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Insulation and ufh in suspended wooden floors

  • 11-04-2022 7:44pm
    Registered Users Posts: 311 ✭✭

    I have a dormer bungalow built in the late 60s which has suspended wood floors.

    i am looking at taking up the floorboards in one room to insulate it with mineral wool in between using a vapour barrier to hold the insulation up and a vapour barrier before I put the floorboards back down. I should get 100mm of insulation down given the joists are about 110mm in height.

    my question is - would this be enough to put down a heat pump based ufh down, potentially on top of the floorboards (the ceilings are high at 2.4m)? And what kind of ufh have people done in similar situations?

    and if not enough, what kind of insulation could I put on top of the floorboards that would make sense?

    my thought process is that if it works in one room I could roll it out across the rest of the house.


  • Registered Users Posts: 1,097 ✭✭✭Roger Mellie Man on the Telly

    You won't achieve regulatory compliance for ufh with 4" mineral wool. You won't even come remotely close.

    Note also that 2.4m is minimum ceiling height for habitable rooms.

  • Registered Users Posts: 1,097 ✭✭✭Roger Mellie Man on the Telly

    ^^^ Not sure why I had to tell you this twice.

  • Registered Users Posts: 31,071 ✭✭✭✭Lumen

    Are you sure you have the energy efficiency (insulation and air tightness) for a heat pump/ufh solution?

  • Subscribers Posts: 41,582 ✭✭✭✭sydthebeat

    What are you going to place the UFH pipes within? A concrete screed?

    That won't work for lots of reasons

    1. You need 2.4m min floor to ceiling height

    2. Assuming a 75mm screed, that won't be structurally good enough for point loading

    3. It's quite possible your existing floor joists won't be able to take the extra weight of the screed without issue / augmentation

    4. You'd need approx 10" of mineral wool to give you the required u value to install UFH over.

    The proper job here is to replace the floor with a solid concrete type, proper rigid insulation, perimeter strips around all rooms

  • Registered Users Posts: 39,321 ✭✭✭✭Mellor

    2.4m ceilings are low. Any reduction will cause you issues.

    Agree will all of these, except for no.2

    An unbonded screed is non structural. Doesn’t need to self support the loads. But moot at the current floor is not suitable for all the other reasons.

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  • Registered Users Posts: 757 ✭✭✭C. Eastwood


    I advise that you ought not do what you are suggesting.

    you mentioned putting a Vapour Barrier (VP) to hold up the insulation.

    the Rule with a VB is that it must always be placed at the Warm Side of the Insulation.

    It must never be positioned at the cold side of the insulation because any moisture passing through the insulation will condense on the VB and will cause interstitial condensation in the cold section of the insulation.

    your house was built in the 60’s, with timber suspended ground floors with the joists bearing on dwarf walls/ sleeper walls bearing on approx 100 mm of oversite concrete bearing on the soil. The floor will have ventilation which prevents Wet Rot and Dry Rot from occurring.

    The joists and tongued & grooved flooring will have a Moisture Content (MC) of approx 17 %. If a VB is fitter it may cause the MC of the timbers to increase above the Dry Rot safety line of approx 22%. All timbers are contaminated with Dry Rot spores, and these spores require a MC above 22% and a lack of fresh air in order to germinate. Dry Rot will cause terrible timber decay in a short period of time.

    Fitting anything under the t&g flooring can cause serious problems.

    Underfloor heating pipes can be accommodated within the floor structure of upper timber floors.

    interfering with the underside of a timber suspended ground floor will cause problems

    my second bit of advice is -do not raise the level of the ground floor.

    I agree with the last comment of sydthebeat above - replace the timber ground floor with a concrete floor.

    I will explain how to do this later.

  • Registered Users Posts: 2,600 ✭✭✭Yellow_Fern

    Some of the posters her seem to be unaware that that there is such thing as dry UFH systems that use tracks. These happen to be some of the thinnest systems available, but often give out less heat, so you need to do you calculations carefully. You could run that with a gas or oil heating system. Your UFH could lay on your joists and then lay you new floor over them. You will get more from PIR than wool, but you need to do your maths to see if you have enough.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood


    Yes there are many methods to fit UFH systems in upper suspended floors.

    As I stated above previously, suspended timber ground floors requires Through Underfloor Ventilation to prevent timber decay.

    I would not recommend interfering with this type of floor, and would recommend removing all the timber and installing a concrete floor and incorporate the UFH pipes in to a 50 mm Screed above the concrete.

  • Registered Users Posts: 311 ✭✭macannrb

    Hi all - thanks for the feedback.

    answering some the questions

    1) @Lumen is there enough insulation? - not at present but part of the upgrade in the room will be adding in 80mm of kingspan PIR insulation To the external walls. Add in the underfloor insulation plus about 120 PIR and 100mm mineral in the attic above ( part attic part bedroom).

    2) @C. Eastwood - your point in VB is spot on. I should have wrote vapour control under the insulation with the VB on the warm side. I plan to wrap the VC over the joists so that the wood has more access to ventilation.

    its also the reason for the mineral wool as this is a bit better breathing than PIR.

