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Green Building Bible, Fourth Edition
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Heating and cooling: Pug/biscuit mix for wet UFH, can it be laid wet?

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    • CommentAuthorjohn37
    • CommentTimeFeb 28th 2016
     
    We are doing wet UFH on suspended floors with very strong timber joists (9 x 3in at approx. 400mm centres, max span 4.8m). We are setting the pipes between the joists on a 45mm thick PIR base and pug mix (8:1 sharp sand: cement) between and around the pipes, as commonly used. I understand the pug mix is usually mixed and laid dry but my builder would like to lay it wet, in fact he has already started and it looks ok.

    Is there any disadvantage or problem laying the pug mix wet at 25mm depth?
    • CommentAuthortony
    • CommentTimeFeb 28th 2016 edited
     
    Where will the water go? Normally it is laid semi dry like floor screed

    The mix will be weaker and will take longer to dry out, could wreck flooring if laid too soon.

    I wouldn't use a biscuit mix, see other threads on this.
    • CommentAuthorjohn37
    • CommentTimeFeb 28th 2016
     
    Thanks Tony. Good point. Would the water be taken up by the cement to some extent? How dry is semi-dry, e.g. how much added water? Would it be better to increase the cement from 8:1?

    The new structural flooring will cement impregnated particle board. Do you think a that could be laid immediately on a semi-dry pug mix?

    What would you use instead of biscuit mix?
    • CommentAuthortony
    • CommentTimeFeb 28th 2016
     
    Sentence

    1) no like screed if you must, I think the Victorians used to lay it completely dry, little as pos water, no need to increase cement

    2) No wait for pug to dry out , I wouldnt chance it.

    3) air is my favoured option, cheapest too :) and it works

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