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Suspended timber floor insulation

  • 18-05-2022 4:11pm
    Registered Users Posts: 3,964 ✭✭✭ muckwarrior

    We're in the process of buying a 1970's semi-d with suspended timber floors. There are gunbarrell pipes that need replacing so I was thinking about pulling the floors up entirely at the same time and insulating them. My original plan was to fix batons to the joists, and place rigid insulation board on top - job done. But I've since been watching youtube videos and reading threads here, and now I'm more confused than ever.

    Vapour control? Vapour Barrier? Fiberglass insulation? As far as I can tell, the risk, which you want to avoid at all costs, is condensation, which would result in dry rot?

    After reading a couple of more articles my conclusion is that the best approach might be to drape netting over the joists and use that to suspend sheep's wool insulation (although I'm not sure if that's readily available in Ireland?). Does this sound like a safe/worthwhile approach? Advice welcome.


  • Registered Users Posts: 30,752 ✭✭✭✭ Lumen

    Inexpert opinions incoming...

    Last time I researched sheeps wool it was some combination of expensive, infestation-prone and fire-risky. It's possible the latter two issues have been resolved with whatever miracle treatment is being pushed now, but I'm guessing it's still expensive.

    Where will the pipes be with respect to your planned insulation?

    I've done a couple of rockwool-and-membrane ground floors and I don't like it that much. The tricky bit is getting a consistent insulation depth, particularly near the joist. Rockwool works much better on upper floors where it can rest on a flat surface.

    For my remaining ground floors (including some awkward voids caused by my sloping site) I'm looking at pouring in foamed glass aggregate. It's not that expensive and seems idiot-proof (i.e. the results are not sensitive to the skill of the installer) and safe (i.e. breathable and non-capillary).

    edit: this looks decent

    Post edited by Lumen on

  • Registered Users Posts: 3,964 ✭✭✭ muckwarrior

    Cheers. That foam aggregate wasn't on my radar at all. It looks like it would be a bit less labour too and easily fit around any pipes or wires under the boards.

    We don't have the keys yet so haven't been able to look under the floor to see how pipes etc. are routed.

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


    You asked for advice

    Do not use sheepswool in the void under the floor.

    The best option with most of these suspended floors with through ventilation area under the floor is to remove all the timber. Block the vents. Fit hardcore if needed. Sand blinding. DPM or Radon barrier. 150 mm insulation and 150 mm concrete.

    There is no need for vapour control. The only vapour is from rising damp in the void. The through ventilation will remove this damp air.

    Fit a net over the joists, fit 100 mm fibreglass on top of the net. Refit the t&g flooring boards. This will reduce heat loss by Conduction. Do not fit a DPM/ vapour barrier under the flooring boards.

    To prevent heat loss by Convection (draughts), fit a sheet of polythene (DPM) over the flooring boards and up the walls. Mastic it on to the walls. Fit laminate flooring on top of underlay on top of the polythene. Cut polythene approx 25 mm below height of skirting boards. Fit Skirting.

    The biggest draughts on these floors is up under the skirting boards. Remove the skirting boards and apply expanded foam to fill and seal the gap between the flooring boards and the walls. Fit new skirting boards. There will be cold air coming up between the flooring boards. The sheet of polythene will prevent this.

  • Registered Users Posts: 3,964 ✭✭✭ muckwarrior

    Thanks C. Eastwood. The first option may be the best in theory, but I'm looking for the best combination of cost, effort and effectiveness. We don't have the time or money to go completely re-doing floors with concrete. I don't particularly like laminate flooring either, so would prefer to keep the existing floor boards.

    With that in mind, what's your opinion of the link that Lumen posted? Is there any disadvantage to fitting the polythene/airtight membrane below the boards?

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


    The example given by lumen is perfect. But it will be expensive and must be completed by a qualified Carpenter & Joiner.

    Also it will prevent draughts up or down between the tongue & grooved flooring boards.

    With the example shown -the wind membrane should not cause any problems of Dry Rot in the timbers if it’s done correctly and in accordance with Manufactures Instructions.

    The t&g flooring boards will get damaged when taking them up.

    Get costs on this before you start.

    Also you could get a cost on the concrete floor to make a comparison.

    You could keep the concrete finished down 20 mm and fit t&g flooring boards on top.

    Just price both options.



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  • Registered Users Posts: 555 ✭✭✭ jonnybravo

    Just to add my experience. We were looking to do either of the options below. We got it priced with 3 builders as part of an overall renovation and all said it would only be slightly cheaper (c.€1k) for option 2 and they all recommended option 1. When we went with option 1 we also put in underfloor heating (which obviously added to the cost).

    1) The best option with most of these suspended floors with through ventilation area under the floor is to remove all the timber. Block the vents. Fit hardcore if needed. Sand blinding. DPM or Radon barrier. 150 mm insulation and 150 mm concrete.

    2) Something similar to this

  • Registered Users Posts: 3,964 ✭✭✭ muckwarrior

    I'm surprised there was so little difference in the cost. That said, I'd be able to do the insulation myself whereas I wouldn't take on the job of concreting etc. Still, I might try to get a quote just in case it's worth it.

  • Registered Users Posts: 1,959 ✭✭✭ Paulzx

    Same here. I got quotes for option 1 and 2 during a renovation. Only 2500 in difference with concrete being the more expensive option. Builder also recommended the concrete option.

    I'm delighted that i got rid of the suspended floor. It also has the added advantage of removing what is a nice dry home for vermin!