Insulating under suspended wooden floor

[Martin Pentreath]

Hi all,

I'm finally getting round to a long-postponed project - insulating under the living room floor.

It's a standard suspended timber floor on the ground floor of a victorian terrace - two rooms knocked together. The boards a bare, sanded, varnished, gappy, drafty, with a few rugs around. Under the floor there is a void about two feet deep before you hit the soil. There certainly seems to be adequate underfloor ventilation thanks to airbricks, hence the fact that the room above is like a field in Alaska.

I'm having some kingspan delivered tomorrow from the seconds place. The plan is to wriggle underneath and wedge carefully cut strips of kingspan in between the joists. Apart from increasing the themal properties of the floor, draft-proofing is a major objective. So I'm planning on using some expanding foam around the edges (applied from the underside obviously). As to the gaps between the boards themselves the quickest and easiest solution would be to use some very adhesive tape, again applied from the underside. Any comments on this? I was thinking of using something like this:

Martin

[David WE Roberts]

If you have sealed between the joists under the floorboards, do you need to do more?

[robgraham]

I should really have done this rather a lot of years ago when I had to clean out under our sitting room floor. Much the same sort of gap. I suppose I could employ my son to do it, but the idea of going under there now at 70 is just not realistic.

Enjoy !

Rob

[mike]

I insulated the roof of a single story extension from underneath with Kingspan between joists. Cut with a saw as accurately as possible, two

50mm layers, covered the joints between boards with duck tape. Tapped thin wire nails into the sides of the joists to stop the Kingspan slipping out. Some was such a tight fit that it wasn't a problem, other bits with a warped joist and a slightly wonky cut line needed it. Didn't bother trying to fill any edges between joists and boards as I thought it was 95% there and considerably better than what was there before.

Don't forget to wear goggles. Kingspan dust in your eyes feels like sandpaper.

[stuart noble]

If you position the insulation over the joist gap, with one edge against the joist, you can run a saw against the other joist so that it's always a tight fit. Of course I only discovered this after much measuring and needless faffing about

[The Natural Philosopher]

you sill need to caulk or foam or tape the gaps.

[fred]

In article , Martin Pentreath writes

so doing those too isn't much of an overhead and you're then guaranteed that it is all airtight.

As another has stated, it's all to easy to negate insulation with cold airflow leaking to the warm side.

Consider how you're going to make the transitions between floor and walls/dwarf walls to avoid a bridge for damp and avoid removing essential from external lath and plaster walls.

[fred]

In article , fred writes

Sorry, should read . . . and avoid removing essential _ventilation_ from external lath and plaster walls.

[Martin Pentreath]

OK, thanks for all replies so far, especially from those with a bit of previous.

My original intention only involved using foam to seal gaps around the edge of the floor (from underneath). I had thought that taping the gaps between floorboards would provide enough draft proofing there, and then the kingspan would just need to be wedged and pinned between joists to improve the thermal properties without the need for more foam. Would this approach be inadequate then?

And if so, could someone explain a bit more clearly about using foam in conjunction with the kingspan between the joists? Am I trying to fill the void between the upper side of the kingspan and the underside of the floorboards?Or just the gap between the edge of the kingspan and the joists which it's sitting between?

I'm really not much looking forward to a couple of days of potholing under my floor, but if I'm going down there I want to get it done properly.

Cheers!

Martin

[The Natural Philosopher]

The latter. you dont want air bypassing the kingspan.

No pint in living in a insulated house if you leave the windows open....

yeah..

[fred]

In article , Martin Pentreath writes

You're not looking to seal the gap between the celotex and the floorboards boards, it's just not feasible, but you must ensure that air cannot pass around the edge of the celotex to under the boards, negating the insulation.

You are looking to:

1. Minimise the gap above the celotex (push up and wedge in place - cut wooden wedges from batters, no need to nail as foam will fix it later)).
2. Seal all round the celotex boards (board to joist and board to board) with foam so there is no way for cold air to pass the board to the warm side.
3. At the end of a joist run you also need to seal the gap above the board at the end (put a little foam on the top and side of the board at the end and push it up into place, then wedge and foam around).

If you do this correctly then the insulation (plus joists) forms an air tight barrier and you do not need to seal the gaps between the floorboards.

I wouldn't use duct tape for any part of this, you need to pay decent money to get long lasting sticky stuff and adhesion to old/dirty/rough wood is not guaranteed long term.

If you buy a case of gun grade foam and a 15quid dispensing gun you will be able to apply well controlled beads of foam as described above, it makes the job really easy (quicker and easier than tape). As others have said you actually want the celotex slightly undersize (5-10mm clear all round) to make foaming easier.

[Martin Pentreath]

OK fantastic, thanks guys, as always just as well I asked in here before going potholing.

As luck would have it Screwfix is offering their foam gun at clearance price:

you're saying a case of foam cans?? The room is a double one (through lounge, as we used to say in 1978), floor area around 30 sq m. I've never used foam before so I've no idea how far a 750ml can goes, but do reckon I'll need 12 cans? Martin

[fred]

In article , Martin Pentreath writes

Well, that depends how big a gap you leave :-).

My guess, based on recently using approx half a can on filling a 5/6mm gap over 12 linear metres is as follows:

Guessing that your space is approx 8x4m with joists on the short side at

600mm centres we have 13 spans of 4m giving 104 linear metres of joists, 2 seals per joist so 208 linear metres of gap fill plus a bit for the butted ends between the sheets. Your 220m /my 12m x 0.5 can = 9 cans.

The bulk buy of a case saves a bit over singles so I'd go for that.

In my case I was using 2" celotex and looking to get a deep fill up the side which was easy with the nozzle of the pro gun.

The gun deal looks good, don't forget a can of gun cleaner.

[thirty-six]

Why not horse hair and linseed putty?

[thirty-six]

I don't own any sheep, yet I still wear a wool pullover.

[Steve Walker]

this:

>>> Cheers!

I am in my forties, but have a very dodgy and painful knee; the gap between the ground and the joists is also narrower than my shoulders, so I can't turn over in most places; hence I phone my dad (aged 75) to get under the floor! He is fit and well though.

SteveW

[thirty-six]

My house is 1930s and I had to get between the joists to breathe when I intalled the downstairs wiring and plumbing. I managed to get a splinter in my arse for my troubles to maintain the floors's excellent condition.

[mike]

What theories are they, Harry? I didn't mention U values or suggest that Kingspan could be made to fit exactly by cutting. I said that "something that friction fits would be more suitable".

I appreciate that, on Planet Harry, that equates to something you have to cut to the wrong size and then gob up with foam.

You're certainly talking something, Harry, but I don't think the word is "practical".

[george [dicegeorge]]

Re: Insulating under suspended wooden floor

putting 6mm plastic sheets on the earth under the suspended floor, perhaps to stop moisture coming up from the ground?

[g]

[thirty-six]

No, that's 6 mil. 0.006" or about 0.15mm