Insulation for suspended flooring

Sat 31 Jul 2021, 11:43

Thanks for the advice and for some fascinating insights into the complexities of insulation and dry rot.  We have had to replace joists elsewhere in the house, largely due to air bricks being covered over or blocked.  Hopefully, though the room in question is well ventilated below floor level and has escaped a similar fate.  I have found a chap in Milton Keynes who specialises in minimally invasive under-floor insulation.  I'll keep you posted on the outcome.

Fri 30 Jul 2021, 22:22

Not to above, underneath! It's OK to put a damp proof membrane on top to prevent spills soaking in.

Co-operators renovating, with ridiculously low money budgets, many old street houses, had a lot of experience in the self-build short-life housing coop movement in the 80's. Rescuing and trying to insulate…

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Fri 30 Jul 2021, 13:44

I can't see any reason why you would need air flow from under to above the floor boards.

We lifted three boards, left five in situ, lifted three boards etc. In my case the void was quite deep, about 1m (and there was the remains of a well), making it quite easy to stand in there and see where to slide in the celotex. Also, my boards are probably easier to lift, as they are not tongue and groove, instead there is an inch wide metal band between the boards and in a groove.

Thu 29 Jul 2021, 21:23

Thanks, both.  There is a question as to whether there needs to be air flow from under to above the floor boards.  Elsewhere in the house, I have used celotex but I don't want lift all the boards - they are original.

Tue 27 Jul 2021, 07:43

eek, Hans.... cellotex foam plastic rigid board doesn't breath, does it(?); wouldn't really want to use solid board myself...

Mon 26 Jul 2021, 19:11

I has this done, Mark Mills helped me https://www.mmillsplastering.co.uk/. House from about 1830. Lifted three boards, left five in situ. Sized 100mm celotex boards carefully and pushed them in between the joists for a snug fit. Taped up all joins with aluminium tape. Floor now 10 degrees+ C warmer. Transformative.

Mon 26 Jul 2021, 16:33

What, exactly, do you mean by suspended? Joists hanging off the wallplates with a void beneath, possibly with small pillars underneath to support midway?

If floorboards or stout sheet material that can be removed, in a number of houses over the last 45 years I've stripped off the boards almost entirely, stapled stout plastic netting to the bottom of the joists, and shoved 4", 6" or 7" mineral wool on top of the netting.

Open underneath - make sure you have vents below the floor level on a ground floor- to breath.

A thick, rotproof sheet of say damp proof course plastic on top of the mineral wool to stop mineral dust working its way up and out is optional.

I know some of the houses with relatives moved into after doing this had no problems over 20 years, with toasty warm barefoot friendly stripped wood florboards on the ground floor so there is no damp/condensation problem with the plastic sheet over option, which I prefer.

Yes it's grunt work to lift most of the boards and put them down... but worth it. Cheap unskilled labour should suffice for this job, if watched carefully for "quality control"...

I recommend screwing the boards down as this cure creaks and makes for easy access to wires. If you don't like the look of crosshead screws there is a Canadian screw type with a small square socket on the top that looks neater. It is also easier to screw in properly and doesn't suffer from being "turned out" like Pozidrive or Philips type cross-head screws.

I think its called something like the "Robinson" head??? I've forgotten. I managed to get a pile some years ago and they were much easier to use - and undo if needed - than the usual cross-head types (but have run out of them now). Check them out.

WHICH REMINDS ME - make sure that where possible electric wires are shoved UNDERNEATH the insulation. If trapped between the top layer and the insulation a loaded wire can get surprisingly hot!

Fri 23 Jul 2021, 19:11

Hi All

I'm wondering if anyone has had their suspended timber floor insulated and can recommend a specialist?  

Many thanks

James