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Housing - Renovation: Electric UFH in lime screed on cork/lime slab

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    • CommentAuthorkaeko
    • CommentTimeNov 12th 2019
     
    Hello, new here, signed up as found some really helpful info in previous threads.

    We are renovating our house in Normandy, France (possibly mid-late 17th to early 18th century) and looking for some guidance on best way forward with the new kitchen floor.
    Not had much luck so far finding what feels like relevant information online, I'm no builder and the builder (my dad) is new to using lime and underfloor heating, so any recommendations welcome.

    Situation:
    - Semi-detached house on a slight slope (neighbour is uphill so to speak)
    - Earth floor (scraped back and levelled, trenches dug out for plumbing and electrics)
    - Laid a 10cm thick cork granule and lime slab (ratio 2 cork to 1 lime) on top
    - Walls have no foundations to speak of, so couldn't go dig out any further, on neighbour side wall we were 10cm below the wall in places…
    - Walls are crumbly, stones barely held together in most places, so doing a lime/cement skim, then wood fibre insulation with plasterboard skin.
    - Idea/aim is to have underfloor heating, electric, in a lime screed of 4-6cm.

    Questions:
    - Do we definitely need insulation around the wall edges?
    - Is the insulated slab enough insulation under UFH/screed (I gather from the way it was done that it equates to having 5cm of cork boards) or do we need something on top of slab?

    TheUFH thermostat sensor needs to be below the heating elements, slab is mostly dry now (7 weeks old, told to wait 8 weeks as it's been so wet):
    - Do we lay a 1-2cm screen to put sensor and get a better level to support the heating elements?
    - Is that just complicating things, should we have planned small trench in slab?


    Sorry for all the questions, hope I'm making some sense and many thanks in advance.
    •  
      CommentAuthordjh
    • CommentTimeNov 12th 2019
     
    Posted By: kaekoQuestions:
    - Do we definitely need insulation around the wall edges?

    The answer to these questions is 'maybe'. If you don't have insulation around the edges then the corners will be cold and may cause condensation and mould/rot depending on what else is in the corners.

    - Is the insulated slab enough insulation under UFH/screed (I gather from the way it was done that it equates to having 5cm of cork boards) or do we need something on top of slab?

    More insulation is usually better, so it depends on other priorities. If you can afford the height then adding some sheet insulation on top will reduce heat loss. Are there any relevant regs where you are?

    TheUFH thermostat sensor needs to be below the heating elements, slab is mostly dry now (7 weeks old, told to wait 8 weeks as it's been so wet):
    - Do we lay a 1-2cm screen to put sensor and get a better level to support the heating elements?
    - Is that just complicating things, should we have planned small trench in slab?

    I don't know what the sensor looks like or how it's connected, but can't you dig a small hole or trench out of the new slab to fit it? If the slab surface isn't flat then you'll need to make it level somehow, either by building it up or scraping it down. But if you're thinking about putting more insulation on top, you need to think about that first.
    • CommentAuthorgyrogear
    • CommentTimeNov 12th 2019
     
    You could just cut a groove with disqueuse towards the centre of the slab, to take the sensor and cable. It will get covered by the screed anyway...

    I would use edge insulation (say 30mm of XPS) around the screed, even if there was none in the slab itself.
    (Because the wall will serve for capillary passage of ground water/moisture so IMO you don't want to be drying it out with your UFH... as the structural walls "need their damp" like they have had for past 300 yrs etc.

    (Here is screeding of hydronic UFH (lime-sand screed, over 30-cm lime-cork slab) : note absence of edge insulation...

    http://www.formaterre.org/archive/2018/10/13/chantier-realise-quand-dalle-chaux-liege-rencontre-chape-cha-6096562.html

    If you have the time, you could consider a self-levelling lime screed, but you are then looking at 110 days drying time... If you really needed access, you would have to use ledgers and planks over the screed...

    Useful vid here, but guess you know it all by now, anyway !!
    https://www.youtube.com/watch?v=SpIOIyfuTk8

    (This slab is 15 cms - so yours sounds a bit on the thin side at only 10 cms but it is in now, so don't lose any sleep ! just try and lay your heating cables as high as poss per manufacturer's instructions...

    Good luck,

    gg
    • CommentAuthorkaeko
    • CommentTimeNov 13th 2019 edited
     
    Thanks djh for confirming direction.

    There's no exact regulations for floor insulation in France, unless you want to benefit from certain subsidies and that's only if you use companies rather than do it yourself.
    However, the recommended ground floor thermal resistance is R = 2.5, I just redid calculations and considering we put down over 3m2 of cork granules for a 10cm thick floor, that gives us R = 2.4 so shouldn't need much more if any.

    Gyrogear, amazing links thanks!

    Posted By: gyrogear(Here is screeding of hydronic UFH (lime-sand screed, over 30-cm lime-cork slab) : note absence of edge insulation...

    http://www.formaterre.org/archive/2018/10/13/chantier-realise-quand-dalle-chaux-liege-rencontre-chape-cha-6096562.html

    Formaterre.org looks familiar but don't recall seeing that page before.
    Interesting no edge insulation but it looks like they are not going right up to stone walls, there's wooden planks (or possibly even wood fibre insulation) for the screed height, so maybe keeping that space to build insulated stud wall?
    I've asked in comments, so we'll see.

    Posted By: gyrogearYou could just cut a groove with disqueuse towards the centre of the slab, to take the sensor and cable. It will get covered by the screed anyway...


    Good point, just seems a pity but I should have thought of the sensor location before hand really.

    So long as we get the doors in first, I think self-levelling lime screed makes sense as we have no real need to get in there for a while, run out of money to do more than get floor down anyway for now.

    Posted By: gyrogear(This slab is 15 cms - so yours sounds a bit on the thin side at only 10 cms but it is in now, so don't lose any sleep ! just try and lay your heating cables as high as poss per manufacturer's instructions...


    Unfortunately due to lack of foundations, we could only dig out the floor so much/little, based on advice we did the slab just cork and lime to ensure the most strength.
    Now to go and decide on best electric UFH product or just go for what's in Leroy Merlin…

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