess the bath is the largest loads, when full of water and person. There's no need for UFH or a screed under the bath, I don't think? Which might reduce the concerns. Ditto for the shower stall. Personally, I'd keep the ensuite floor a few mm under the main floor level to prevent accidental runoff.
    • CommentAuthorrevor
    • CommentTimeNov 11th 2022
     
    Posted By: djhJust rereading the thread, I would guess the bath is the largest loads, when full of water and person. There's no need for UFH or a screed under the bath, I don't think? Which might reduce the concerns. Ditto for the shower stall. Personally, I'd keep the ensuite floor a few mm under the main floor level to prevent accidental runoff.


    I would not do UFH under the bath nor any of the sanitaryware but would have screeded and tiled as easier to clean under. But I will rethink that as it would save work. I am not looking forward to carrying buckets of screed upstairs so not doing under the bath would help. Good point about leaving the ensuite floor a bit lower, an ensuite already in use is level and not been any issues with that but there is no bath in that one. In our previous house, our main bathroom had a reclaimed maple flooring on top of the original floor and in 30 years or so never had an issue with it and never had to revarnish it. Old houses never had the "sophisticated" materials we have today and if floor got wet you mopped it up. Sometimes think we use stuff because it available or its design appeals.
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      CommentAuthordjh
    • CommentTimeNov 11th 2022
     
    Posted By: revorI would not do UFH under the bath nor any of the sanitaryware but would have screeded and tiled as easier to clean under.
    Ah, you're planning a freestanding bath? We (chiefly SWMBO) won't have such a thing because of the extra cleaning required, so ours has a panel covering the visible side. BTW, SWMBO won't have tiles anywhere either, again because of the cleaning, so we have 'hospital' vinyl in the wteroom. A commercial installer did an excellent job. In a previous bathroom we used bamboo for the floor because it can handle water better than wood. All warmer underfoot than tiles, as well :)

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