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Housing - Renovation: insulated slab, UFH and perimeter insulation

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    We're refurbishing the kitchen to the rear of the house (mid-terrace victorian), and planning to dig up the old concrete there, laying insulation and underfloor heating. This is part of a wider refurb and we'll be building a new rear external wall, installing some new low-u triple glazing, and joining our neighbour's party wall on the side return.

    Have read a lot of different threads on here and elsewhere and if money was no object I'd be going for a limecrete/glasscrete floor, we have to be a bit leaner on the spend here so looking now at XPS, topped with underfloor heating and a thin concrete slab.

    Some questions I'm looking for guidance on are:

    1) The main part of the house has a suspended floor, which I've already insulated - I need to make sure it's ventilated to the rear - at the moment there are vents under the existing slab so we'll need to lay some new very long pipes to provide ventilation through the slab, which I am conscious will be thermal bridges through the insulation layer. Any suggestions on how this should be achieved? Is it best for the pipe to be as low down as possible in the insulation layer?

    2) We're going to have quite a few pipes running around the place to the WC under the stairs, the new HW cylinder to the heat pump, and obvs the usual plumbing as well - all of which I guess presents a bit of a thermal bridge to the insulation layer. Other than making sure these penetrations are well sealed is there anything else to consider?

    3) Do we put the XPS under the concrete slab or slab first, then XPS, then screed? Seen advocates for both options with no obvious winner...

    4) What would you do for UFH in this context?


    For further context, the finished floor is going to be marmoleum (Forbo's lino).

    Thanks in advance :)
    • CommentAuthorMike1
    • CommentTimeJul 4th 2023
     
    Posted By: number_thirty_three1)...Is it best for the pipe to be as low down as possible in the insulation layer?
    Yes, as long as there's no risk of it filling with water due to flooding or the like.

    Posted By: number_thirty_three2) We're going to have quite a few pipes running around the place
    Difficult to envisage the issue without a sketch...

    Posted By: number_thirty_three3) Do we put the XPS under the concrete slab or slab first, then XPS, then screed? Seen advocates for both options with no obvious winner...
    You' may find the same again. Personally I'd skip the screed and add additional insulation if I could. You could plan to add a thin self-levelling compound over the top before laying flooring (as long as it's one suitable for use over heated floors) if you're concerned that there may be a problem.

    Posted By: number_thirty_three4) What would you do for UFH in this context?
    Pipes? I guess that's not what you're asking?
    • CommentAuthortony
    • CommentTimeJul 7th 2023
     
    I like eps under the concrete and lots of it
    Do you have a U-value in mind?
  4.  
    insulation then concrete. Use steel mesh in the slab, and cable tie the UFH pipes to the mesh. The issue can be that the concrete would be thicker than a screed, so would take longer to dry out, and result in the water damaging the finished floor, or raising bubbles in the vinyl in your case.

    Standard answer is to apply a liquid DPM over the concrete, prior to the finished floor going being laid. That's what I did on on commercial projects, and what I do now on domestic projects. As Mike1 says, allow a £100 for someone to fix any low bits with self level, as insurance, but hopefully won't need if the concrete guys do their job.

    If you go for say 2 layers of 100mm XPS, that shd get you to around Uval of 0.13 W/m2K (need to calc with area/perimeter ratios etc.)

    Air ducts for u/f ventilation, being below the new solid floor insulation (dug into the solum), will likely be below external ground level. To get back above ground level when you get beyond the new solid slab, you could connect the ducts to periscopic vents, which you build in to the block/brick underbuild, here's an image for info....

    https://sydenhams.s3.amazonaws.com/Building%20Materials/Roofing%20and%20Ventilation/Building%20Ventilation/g960swl.jpg
    • CommentAuthorwookey
    • CommentTimeOct 24th 2023
     
    You definitely want the thermal mass above the insulation, not below it. So concrete on top of insulation. I'd avoid concrete and use lithothem clay tiles (greener but presumably more expensive) or just 28mm T&G screedboard.

    Doing the same in the rest of the house would have saved you having to put in annoying vent pipes, but a bit late for that now :-) Yes vent as low as possible then a periscope vent back up to air-brick height.

    As for pipes being thermal bridges. Not much to be done about that except use plastic pipe, and grommet every entry to the airtightness layer (top of the insulation? top of the concrete/screedboard?
    • CommentAuthortony
    • CommentTimeOct 24th 2023
     
    I like mass above the insulation = greater comfort, less temperature swings and can store more incidental solar gains

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