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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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Heating and cooling: Underfloor heating + dry screed

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    • CommentAuthorjules
    • CommentTimeApr 3rd 2009
     
    I am looking at a ground floor UFH system involving 75mm high battens at 400 centres with 50mm insulation between, and pipes at 200 spacing laid on the insulation under an engineered wood floor. Rather than containing the pipes in a 25mm high air gap, I was thinking that I would get better heat output by filling the gap with a dry screed. Anyone any experience of using dry screeds in this way? If so, what composition is the screed?
    Thanks
    • CommentAuthorPeter C
    • CommentTimeApr 6th 2009
     
    Hi,

    I would like a 50mm screed as it is the screed that will be retaining the heat to be released long after the boiler has been switched off.

    A form of dry screed can be a 5 : 1 mix of sand / cement, it's not 100% dry a guide is about 25% of the usual amount of water used for the mix as it is not structural.

    I would also like at least 100mm insulation the last thing you need is to be losing the heat into the sub floor.
    • CommentAuthorCWatters
    • CommentTimeApr 7th 2009
     
    <blockquote><cite>Posted By: jules</cite>I am looking at a ground floor UFH system involving 75mm high battens at 400 centres with 50mm insulation between, and pipes at 200 spacing laid on the insulation under an engineered wood floor. Rather than containing the pipes in a 25mm high air gap, I was thinking that I would get better heat output by filling the gap with a dry screed. Anyone any experience of using dry screeds in this way? If so, what composition is the screed?
    Thanks</blockquote>

    Why not use 75mm Osma "Foiled polystyrene" between the 75mm battens? We're using that under 21mm engineered oak secret nailed to the battens and works well. The OSMA profile insulation has foil heat spreader plates built in. No need to for a dry mix with all the dust it produces.

    This is the stuff. They show a floating floor but I would stick with the battens..
    http://osmaufh.wavin.com/master/master.jsp?FOLDER%3C%3Efolder_id=2534374305445917&middleTemplateName=oc_middle_service_sub

    You might find it better to go with their thinest version and put Celotex or similar underneath to make it upto 75mm.
    • CommentAuthorjules
    • CommentTimeApr 8th 2009
     
    CWatters
    It's a complicated story involving an incompetent builder, but originally the Osma system is exactly what I was going to go for. However I have been left by my builder with a manifold from another manufacturer and lots of 50mm floor insulation, so we are working to make the best of what we've got. I did get a quote from Osma, but it involved basically starting again and was about 4 times more than my plan will cost. We are where we are as they say (and the builder has been sacked).

    Peter
    Ceiling height issues are the main reason I can't use more insulation.
    • CommentAuthorBowman
    • CommentTimeApr 8th 2009
     
    Jules - having done it I wouldn't go too dry - it takes too long too set and even then is very dusty for a long time. I'd use a completely normal fibre screed.
    • CommentAuthorCWatters
    • CommentTimeApr 8th 2009
     
    What he said. Just make sure you allow enough time for it to fully dry out before laying the floor. Engineered wood flooring is more tollerant than solid but we still see a little some seasonal movement. Our floor was laid in summer and we have one joint (just one!) that opens and closes. It opens to about 3-4mm wide in the winter when the air is dryer ad the heating is on.
    • CommentAuthorCWatters
    • CommentTimeApr 8th 2009
     
    Aside: 50mm insulation should be fine for UFH but might not be quite enough to meet the building regs (at least under the old method of target U-Values for elements of the structure. What did your approved drawings specify?). Perhaps best just get it down and covered up before the BCO visits.

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