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How to lay new concrete floors with UFH in a 1980s bungalow

  • 21-03-2022 10:34pm
    Registered Users Posts: 194 ✭✭ Capra

    We are doing an extension on our 1980s bungalow. As we are putting in a heatpump we are going to be doing unfloor heating and we are ripping out the old floors.

    Are there other ways of doing this than the standard 50mm sand, 150mm slab, 150mm insulation and 50-75mm of screed with pipes? I was watching a video of a renovation in Denmark where they put a sand base on top of the hardcore/dirt and then 300mm of EPS directly on top of it followed by 100m of concrete with steel mesh and underfloor pipes all contained within. It looks extremely easy to do and by all accounts this is the method they use throughout Denmark which I'd imagine has a similar climate to here. Has anyone here done anything like this or a similar alternative?


  • Registered Users Posts: 751 ✭✭✭ C. Eastwood

    150 mm concrete, on DPM or Radon Barrier, on sand blinding ( to protect the DPM) on Minimum 150 mm compacted hardcore.

    Irish Building Regulations requires a 150 mm concrete floor slab.

    100 mm insulation is fine.

    I would only have a maximum of 50 mm screed for the UFH pipes.

    Building Regs requires a Radon Sump.

    To check if you require a Radon Barrier, go to this website:-

    select your County, put in your Eircode (do not leave a space in the Eircode). Read the instructions.

    If your area is less than 10 % - a Radon Barrier is not required.

    Use a 1200 gauge DPM.

  • Moderators, Home & Garden Moderators, Science, Health & Environment Moderators Posts: 16,655 Mod ✭✭✭✭ DOCARCH

    Not sure 100mm insulation is fine?

    For underfloor heating need to achieve u-value of at least 0.15. That usually requires more than 100mm of ('standard') insulation.

  • Registered Users Posts: 1,535 ✭✭✭ Dudda

    I suppose technically as an existing building it might be exempt from needing to reach required TGD targets of 0.15 but if going to all the effort of digging out the floor (which is a massive job) only putting in 100mm of insulation is madness. The price of electricity (which a heat pump uses) is going up. €500 spent on electricity won't last long but €500 spent now on insulation will last forever.

    Digging out a floor in an existing building is labour intensive as you've usually a tiny digger so it can fit into the house. You mentioned 300mm of EPS insulation and while EPS insulation is cheaper than PIR you need more of it. The effort and cost required of digging down further wouldn't offset the cost of the cheaper EPS insulation.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood

    Any person carrying out works on an existing building is obliged to comply with the Building Regulations (BR).

    Any remedial works must be completed in accordance with the BR.

    Replacing a ground floor in an existing dwelling - the BR states - “replacing the ground floor in an existing dwelling is limited insofar as is reasonably practicable”. This should only be decided by the Consultant who will provide a Certificate of Compliance with the BR.

    In some existing dwellinghouses it is not possible to excavate down the required 425 mm to put in 150 conc on 100 insulation on DP or Radon barrier on 25 mm sand on 150 compacted hardcore

    The BR Maximum U Value for replacing an existing ground floor is 0.45 W/m2/degreeC/hour. (0.45 W/m2·K)

    100 mm of Kingspan Kooltherm K103 Floor Boar, will give a U Value of 0.13 W/m2·K.

    Why any person would want to put in more than 100 mm of insulation under the concrete is a mystery to me, and a complete wast of interest to pay back on a loan to a lending institution.

    This is Ireland where we may have -1 degree C one or two nights a year.

    Stand in the middle of your concrete ground floor, and ask yourself - where is the heat loss through the floor going to go.

  • Registered Users Posts: 9,769 ✭✭✭ Effects

    Does it cost that much more to go the extra 50mm? Or is just that the diminishing returns isn't worth it?

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  • Registered Users Posts: 290 ✭✭ Biker1

    Done a payback calculation for 150mm versus 125mm PIR in a new house and it would take 80 years recoup the investment for the extra 25mm. And that's not taking into account the rise in temperatures that is coming.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood

    100 mm of Kingspan Kooltherm K103 Floor Boar, will give a U Value of 0.13 W/m2·K.

    150 mm of Kingspan Kooltherm K103 Floor Boar, will give a U Value of 0.1W/m2·K.

    Difference is 0.03 W/m2.K.

    assume a ground floor of 15 m x 8 m = 120 m2 of floor.

    Heat loss is approx 0.03 watts x 120 m2 x (20-9) degree C per hour = 40 watts loss / hr.

    leave off one of the 40 watt bulbs 💡 in the house periodically, and you get the same saving. Or open the external doors for shorter periods of time. Don’t forget that we live in Ireland.

    Most people cannot afford to put in more Insulation than is required by the Building Regulations.

    Unnecessary overspend on hundreds of other items in the refurbishment of the house and you end up paying a lot of interest to the lending institution for a large portion of your life.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood


    Agreed - You are correct


  • Registered Users Posts: 1,285 ✭✭✭ dathi

    The BR Maximum U Value for replacing an existing ground floor is 0.45 W/m2/degreeC/hour. (0.45 W/m2·K)

    part L Where the source of space heating is underfloor heating, the maximum floor Uvalue should be 0.15 W/m2K.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood


    Therefore 100 mm of Kingspan Kooltherm K103 Floor Boar, will give a U Value of 0.13 W/m2·K which complies with the BR for underfloor heating

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  • Registered Users Posts: 9,769 ✭✭✭ Effects

    Can I ask what that factors in? I'm presuming extra cost of digging, removal of material and more expensive insulation. Is that about it?

  • Registered Users Posts: 290 ✭✭ Biker1

    New build.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood

    Yes. Extra works.

    They are removing the floor and excavation of the ground under the floor of a house approx 40 years old.

    The extra 50 mm of excavation, and 50 mm insulation will add an extra cost and will take approx 80 years for payback.

    100 mm of certain types of insulation complies with the Building Regulations.

  • Registered Users Posts: 2,272 ✭✭✭ Yellow_Fern

    Btw there are other insulation types that are superior to PIR allowing a much thinner floor, but they are more expensive, so they are relatively rarely used.

  • Registered Users Posts: 2,709 ✭✭✭ ?Cee?view

    What are those please? Our house is built on granite, so anything that results in less excavation is very welcome.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood


    If you contact any of the Irish Insulation manufactures - by phone or email - they will give you excellent free advice.

  • Registered Users Posts: 2,272 ✭✭✭ Yellow_Fern

    One type is aerogel insulation, a lot more expensive but with a modest reduction in thickness. What is even better are vacuum boards where you get a very substantial thickness reduction but you would be paying a lot for it. I think Kingspan sells vacuum boards. It is very niche product to be honest.