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Housing - Renovation: UFH on suspended floor - airtightness?

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    I have a quote for UFH with the pipes clipped to rigid insulation between floor joists. Screed layer on top. This seems a standard installation method for suspended wooden floors. Given that the joists need to be notched to take the pipes I can't see any way of installing an airtight membrane with this setup or am I missing something?

    The alternative is to lay the pipes on top of the joists (on a new floor layer) but would result in a raised floor and I'm not sure I have the ceiling height downstairs (but OK upstairs).
    • CommentAuthorrevor
    • CommentTimeJan 24th 2020
     
    Why are you putting a screed layer over the pipes and how thick. Will joists take the weight. There are alternative methods which minimise the height increase and don't need screed. There are a lot of different options you can consider. Have a look at Robbens website there will be a better solution there. The tech staff are also very knowledgeable and helpful.

    https://www.underfloorheating.co.uk/PBCPPlayer.asp?ID=1889187
  3.  
    I'm keen on the screed layer to create more thermal mass. The other options could be laid over the existing floor but

    To quote from the link you sent "The Pro-mix system allows a suspended or batten floor to be heated from a very low heat source. Using 30-35mm of dry screed, the system will build up and hold heat. It also allows closer pipe spacing's if required to meet the heating demand of the rooms".

    I guess I won't know if the joists will take the weight (25 - 30 mm dry screed mixture) until I uncover them and see what condition they are in?

    Any opinions on if I can use the screed layer as the airtight layer?
  4.  
    Won't the rigid insulation, that you're attaching the UFH pipes to, provide airtightness? Especially if they're foamed into the joists for a snug fit, and perhaps taped to the joists as well before the screed is fitted.

    If you also tape and insulated the walls that meet the joist ends, I'd imagine that'd be sufficient? You want to stop air movement from the underfloor void to the UFH pipes rather than from the pipes into the room.
  5.  
    Posted By: Pile-o-StoneWon't the rigid insulation, that you're attaching the UFH pipes to, provide airtightness? Especially if they're foamed into the joists for a snug fit, and perhaps taped to the joists as well before the screed is fitted.


    Hopefully but I'm worried about shrinkage and also how careful the builder will be to get a tight fit. Is is OK to foam the joists? All I hear is horror stories about use of spray foam. I like the idea of taping the insulation to the joists.
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      CommentAuthordjh
    • CommentTimeJan 27th 2020
     
    Posted By: modernvictorianAny opinions on if I can use the screed layer as the airtight layer?

    No. There's no reason to expect the screed layer itself to be airtight, I think, plus it isn't continuous since it's only between joists. Joists and other timber can't be guaranteed airtight either. Only processed sheet type timber that is stated to be airtight.

    A membrane or airtightness paint at the bottom of the construction or near the top just under the floor deck should work, or maybe the floor deck itself can be made airtight?

    It all depends on what airtightness target you are aiming for, of course.
  7.  
    Another resurrection. I have decided on the buildup for UFH but still struggling with the airtight layer. UFH will be the Omnie foilboards that fit between joists. Under this will be Proclima Solitex membrane draped over and between joists and thermojute flexible batts between joists (and under the Omnie foilboards). I could put Proclima DA vapour control membrane over the top when finished but not sure if this is compatible with UFH pipes directly underneath it. Is it standard practice, or not, to place a vapour control membrane between the UFH pipes and sub-floor?
    • CommentAuthordickster
    • CommentTimeJan 13th 2021
     
    After worrying (a little bit) about our suspended ply floor air tightness, the vinyl tile man put down a primer coat and a green layer of glue all over. No leaks there then.
  9.  
    Yes that would do it!

    The concern is about the efficiency of the UFH having to pass heat through a membrane.

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