Insulating a floor

[GMM]

Just reading the thread on the insulating effect of curtains has prompted me to post a question I've been mulling for a while.

For good reasons, though contrary to the high principles of this group, I had our living room floor replaced by a builder last summer. new joists, new T&G boards and 75mm of celotex. This has made a significant difference to the temperature in the room, clearly down to the celotex (although we did replace the radiators too).

Now, the dining room would benefit from some insulation too. It's over the cellar, so quite accessible from below. The question is, what thickness of celotex is best?

The 75mm in the living room was calculated by the regulations (although no BCO etc was involved). Talking recently to another builder (socially) who specialises in insulation, he reckoned 100mm would do a better job.

On the other hand, it seems to me that much of the advantage is from excluding draughts so, in principle, thinner celotex (say 50mm), well cut to meet the joists closely, would do most of the job. (Being over the cellar, the temperature below the floor isn't as low as under the living room.) Anything too thin would probably sag over time.

Of course cost varies with thickness so there must be a sweet spot. If we can agree that the regulations are irrelevant (except for guidance), is it a good or bad use of cash to use the thicker stuff?

[Jules Richardson]

Doesn't that make it all uneven and lumpy?

[meow2222]

Each extra inch gives less return. The only way to answer your question, other than by a wild guess, is to calculate the cost of heat loss at each contemplated thickeness and compare to the extra cost of adding it.

If you're putting it between the joists, once you get to 2 or 3" of celotex, most of the heat loss is through the joists, so more between them doesn't achieve much. If you can also put some under all the joists, you can get much lower heat loss.

NT

[Owain]

But somewhere to park the bicycle ...

Owain

[harryagain]

There's no such thing as too much insulation. Even if not strictly viable now, it soon will be. I have 8"/200mm below my floors.

To fit between joists, cut an inch or so undersize, put into position supported with nails and fill the gaps with the aerosol canned fixing foam. (Cheapest place I know is Screwfix). Cut with handsaw.

You need to make sure there are no gaps. Any little air leak negates the whole exercise.

You can also stick insulation on to the walls immediately below your horizontal insulation. This reduces the thermal bridging in the walls, (ie bypassing your floor insulation.) Good use for any offcuts.

[harryagain]

BTW Google:- "insulation board seconds" to get insulation half price. This has slight damages/voids easily repaired with the canned foam.

There will be a local place round you flogging it.

[Tim Watts]

Try ebay too - there are good priced for even non seconds. Do not buy at B&Q or a builders merchant - unless you like haggling :)

[GMM]

Well I just like to get my own back for what he charged me by walking over him each evening....

[GMM]

Sounds advice Mr W. I'm not sure what B&Q are for really, apart from something you need on a Sunday afternoon (my local BM is open Sunday mornings). The BM isn't too bad for insulation board (not celotex but simialr) - about 2/3 the price of Wickes, and much the same as many online sources, and they deliver anything over £100 for free. Will have a look at ebay/seconds too of course.

[Tim Watts]

They are good when you want sheet wood cuting into lots of rectangles with random sizes but parallel edges :)

give you some ball park pricing (for various quantities) so you know if a particular price is sensible or not.

[GMM]

than by a wild guess, is to calculate the cost of heat loss at each contemplated thickeness and compare to the extra cost of adding it.

most of the heat loss is through the joists, so more between them doesn't achieve much. If you can also put some under all the joists, you can get much lower heat loss.

That's very much my feeling as the gain in benefit is with the reciprocal of the thickness and, as I said, I suspect that lot of the advantage is from reducing draughts. I think adding any below the joists would lower the ceiling too much as I (6') have to duck between them as it is. If that weren't the case, and I could get whole sheets down there, I would serously think of just putting them on the bottom face of the joists, like you might with plasterboard. I suspect it would be pretty dented after a short time though, with impressions of the top of me 'ed!

[GMM]

and carefully cut to fractionally more than the gap between the joists so the celotex was a very tight fit. Of course, the outcome is the same.

Not sure I can see the purpose in going beyond about 100mm of PIR though. I might if it was free of course...

[GMM]

> will give you some ball park pricing (for various quantities) so you know if

Indeed - that's the last time I bought anything there: A sheet of ply cut down to spec. They do also sell spare nozzles for expanding foam (sometimes) so I stand corrected!

I did take a look at ebay earlier: Actually, the going rate there for an 8x4 of 50mm is a shade over £20. At my local BM (Selco), Ecotherm that size was around £16 + VAT (a shade under £20) last time I looked with free delivery when you spend over £100 (I need 7 sheets, so no problem).

Some of the seconds deals look not half bad though, if I'm reading them right.

[Man at B&Q]

, other than by a wild guess, is to calculate the cost of heat loss at each= contemplated thickeness and compare to the extra cost of adding it.

otex, most of the heat loss is through the joists, so more between them doe= sn't achieve much. If you can also put some under all the joists, you can g= et much lower heat loss.

If you forget, Cellotex is softer than a joist :-)

MBQ

[meow2222]

If you can spare 9mm of headroom, you could pack the spaces with fibreglass and fit 9mm PB. Much cheaper than PIR foam. You'd need to do the calcs to see which option works out better - fibreglass needs more depth for the same effect.

NT

[The Other Mike]

and a complete void between your ears

Spray foam isn't a particularly good insulator and isn't a substitute for accurate cutting

[The Natural Philosopher]

actually it is a very good insulator. Not quite celotex, but its pretty darned good.

[Martin Bonner]

Of course there is. Apart from anything else, insulation consumes energy in its manufacture and installation. An upper bound for "too much" is when the embedded energy of the insulation exceeds the energy saved (over the lifetime of the insulation).

A more realistic bound is probably given by the point when the extra money involved in buying the insulation would be more effectively spent on some other sustainability project.

(Oh, and if you are inclined to dismiss me as a "we don't need to worry about global warming" nutter - I'm on the committee of the local Green Party.)

Well it depends how much you put on.

If that is 8" of wool/rock-wool/wood-fibre, then that's a lot, but not ridiculous. If it's PU foam, then I think you could have spent the money more effectively. If it's 8" of PU foam *between* the joists, then you have definitely wasted your money (as another poster has said, most of the heat will be leaking through the joists).

To the OP: My feeling is that given the installation cost is (roughly) independent of the thickness, it is usually better to over- rather than under-insulate. On the other hand, if you are insulating over a cellar, it isn't nearly so difficult to go back and add some more if you under do it. So I think

50mm ought to be enough.

Also: Don't skimp on the air-tightness. When we put insulation under our floor, it made very little difference. Then the installer sealed round the edge and installed the skirting; suddenly the room was massively warmer.

[GMM]

Wise words Mr B. My impression (which is really where I started) is that draught prevention is easily as important as the absolute level of insulation.

I guess my installation cost (in folding money terms, though not in time) is zero as I'll be fitting. certainly going back for another shot would not be impossible, though a tad inconvenient once everything is well sealed. Maybe the trick is to mount it high enough (and the joists are quite deep)to just put another layer directly beneath.