Extension screed with UFH or insulation + boards?

Discussion in 'Builders' Talk' started by SuperPav, Jan 11, 2019.

  1. SuperPav

    SuperPav New Member

    I'm at the point now where the roof is going onto the extension. It's approximately 6x6m at the back of a 1920's semi.

    The concrete slab (sat on a DPM lapped into DPC, and insulated underneath) is approx 60-70mm below the timber floorboard level of the house which it will join. The final floor on the whole ground level will be engineered wood.

    I was going to just get the extension screeded up to the level, but am now rethinking options as 200mm slab + 60mm screed is a lot of (cold) thermal mass to heat up?

    1) I've never installed UFH, would I be able to just lay a wet UFH system in the extension over the concrete slab, and then screed over it up to the FFL? Would I need any insulation or boards between the pipes and the concrete slab? Would quite like to give UFH a go... (Rest of the house will just be radiators off a gas combi)

    2) If I'm not going to install UFH, as the concrete slab is relatively level, albeit not very even, is an alternative to just lay e.g. 40 or 50mm insulation boards with T&G flooring boards or OSB on top? Would they need fixing down into the concrete, or just left as completely floating?

  2. Dave Marques

    Dave Marques New Member

    Take a look at Uponor's Minitec system, 15mm height total, 12mm panel, 3mm screed on top. Sits on top of a solid screed.
  3. rogerk101

    rogerk101 Well-Known Member

    Wet underfloor heating systems needs the pipes to set in about 70mm of screed anyway - so no problem with heights there.
    You say that the 200mm thick concrete slab is insulated underneath ... why did you do it that way round?
    The usual (correct) way to do it is to cast the 200mm slab, then put on at least 100mm (preferably more) of insulation and then the UFH pipes set in 70mm screed.
    You're right to be concerned ... the UFH system is going to have enormous delays in getting 270mm of concrete up to a temperature that is capable of heating a room. It'll be one of those systems that turns on in autumn and turns off in spring because the thermal lag will just be too long for any 'control'.
  4. SuperPav

    SuperPav New Member

    Why it's done is because it is one of several things that the builder did that weren't ideal, let's leave it at that. Unfortunately, it's also one where corrective action is a bit tricky due to its nature!

    In this instance, would you therefore reckon that sticking some 50mm Celotex on top of the slab, and then just tng chipboard would be best, and leave the UFH out?

    Or can I go for a "dry" UFH install e.g. 25mm Celotex over slab, 25-40mmmm insulated UFH formers with pipes, then a thin skim of screed, or even just covered in flooring?)?
  5. Jimbo

    Jimbo Well-Known Member

    On the other hand all that thermal mass will help keep the room from overheating in the summer.
  6. Isitreally

    Isitreally Well-Known Member

    He probably did it that way as it was always done that way, my own extension is, 150mm celotex, 100mm concrete, 50mm screed, that's what was on drawings.
  7. Jimbo

    Jimbo Well-Known Member

    Where an extension has dramatically different thermal mass and insulation than the rest of the structure, the heating system should really be split into separate zones IMO.
    rogerk101 likes this.
  8. Isitreally

    Isitreally Well-Known Member

    Mine is on two zones, one as was original and one for extension, i did this not because of thermal mass, but simply the extension wouldn't be used all the time, so heating up as much would just be a waste.
    SuperPav likes this.
  9. Jimbo

    Jimbo Well-Known Member

    Cool :)
  10. SuperPav

    SuperPav New Member

    Thanks both. Isitreally, your ground floor construction is very similar to what I have, so must not be all that unusual.

    So do you have UFH on the extension, or just rads?

    I'm tempted to just lay UFH in the screed and then have a (smaller) radiator to top it up if required. The extension will house a kitchen diner so will be used regularly. If it takes ages to warm up but just provides "background" heat during the winter months, that'll be fine. I can then "switch on" the rad via a TRV if required?

    Alternatively, does anybody have any experience of laying flooring boards over a cast slab - it seems pretty straightforward and with some insulation should provide similar performance to the rest of the ground floor.. or am I missing something?

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