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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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    • CommentAuthorTriassic
    • CommentTimeFeb 12th 2015
    The architect has said we can't place UFH pipes, zip tied to the reinforcing mesh, in the structural raft and that the UFH can only go in a screed layer above the completed raft. What are your thoughts, is he right?
    yes, well you could, but it ud be silly/less than efficient.
    Pour a thicker insulated screed above the structural raft.
    i PLACED some insulation, 50mm polystyrene, below the raft as well.
    then more above, below the screed, which was 100mm to 125mm
    defence in depth if you like.
    • CommentAuthorTriassic
    • CommentTimeFeb 12th 2015 edited
    The slab is based on the Vikinhg House passive slab design and insulated with 200mm of EPS 300 below the slab. I'm looking to do away with the 50mm of insulation and 75mm of screed above the slab.
    • CommentAuthordickster
    • CommentTimeFeb 12th 2015
    We had structural raft, 100mm insulation and then UFH pipes in screed. Building inspector very hot on structural rebar, ensuring proper spacing etc. No way would he have allowed UFH pipes in there!
    • CommentTimeFeb 12th 2015
    How come Viking House does it with no problems???
    • CommentAuthorGreenfish
    • CommentTimeFeb 12th 2015
    There are situations where EPS under raft is not a good idea, all depends on the site. I know I initially suggested it to my builder and he was dead against it, and I can see now that it would have been very impractical for my build. Amidst the bed rock and clay mix it would have been impossible to get a flat area to lay EPS on, let alone stop it floating away (site was like the Somme). Structural engineer needs to sign it off as well. We went for EPS in the cavity below slab level acting as an insulating skirt instead. To go for VH style passive slab I suspect you need both an architect and a builder that are familiar with it.

    As for UFH in slab, how are you considering heating the water and what is your lifestyle? In slab will be very slow to respond. Great if you have GSHP and are there all day, permanently circling flow just above room temp.

    Posted By: joe90How come Viking House does it with no problems???
    Hope he will come along and say. Is it a suitable approach for all situations?
    • CommentAuthorcjard
    • CommentTimeFeb 12th 2015
    Maybe he has concerns about thermal expansion destabilising the structure bearing on it

    Hang on.. doesn't your architect work for you? If you don't like the answers and aren't being given an appealing justification, switch
    Posted By: TriassicThe architect has said we can't place UFH pipes, zip tied to the reinforcing mesh, in the structural raft and that the UFH can only go in a screed layer above the completed raft.
    Did you ask him why not? We've done 100's without problems, there's about 50,000 done that way every year in Scandinavia.
    • CommentAuthorCWatters
    • CommentTimeFeb 12th 2015 edited
    The architect has said we can't place UFH pipes, zip tied to the reinforcing mesh, in the structural raft and that the UFH can only go in a screed layer above the completed raft. What are your thoughts, is he right?

    I forget what stage you are at? Two options...

    Screed with UFH

    Slab with UFH

    Normally screed is around 65mm thick and the slab 100mm thick. So the latter has more thermal mass/slower response time than the former.
    • CommentAuthorTriassic
    • CommentTimeFeb 12th 2015
    Posted By: CWattersSlab with UFH > Insulation > Ground
    Our current home has the UFH pipes zip tied to the reinforcing mesh, so I'm happy with a slow response.
    • CommentAuthorborpin
    • CommentTimeFeb 13th 2015
    Posted By: Triassicin the structural raft
    The fact it is structural probably does it. Lots of pipes running through the concrete inevitably reduce the strength. Most houses have a concerete 'slab' rather than a full on raft. Just drop it by however much and screed it.
    • CommentAuthorwoodgnome
    • CommentTimeFeb 13th 2015
    If you run the UFH as one zone so the temp is even across the slab i cant see a problem.
    The architect may be worried about different temps across the raft.
    I would be interested to know what problem the architect expects.

    If he's concerned about the voids in the concrete then you can just make the concrete 20mm thicker to accommodate the pipe thickness. This should be at least as strong, after all, don't they often include voids in concrete to improve the strength to weight ratio?

    If he's concerned about the increase in temperature of the slab causing differential expansion at floor/wall junctions then don't you get this problem in any case where the insulation is outside the structural component? You could argue that the problem is minimised by having both the load bearing slab and the walls on the warm side of the insulation layer.

    If he's concerned by the temperature of the water then what temperature would be happy with? Does he realise that well insulated buildings can be heated with water just above room temperature?

    • CommentAuthorringi
    • CommentTimeFeb 13th 2015
    Is there a BS standard that says how to do the calcs when there are UFH pipes in the slab?

    At the very least the pipes will make the concrete connection to the steal weaker.
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