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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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    I am installing underfloor heating (UFH) in timber joists on the 1st floor. I have decided to set the pipes in a dry cement mix or pug mix. This is really on the advice of others. Using the cement mix I can avoid using expensive alternatives such as aluminium heat spreaders and I believe 35mm of cement mix will act as a better heat sink/radiator.
    But can any one tell me why it has to be a dry mix? I am slightly concered that the finished product will be "dusty". I would prefer to use a wetter mix (like a screed), but Im not sure wether this would be wise. Also I understand that it is a weak mix perhaps 8:1.

    • CommentAuthorTheDoctor
    • CommentTimeSep 16th 2008
    have you checked the weight of the screed against joist depth / spacing / span?

    a wet mix WILL crack all over the shop if it is only 35mm deep. The pipes will be 15mm dia, so that is only 10mm cover top and bottom in a timber structure that inherently moves.

    over a fairly 'typical' first floor, you are going to have approximately 75 sqm of 'screed' at 35mm thick.
    That is 2.65 cubic meters of concrete, at approx 2 tonnes / m2, giving you a weight of just over 5 tonnes!

    have a chat with the structural Engineer
    Why not use a fine, dry sand instead of cement?
    • CommentAuthorTerry
    • CommentTimeSep 16th 2008
    Our UHF supplier recomends this method, but only 25mm. Their recommended method of installation - batons fixed to joists to support insulation board with the pipes fixed into it and surounded by 25mm dry mix means that the dry mix is purely a diffuser and has no structural strength, therefore, as far as the flooring goes you still only have the joists as bearers. For tile finishes etc you still need to install a suitable substrate such as board. A bit of cracking or dust is therefore not an issue.
    As the Doc says, check on the extra loading.
    Ive worked out that there is just under a cubic meter. The coverage is just one room upstairs. Ive just looked up and it says dry sand is about 1600 kg /m3. Im using 9' Joist over a 3m span at 400mm centers so I figure Im ok on that sort of weight but will make sure.
    What will the cement do in a 8:1 dry mix? Would it help bind the sand better? I dont suppose the cement will improve the performance of the mix.
    • CommentAuthorTerry
    • CommentTimeSep 16th 2008
    suspect the cement is mainly to solidify the sand a bit. you would have to seal everthing up well to stop loose sand pouring out round the edges of the insulation or at penetrations.
    Mind you perhaps the cement bonds help to transfer the heat more uniformly???
    What ive decided to do is lay a very weak sandy mix. In order to to stop loose sand pouring out ive had to go round and seal gaps and larger holes around the insulation and ply. I suspect I wil not be able to get hold of any 'dry' sand, so I will use builders sand which I suspect will be damp.

    I am a little worried however that mixing damp sand with cement will affect the mix adversely, and perhaps cause what is supposed to be a dry biscuit or pug mix to harden too much. Perhaps I should not bother using cement at all.

    Either way I will have to rely on it drying out in situ and then adding to it if the bulk has shrunk because of water loss.
    Places like Covers or Travis Perkins should supply kiln dried sand no prob, its a fairly standard material used in latex mixes and sand blasting.

    Theres no prob using damp sand with the cement, it will still be very dry mix.

    How about lime instead of cement, or even gypsem plaster, mixed in dry? Over time the plaster, lime or cement will absorb ambient humidity and start to set on its own.

    To seal the gaps you could mix a little plaster or mortar and stop up the gaps.
    • CommentAuthorTheDoctor
    • CommentTimeSep 17th 2008
    you could lay a breather membrane to fill the gaps?
    • CommentAuthorCWatters
    • CommentTimeSep 17th 2008
    Why not use OSMA profile insulation that comes with foil heat spreader plates already fitted? You just push the pipe in the slots and fit wood floor over the top. No additional heat spreader plates required.


    We have this under 21mm engineered oak flooring (between joists) on a ground floor and it seems to work fine.
    • CommentAuthorabomb1969
    • CommentTimeSep 17th 2008
    CWatters any idea of the cost of doing this Osma profile stuff? Spreader plates are going to be about £3k for my first floor.
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