Sealing gaps around under floor insulation and celotex/kingspan/cheaper alternative?

[David Robinson]

I'm thinking about adding insulation under our suspended timber floors (floor boards).

I reckon, with effort, there's enough space beneath (maybe 2-3ft) to lift a few boards and crawl around underneath to push celotex/kingspan- like material between the joists. I think this will be easier than lifting all the boards and doing it from on top, though I could be wrong.

Three questions...

1. Worth doing? We're having laminate flooring and I'm hoping insulation will help it feel less cold.
2. Source for cheaper version of celotex/kingspan-like material? 50mm depth. Local merchants has 8' * 4' at ~=A315+VAT. Anyone know better / cheaper?
3. I intend to hold it up with some nails, or maybe no-nails? I think battens would be fiddly. What can I seal the inevitable small gaps up with? I reckon expanding foam might be too messy in the cramped conditions. I'm thinking something like chunky-applied sealant or whatever, but don't know what! Something that goes on with the consistency of filler or tile cement or something, fills the gap and helps hold the boards up.

Any advice on this topic gratefully received. e.g. am I mad even contemplating this?!

Cheers, David.

[dom]

You're making the celotex thing too complicated.

Get under the floor, and measure the spacing between each pair of joists.

Slice up the celotex to this width, and push-fit it into place between the joists - you'll soon learn how to cut it to get that push fit.

Use foil tape between the celotex and the joist (i.e a strip of tape roughly bridging the right-angles corner) , and between successive pieces of celotex between a single joist.

I bought some of my celotex from an ebay supplier, but my builders merchant (Ridgeons) was able to match the price.

[george [dicegeorge]]

If there's laminate flooring on top then that is a vapour seal, isnt it? so there's no need to seal against condensation so you dont have to fill every gap (if my understandng is correct)

However there are foams like pinkgrip dryfix which only expand a little bit not ten times like the common or garden expanding foams.

Maybe you could wear a fullface motorbike helmet when crawling underneath to stop foam falling in your face!

[g]

[David Robinson]

It'll stick OK to the joists then? If so, that should be fine.

I'll try price matching locally - it'll make it easier to go back for more!

I was hoping that it (or maybe some kind of foil underlayer?) would be good enough in this respect, yes.

It's the draught I'm trying to keep out. It's _very_ draughty under the house (which is fine and good of course), but if these draught can get around or above the celotex, then it'll make it pointless.

I'm worried the "push fit" might work lose eventually, though I suppose that depends how tight I make it. I'm thinking the combination of the winds (like last night!), people walking around, and five years of gravity might make it drop.

Cheers, David.

[Martin Bonner]

Absolutely. We lifted our floor and fitted celotex, and laid a new floor. Not much difference to the warmth of the house. We then sealed round the edge of the room. Suddenly it was lovely and warm!

Having said that, if you've got laminate flooring, the actual floor will be pretty draught-tight. You only need to worry about round the edges. For that, I would tend to go for expanding foam and be certain.

[The Natural Philosopher]

been there done that.

If you are layng carpet, hardboard tacked over the boards is relatively non destructive and seals all gaps. Unless you realy go to town and seal the celeotex, this is needful.

I had some caos here in the new build..due to wood shrinkage. Icy draughts were coming in via the general connection to the loft space. Sealing those with caulk really mode the rooms warm again.

One thing I learnt buildng power electroncs; the heat loss from a fan blown heatsink its 5-20 times better than the sim0ply vonective and radiative losses in still air.

draughts make insulation WORTHLESS in any wind.

airtight is what you need.

[js.b1]

If any thermal looping can occur, the insulation is merely "warehoused" versus effective.

The acrylic adhesive foil tapes, Celotex included, will not stick well to wood particularly if tannalised. Leave it overnight and the tape will lift, with rough sawn tannalised it will lift before your eyes. Just gunge in a thick seam of sealant if you have miss-cut - and cut so the insulation is a tight fit (use a board and thump the stuff into place).

Thermal looping is a big problem with insulation, attention to detail really does matter - something you are not likely to find with new- build.

[David Robinson]

Just standard silicone sealant?

Yes, I have this theory that many of these houses and extensions built to modern insulation regs do have the right _amount_ of insulation, but there's a gale blowing just behind the dot-n-dab plasterboard (and just under the chipboard) which makes all the insulation in the cavity (and between the joists) completely useless.

Shame because, done properly (i.e. almost no draughts, no thermal bridging), I reckon the insulation demanded by current regs can create a house that needs very little heating.

Cheers, David.

[js.b1]

Whatever says it will bond to metal (foil) and wood.

I did my mother's "box room" because it has 2 outside walls, subject to prevailing wind, over a large porch. One wall 30mm Marmox Keraflex'd & 3mm plaster skim (=3D20mm Celotex). Other wall 40mm Celotex (20mm between battens, 20mm over).

The difference was hilarious. It is quieter, it is warm with your own body heat reflecting it back. Just one point, do not try to plaster on a 16oC day, because the insulation will mean your own hot body heat flashes the plaster dry faster than you can work.

Marmox took a few hours - butter toast & stick to wall, PU sealant joints, wait a few days and one hour for a flash plaster skim. Celotex took forever, cut tannalised battens, treat cut-ends, drill stainless screws & plugs, level with spacers, expanding foam behind battens to prevent thermal looping & solidify, celotex cut & push in, foil taped joints, sealant all joints, celotex over top, foil tape joints, expanding foam around edge & cut back, plasterboard cut & screwed over top, taped & filled joints, plaster skim over top.

Despite Marmox being much more expensive than Celotex, by the time you have added the "extras" that Celotex needs you end up at a similar price. This is obviously not true for timber frame, all the studwork etc is "free", just stuff it in.