Insulating a suspended floor

[andrewpreece]

I insulated my dining room floor from underneath, so rather more difficult than your situation, but I used rockwool and held it in place with criss-crossed galvanised wires wrapped around nails I bashed into the underside of the joists. It worked, but it's not a method I would whole-heartedly recommend. For one thing you need to wear a hat, goggles, gloves, boiler suit and neckerchief to stop the evil rockwool falling into your face and down your neck. Also if you don't take care of the rockwool it disintegrates and falls through the wires anyway. The garden netting suggestion seems an improvement. You have the advantage of being able to work from above so rockwool is not such a pain. I've never used cellotex, so I can't comment on it.

Andy.

[Peter Andrews]

I shall shortly be replacing the entire floor of a 2.2 x 3.5m kitchen, with chipboard, due to some (old) water damage. It is a conventional suspended floor on 2 x 8 joists. While its all up it seems sensible to install some insulation, but what and how?

Peter

[The Natural Philosopher]

celotex pressed between the joists and all joints taped over with the Al tape.

[Set Square]

What stops it falling out?

[MikeS]

MikeS

[Mike]

The potential energy you have stored up in the foam by pressing it between the joists. Hint : The amount required is >1 woman strength. We did it from below once and as I couldn't fit below my wife did it. But she wasn't strong enough to compress the Celotex as much as I normally do for roofs and a few have fallen out.

[Chris and Patsy]

Peter

I did this by fitting Celotex (40mm) between the joists as others have suggested but banged nails along each side of the joist to support the Celotex. I think I used 2" wire nails hammered half way in spaced every foot or so and at a height to leave a 1-2" gap at the top, my joists where only

4" high.

The most difficult part was running the central heating pipes for three radiators, it was a much larger room than your kitchen. I ran them under the Celotex when running across the joists and on top when running in-line with the joists. The pipes were well insulated when under the insulation and partially insulated when above to avoid warm patches on the floor.

I am very pleased with the results, the room is definitely cosier than it was, well worth the effort and additional cost (I was replacing the floor anyway).

I plan to repeat the whole exercise for two more rooms sometime in the future (plenty of other things to finish at the moment !).

Chris

[Dave]

I am trying to learn something here, so don't jump to any conclusions just yet :-)

I can understand insulation above a room, as heat rises, but what benefit do you get from insulating below a floor?

Dave

[The Natural Philosopher]

friction and the tape.

[The Natural Philosopher]

heat doesn't rise.

Hot air does.

heat is conducted way through floors to cold ground or even colder unederfloor spaces.

A suspended floor over a ventilated space can put no more than 1/2" of wood between you and a sub zero arctic gale.

A LOT of heat loss.

[Roger]

The message from "Dave" contains these words:

Heat is transmitted by conduction and radiation as well as convection. Hot air rises but the other 2 just go in the direction of cold.

[Christian McArdle]

I wouldn't even consider chipboard in a kitchen. It has considerable disadvantages. Firstly, you can lift boards to access services (of which there are a multitude in a kitchen). Secondly, get them wet and they'll disintegrate. Kitchens floors WILL get wet at some point.

Use real wood floorboards, I'd say. If you're tiling over, so the lifting won't be possible anyway, and you want a flat surface for easier tiling, then use plywood sheet, which will be several orders of magnitude more resistant to the inevitable water damage.

Christian.

[Rob Morley]

What about moisture-resistant chipboard?

[Christian McArdle]

You mean like a hacksaw resistant padlock or a stain resistant carpet?

Not that the ideas don't have merit, it is just the execution doesn't always live up to the reality.

Obviously, if you must have chipboard in a kitchen, it absolutely must be moisture resistant. It'll still look like reconstituted witch's sick after the second flood, even if it means it does survive the first.

I see no reason to favour it over plywood except for cost. The ply will still be a small fraction of the kitchen cost and its absence will be regretted when several thousand pounds worth of stone tiling work has to be pulled up to repair the rotten chipboard.

Christian.

[Rob Morley]

True :-)

[Dave]

Thanks Roger and TNP for getting me up to speed on this point.

I have lived with solid floors all my life, hence my question about the insulation on suspended flooring.

This will explain why No. one son's floors have so much celotex (sp?) under them then. (Thurso, North Scotland)

Can I do anything with my solid floor that is bitumen based, so as to reduce the heat loss? Cos I am in the process of minimising all heat loss from my house as I am not too far from retirement age. Just put up another 4 inches (100mm for the eurocrats ;) of loft insulation and it is paying off already.

Dave

[Mike]

It is possible to lay Celotex against the outside of the foundations but I can't believe the effort of excavating and back-filling would be worth the benefit. Try yet more loft insulation.

[The Natural Philosopher]

I thnk its fine, especially if tiled or vinyled over. Ive got a bathroom like trhat - its fine.

[The Natural Philosopher]

On solid floors, yu at least have a huge lump of soil to go through to get to the cold air - and in winter the air is gerenerally colder than the soil.

Building regs like to have insulation in even solid floors. The normal way to do this is to use about 3-4" of screed over 1-2" of BLUE or PINK polystyrene. The higher density stuff.

Mostly, unless you have high ceilings and doors, this means jack hammering up whats there, re surfacing, DPM ing and relaying the insulation and screed., BIG job.

Smaller amounts of e.g. cork tiles under vinyl or under a capret and decent carpe insulation are the quicker and easier option, but noweher near as good.

However if you ARE redoing a floor, consider teh optiojn of ulling up tey solid and relaying with insulation.

Its the fouryh most important heatloss area in old houses.

The worst is draughts. Then the upstairs ceilings Then the walls Then the ground floors.

Glazing and so on is minimal by comparison, particularly if lined curtains are used.

[The Natural Philosopher]

It can be. I haven;t got te figures to hand, but once teh loft goes below a U value of 0.3 or so, then a floor at U value 1 or so, becomes more dominant. Same area. So if you double loft nsulation and go ato U vale of say 0.15 on the roof, you are gaining an effective .15 U times floor plan are.

To go from 1 to 0.85 on the floor, might be no more than decent carpet underlay..to gain you as much.

Celotex outside the foundations does (almost)nowt. The dominant path is not by the footings, its via the soil under the footings. If you have seen polystyrene laid alongsie footings, chances are its there for the same reasons I have it. To absorb soil heave.