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Replacing floor joist with concrete slab

Discussion in 'Floors, Stairs and Lofts' started by Ben-v, 23 Nov 2019.

  1. Ben-v

    Ben-v

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    Hi,
    My partner and I are having an extension on the back of our house and we would like UFH throughout the downstairs.

    Most UFH systems I have found aren’t the best for wooden floorboards and the floor height would raise substantially, so I was wondering about the feasibility of replacing the whole downstairs (approx. 56m2). Final finish will be tiles also.

    Any advice would be appreciated.
     
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  3. bobasd

    bobasd

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    No way no. what you propose doesn't make technical or financial sense.
    not least being the cost of running elec UFH.

    fwiw: fully tiled throughout floors are OK for car showrooms but are so 1990's in residential situations.
     
  4. jonbey

    jonbey

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    House next door did this. Ripped out all joists, removed sleeper walls, removed all electrics and plumbing, basically removed everything, then put down dpm, jablite insulation, wet underfloor heating and screeded. No radiators.
     
  5. bobasd

    bobasd

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    of course people do it but its the cost and consequences that are the point.

    a few years ago there was a surge in burrowing under houses for garages and "play rooms" etc. nowadays the leaks and structural cracks and re-sale losses are coming home to roost.
     
  6. bobasd

    bobasd

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    OP,
    at the least, your proposed extension will presumably be solid floored?
    consideration must be given to any existing air bricks in the rear elevation that are presently venting any main house suspended floors - the extn will block them.
    supplying ventilation through tubes is the typical remedy - google pics.
     
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  8. Ben-v

    Ben-v

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    I was more trying to establish the extent of the works to facilitate it and possibly an idea of cost. The extension will be slab and looking to match the floor level throughout. Will keep a ventilation system in place there is a few rooms with a low tog carpet in.

    Also bobasd wet UFH system with a solid concrete slab is more effective economically as the slab as a solid mass retains the heat more effectively, overall keeping bills down. I would never consider electric above 5 metres.
     
  9. bobasd

    bobasd

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    i'm dont want to go into this much further that this but:

    i well understand the claims made for wet UFH but, even if they come near whats claimed, and even if its installed correctly, wet UFH has its own problems & issues in practice.
    i've also read, mostly going south, reports on heat retention in concrete mass ie heat sinks - concrete is not a good heat sink, it takes ages to warm up, and retains very little heat.
    the return for installation and running costs of UFH is small.

    none of our heating systems are up to much or come without various significant costs but, for my money, UFH is not the way to go until its much more advanced.
     
  10. Notch7

    Notch7

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    I did a job for a client, a 25 sq m orangery. We did wet UFH in their and dug up most of the groundfloor screed, fitted insulation, pipes and laid new screed.
    It worked very well.

    Im sure removing a suspended floor and replacing with a slab would work fine.
    A few points:
    -ground floor slabs are only suitable for load bearing ground -not suitable for shrinkable clay soils and or nearby trees
    -lay hardcore, blinding, dpm, concrete, insulation, pipes, screed, ditra mat, tiles.
    -fit the thickest insulation you can afford, at least 100mm
    -use a screed such as mapei top cem or a liquid anhydrite screed -they are stronger, dry quicker and transmit heat faster.
    -decouple tiles with ditra
    -use perimeter expansion foam
    -use Alpert pipe it lays much quicker
     
  11. bobasd

    bobasd

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    thanks for that great install list - it should be a great help to those intending to do it.
    But the issues are not merely if the UFH "would work fine" but installation costs and knock-on practicalities - operating costs - payback time? - and environmental costs.

    i obviously hope that your Orangery installation works well i dont want anyone's work to fail or anyone's fabric to be damaged.
    but in my experience i've come across a number of failing or full stop failed UFH systems to the extent that we've been sometimes asked to dig them out - leaks had created pools of slimey mould on the insulation.
    we purposely never install these systems, and refer repairs & guarantees to subbies.
     
  12. Notch7

    Notch7

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    Thanks for that.

    Underfloor heating is certainly a current trend and I agree with your comments -people must do their research to decide.

    I dont have enough experience to comment on reliability -I personally think UFH is like anything else -it depends on quality of install more than the technology.
    I hate the scheduling of underfloor heating

    Feedback Ive had from customers has been good in terms of effectiveness and running costs -for wet.

    I would point out that customers are generally surprised by how warm an orangery can be and when open to the house, dont impact on heating costs much. But then new extensions are far better insulated than most houses and the lantern does help to provide significant solar gain through winter months.

    The key to successful wet UFH is having a thick layer of celetex below the screed, insulation around the edge. That way the screed is a fully insulated thermal store.
     
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