Insulating under a floor from beneath

I would want to establish with some certainty that was where the heat was being lost before putting in the effort. The floor is usually the place least heat is lost, unless it's bare boards with gaps allowing a draft to blow through.

Otherwise you risk putting in all the effort, and not seeing the gains you wanted.

Can you use PIR insulation board *under* the joists - that would save much cutting.

However, make sure the space under this all is ventilated (rather than the air bricks/vents opening between the joists) or you'll get damp problems.

In article , Adam Funk writes

All points have been asked and answered a hundred times before.

I've contributed many times on this subject sharing long experience, the latest being easily within the last couple of months, but I'm afraid I just can face rehashing the same old script again.

Please have a search on google groups for the pertinent discussions.

Get the special saw; it is very sharp but has no teeth. If you use an ordinary saw you get annoying bits everywhere. Also, chickens (if you have them) enthusiastically eat the little bits thinking they are food and it's not good for them.

Robert

When you look at the separation of the joists and distance down to the earth it's often not easy to get whole thick boards down there.

Robert

Ooh - that sounds interesting.

AKA: bread knife :-)

Granted - had the same problem with my roof - had to cut the board lenghtways in 2 halfs

Apart from small cuts, I found bread knives to be horrible - dangerous even.

The foam tends to grip the blade because the blade has no kerf to speak of.

I assumed the PP meant:

which does have teeth, but claims to produce less dust.

TBH the dust is not too bad if you wear a basic mask - it's heavy and in still air tends to fall quickly (unlike plaster dust) and be easily hoovered. But it's horrible if you get it in the eye - feels like sand.

I would:

- cut PIR board to fit between joists

- push into place (metal side up)

- temporarily pin with a couple of nails.

- seal gap with expanding foam from a foam gun (which will also glue the PIR in place) This point is critical. Assuming your void is ventilated (and if not, your joists will rot), most of the insulation effect comes from stopping draughts.

- lather, rinse, repeat.

The important thing is No Gaps. Slightest gap=draught defeats all your work. I used rigid foam cut 2cm smaller than the gaps & held temporarily with a few nails. Gaps filled with canned foam,

Look on the internet for "insulation+boards+seconds+locality" for a local seller. Seconds come half price. Needs a few small repairs you can do with the canned foam easy. Eg:-

Foam best full depth of joist.

Any scraps, secure to walls vertically tight against your floor with foam. Again no gaps. Reduces thermal bridge effect of heat creeping through the brickwork.

/- push into place (metal side up) /q

Metal side up????

Jim K

I'm convinced this is a worthwhile evidence, from my experience of two downstairs floors. How you do it is, however, subject to local conditions.

The first of mine was done when replacing the entire floor and was a fairly straightforward job of nailing roofing battens along the insides of the joists at the bottom then dropping tight-fitting PIR boards onto that before adding floorboards.

The second was done in situ from the low cellar under my dining room. The pipes, cables, etc here would have made PIR almost impossible to fit, except under the bottom faces of the joists and the headroom was already low. Also, access would not have allowed an 8x4 board into the space, so there would have been a lot of joints to tape over. Instead, I pushed loft insulation into the spaces and suspended it 'hammock-style' with a zig zag of nylon string between nails (which was largely unnecessary as it turned out as it has stayed put by friction). The result was pretty much the same and both these rooms went from cold to warm immediately.

I suspect that most of the benefit is due to draught-proofing/baffling rather than isulation as such. Although I tried to fill every available space, I didn't get paranoid about curing every possible draught, working on the basis that 99% of the job gives 99% of the effect and the other 1% is too much of a PITA to deal with. Undoubtedly, someone will tell you that if you don't seal the floor hermetically the entire job is a waste of time but experience says that simply isn't true, at least in my case.

In message , JimK writes

Foil covered side up I assume

Interesting point. The floors in those two rooms do feel cold when we walk on them. The kitchen is vinyl on hardboard on floorboards; the dining room is carpet on underlay on floorboards. There's currently no insulation in the floor, and the crawl spaces are ventilated (air bricks). I can't think where else the heat could be going out: the 2 windows & the back door are modern double-glazed ones. I haven't been able to feel any distinct drafts, though.

Yup. Specifically, the foil vapour barrier needs to be on the warm side of the insulation to avoid interstitial condensation.

All the celotex I've seen is foiled on both sides.

PIR is certainly available without a foil facing, although not so common. Don't know about one side only.

I'm sure the stuff I used to do the bathroom wall was only foil on one side.

It had Celotex printed on the other side, on the paper or whatever it is facing