    3) as a DIY screed isn’t really an option. I may end up going down that later in a few years (likely a good few) but initially I need to increase comfort levels in a few rooms. If it works well it could be cost effective to do it DIY.

    4) @sydthebeat the required u value for under UFH - is this for in wooden floors, or where the it is submerged in the ground?

    5) @Yellow_Fern those boards are exactly what I’m interested in. They are about 22mm and have PRE routed groves for the Pex pipe. Have you any idea what insulation would be required under them? Any experience with them?

  • Registered Users Posts: 2,600 ✭✭✭Yellow_Fern

    I have not use them. They are lot less popular than screen approaches. They vary a lot too. Ca be quicker to heat than screed. The metal spreader plate ones can be noisy I hear, but they are one style of many. The suppliers usually can do insulation analysis.

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  • Registered Users Posts: 757 ✭✭✭C. Eastwood


    a Vapour Barrier (VP) should not be fitted under the t&g flooring boards as it will prevent the underfloor through ventilation from keeping the Moisture Content (MC) of the flooring boards to a safe MC. Usually the timber in the floor would be approx 17% MC. If you fit a sheet of polythene under the t&g flooring- when there is a spillage of water on the floor, the water will be held by the polythene sheet. The t&g flooring which is Hygroscopic (like salt) will absorb the water which will increase the MC of the flooring boards above the Dry Rot safety line of approx 22% MC, all the timber is contaminated with Dry Rot spores.

    For the spores to germinate they need 3 things:- Timber with a MC above approx 22%; lack of fresh air; and the spores. The Damage by Dry Rot is very expensive to reinstate the flooring.

    If you decide to insulate the floor by this method, (I would not certify this) :-

    1. Remove the t&g flooring,
    2. drape an approved plastic mesh net over the joists, sagged 100 mm between the joists
    3. fit 100 mm fibreglass insulation on the net between the joists and fit the flooring.

    There is no need to fit a VB on the warm side of the insulation. A VB is fitted on the warm side of the insulation so as to prevent interstitial condensation occurring within the insulation. There will not be any vapour passing down through the floor. This will drastically reduce the heat loss by Convection through the floor.

    Next you need to stop the heat loss through the floor by Convection (by air movement ).

    If this is carried out to suspended timber floors it would prevent a lot of heat loss by Convection.

    1. Remove the skirting boards
    2. seal all around the perimeter of the floor between the floor boards and the wall with expanding foam.
    3. you could check this by shaking talcum powder down over the area to see if the powder is blowing in the wind.
    4. This is to prevent cold air from the ventilation under the floor from causing draughts in to the room in these gaps.
    5. Ensure that there are no gaps whatsoever.
    6. apply a bead of silicone mastic around the perimeter of the flooring boards in the location of the underside of the where the skirting boards will fit
    7. fit a sheet of airtight membrane from wall to wall over the floor and press it down on to the bead of mastic
    8. fit new skirting boards pressed down tightly on the membrane to compress the fresh mastic
    9. Fit some type of underlay and flooring (laminate) on top of the airtight membrane
    10. Heat loss by Convection gone.
    11. Hope the feck that Dry Rot does not occur.

    I would also recommend increasing the underfloor ventilation by installing another few vents on the external walls

  • Registered Users Posts: 311 ✭✭macannrb

    hi @C. Eastwood

    That’s great suggestions. Putting the barrier above the floorboards is a great idea.

    would it make more sense to put in PIR? As I would assume this would have less vapour flow with that vs mineral wool.

    on the ventilation that’s an interesting point. Taking off the plaster I realised that the vents go over the level of the floorboards, by 1.5 inches. What I intend to do is put a new air vent in the existing hole that will allow much more air due to wide vents and I’ve also noticed that the existing vents are full of insect droppings /eggs. I will put in insect mesh to stop this. I picked up a airbrick and a telescopic vent from screwfix that means it should have a good flow without any interference from the insulation.

    overall the insulation and the work on the vents should have a massive improvement in comfort and really needs to done to the rest of the ground floor.

  • Registered Users Posts: 757 ✭✭✭C. Eastwood


    The sheet on top of the existing t&g floor boards would not be a VP. It would be a draught stopper to prevent draughts from the underfloor ventilation escaping up through the joints in the t&g floor boards.

    If you use PIR Insulation then I recommend having it tight between the joists at the top, but I would cut a slight splay at the sides to ensure that the sides of all the joists are well vented.

    Do you know how much space there is under the t&g floor boards.

    I would usually advise clients to remove a timber suspended floor and remove the sleeper walls which support the floor joists. These walls are built on 100 mm oversite concrete. This would depend on the clients budget and what they want. It would also depend on the height of the void from the existing finish floor down to the top surface of the existing oversite concrete.

    You could then install a concrete screed with underfloor heating pipes, on top of insulation, on top of a Damp Proof Membrane (DPM) or Radon Barrier.

    Let me know what you think.

    I have been accused on this forum of touting for work and I want to make it very clear to everyone reading my professional advice here,- that I do not want any work from anyone under and circumstances.