Insulating a suspended timber floor

[Bert Coules]

My sixties bungalow has a suspended timber floor - t&g floorboards on wooden joists over a void some 3 or 4 feet deep with airbricks to the outside world. The floor covering is thin (and cheap) laminate laid on the usual fibre mats (laid casually, with no bonding or sealing between the sheets).

Am I right in thinking that I'm probably losing a lot of heat through this floor? Compared to the cavity walls and their blown insulation, the uPVC double-glazed windows, and the loft with its 6 inches of fibreglass (or whatever it is) the floor must surely be leaking heat like an open door (or almost). Certainly it always feels extremely cold to the touch.

If that is the case, what's the best way of stopping it? Celotex slabs jammed between the joists, with the edges filled with foam? Some sort of loose-lay material? (But held in place how?) Or something else entirely?

The laminate is coming to the end of its useful existence, so this is the time to do it. A Google search turns up quite a few specialist companies, but unless the best method is something hugely complex that I've not thought of, this sounds like a feasible DIY job.

Many thanks for any thoughts.

[Bob Minchin]

Nail some cheap thin ply to the underside of the floor joists and ensure there is plenty of ventilation underneath. When you take up the laminate and boards fill the voids with you choice of insulation. Fibreglass is almost free via local green energy initiatives or you can buy reject foam (just with damaged corners) etc. Whilst you are at it you might want to consider under floor heating??

[fred]

In article , Bert Coules writes

Although the most expensive, this really is the easiest way. Contrary to common advice here I would cut the slabs a tad undersize (5mm or so) and hold them in place with a nail or 2 below into the side of the joist until you can use the obligatory builders' foam to seal the gap and permanently fix the slabs in place. The reason for the small gap is to make sure there is somewhere for the foam to go and therefore ensure a seal. Use a foam gun for controllability. 100mm PIR, foil faced sheets, buying from seconds supplier.

That's how mine was done, fibreglass held in place with hardboard and edge on battens nailed into the side of the joist. Verdict, effective but a total PITA, don't do it.

[Tim Watts]

On Wednesday 16 October 2013 20:14 Bert Coules wrote in uk.d-i-y:

That's probably the best idea. MAke sure you leave air flow under the celotex to avoid damp.

This both gives you isulation and lack of draughts which will make everything a *whole* lot better.

Can be done if supported on chicken wire - but that may be as tricky as installing celotex.

Another method is to bang a few 1" nails in along a horizontal line down the length of each joist, lay undersized strips of celotex on (5mm gap each side) then foam in. This will be less fiddly and the foam guarantees an air tight joint.

[GB]

I'm going to have to do this sooner or later, but I was thinking of working from underneath, as we have parquet flooring, although currently carpeted. It's a total PITA job, which is why I've been putting it off ofc.

[Jeremy Nicoll - news posts]

Every time I read something like this I think about what the underneath ofmy floors look like: there's all sorts of pipework, conduit, alarm & phone cables etc strung from 'room' to 'room'; some of it crosses from joist to joist so is - possibly - below the point where one would try to insert celotex, but quite a lot of stuff runs parallel to joists but above their lowest points. I think the whole job would be trickier than it sounds, especially if one wanted to maintain access to pipes & cables that would otherwise get sealed into inter-joist gaps.

[Geoff Pearson]

The Energy Savings Trust, under contract to Edinburgh City Council, insulated under my floors for free in April this year (90 sq m). Many rolls of fibreglass went in and were suspended between the joist bys netting. We think we have already noticed a benefit - and it was free. Ask the Trust.

[harryagain]

You will losing lots of heat and there will be draughts too. You can use celotex between the joists working from underneath. In practice it is near impossible to get a good fit, the stuff is quite brittle too. Cut the celotex about 1" smaller than the required gap in the joists.. Support in position with a few nails and fill the gap with aerosol fixing foam. (Messy job) It's important that the job is completely airtight or all your work is wasted.. If you have any over, stick it to the walls abutting the horizontal insulation, it reduces thermal bridging, ie heat tracking down the brickwork from above. Go round all the skirting/floor joint and seal off either with the fixing foam or silicon.

BTW, you can get "seconds" in this rigid board insulation for half the price. Available everywhere. Slight malforms/voids and mechanical damages, easily fixed with the fixing foam.. Google "seconds rigid board insulation" I have 200mm of foam under my house. (Two layers)

[stuart noble]

You can get the perfect fit between joists, however irregular, by running a handsaw against the joist edge and cutting the celotex at the same time i.e. "feeding" it in gradually. No gaps and no need for any fixings. Another pair of hands is useful when handling a full sheet but it gets easier as the sheet gets smaller

[Bert Coules]

That's > I think about what the underneath of my

Yes, I've wondered about that. What if the Celotex were fixed in place but set say 1" below the top of the joists, to allow for cabling and so on? Would the effectiveness of the insulation be compromised if it wasn't right up against the underside of the floorboards? (Apart from because it might have to be thinner?)

Geoff Pearson mentioned:

Thanks for that: I'll check them out.

[Tim Watts]

On Thursday 17 October 2013 11:45 Bert Coules wrote in uk.d-i-y:

No I didn't... Is this a victim of crap quoting some levels up?

[Bert Coules]

Tim Watts really did write:

No, it was the consequence of my own incompetence. Apologies to you, and to Jeremy Nicholl, whom I actually was quoting.

[fred]

In article , Bert Coules writes

If there is significant air movement above the insulation then much of its benefit will be lost. If you do this yourself then you can plan and block draughts passing over the top but if you get somebody in to do this then you can never be sure it will be done right and you could have spent all that money on insulation for nothing. Don't underestimate how little a top side draught could cut the effectiveness of your insulation in half.

Personally I wouldn't do any cutting under the floor at all, I would survey the joist spacings to see how much they vary and either cut all to one size (very slightly undersize - 5-10mm overall) (very quick) or cut them to the map that you have surveyed. This will require you to get over your preference not to use foam as a fill-in[1] as it will be required to do the job right. This will be particularly important if you do level a gap above the boards as you'll need to add a little above at joints and definitely at the ends to avoid mini wind tunnels.

[1] I think once you have seen how controllable a proper foam gun (under 20quid) can be, you will see the light (I may even buy you one and send it down to remove further prevarication ;-)

[Bert Coules]

I didn't know that; thanks.

Actually, I have no such preference. It's true that I haven't had much success with foam (in other applications) in the past, but that was always using the one-off applicator cans; I've been told several times that the guns are better.

A most generous offer, thank you. Shall I send you my address?

[Bert Coules]

I just spoke to them and sadly there are no current grants available for this.

Curiously, the chap on the phone had considerable difficulty in grasping what I was talking about. He persisted in assuming that I wanted to install underfloor heating - and he wasn't too keen on that. "Our official stance is that the savings are not worth the expense of putting it in".

[Tim Watts]

On Thursday 17 October 2013 12:41 Bert Coules wrote in uk.d-i-y:

No worries - not like it made me look stupid - I do that without any help :)

[stuart noble]

And, given that heat rises, perhaps that stance makes sense

[Bert Coules]

I'm sorry, I'm not with you. If the heating is under the floor, surely rising is exactly what you'd want it to do? How does that make it not worth the expense?

[Martin Bonner]

Harry has a bit of a reputation on this NG, but on this occasion he's right. We insulated under our suspended floor and were very disappointed when the new T&G was down at how cold the result was; then the gaps round the edge were filled, and skirting boards fitted and it was toasty warm.

It really *is* the case that you need to fix all the drafts. Also, be warned that filling 90% of the holes does *not* fix 90% of the drafts.

[Bert Coules]

Right, thanks for confirming that.