Under floor insulation yet again

I've posted a couple of times before with speculation on how to insulate under my suspended timber ground floor. Now I've finally taken up half the floorboards in the room I'm starting on (the rest will follow tomorrow). I have 4 x 2 joists crossing the room, supported mid way by a block dwarf wall. There's about 8 inches under the bottom of the joists, above concrete.

I'd planned to suspend space blanket between the joists, on netting, but it's looking a lot simpler to fix sheets of 50mm Celotex under the joists. I have a few air bricks on the two external walls, which are level with the adjacent joists and will need ducting to the underfloor space ( which I could do with a bit of Celotex cratfing).

It seems to be that the masonry in the void will still be vented, and the joists are unlikely to suffer damp from below, due to the foil on the Celotex, nor condensation from moist air above, due to being on the warm side of the insulation.

Can anyone think of anything wrong with this plan?

There is a small prize for the first person to say "notifiable work".

Cheers Richard

Reply to
geraldthehamster
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There's this...

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you don't fancy home-brew. You could even lay fibrous stuff in the "tray" formed by the film.

Reply to
Skipweasel

That goes over the joists. Fixing Celotex underneath has the added advantage of giving me a space between it and the floorboards for the existing wiring and central heating pipework.

Cheers Richard

Reply to
geraldthehamster

How are you going to get bits of 50mm Celotext under the joists? It doesn't bend very much... How do you intend to fix it to the joists, particularly the last bit?

Netting and space blanket seems a lot easier and won't leave a void directly under the boards through which drafts can blow taking away heat or possibly cooling the lower timbers below the dewpoint of the warm moist air from above.

Reply to
Dave Liquorice

geraldthehamster ( snipped-for-privacy@hotmail.co.uk) wibbled on Saturday 08 January 2011

22:23:

Sod the notification - there, said it...

Now - I see what you are doing, and it seems fine, damp wise. But how are you going to get 8x4' sheets (or any sheet size) *under* the joists? Are you hoping it will bend enough to go through a 2' gap with 8" of depth below? I don't know - never tried but it seems tight...

Or are you planning to use smaller sheets?

Great if you can - you'll get tight butt joints which would be worth taping the top side to reduce draughts (a small draught into the floorboard/celotex void will quickly ruin your efforts).

I assume you plan to screw them up into the bottoms of the joists using hand reach?

A bit more elaboration would be useful here...

Reply to
Tim Watts

All easily available insulation works by trapping air (or another gas), these foils aren't thick enough to trap the air and are not going to provide the insulation they claim AFAICS.

Reply to
dennis

[Foil-Tec Double is then nailed in place through the Foil-Tec Saddle Clips to maintain required air gap - 4 per m²] From their diagram it looks like the floorboard over the saddle will be higher than the others! and [The horizontal joints are overlapped by 100mm and are left open to allow any moisture accumulation to dissipate] So there could be a draught through there, and dewpoint condensation. ? [g]
Reply to
george [dicegeorge]

It'll be difficult to lay I suggest, where do you stand when nailing down the clips, the film has to move to accommodate the added lengths imposed by the clips. Don

Reply to
Donwill

I'll cut it into large sections that will slide between the joists - there's enough waggle space underneath for a section to bridge maybe three joists. Then I'll fix them from underneath with 75mm drywall screws. To fix the last I'll cut a couple of hatch in it to work though, that I'll mend afterwards by refixing with foam. I'll seal round the edges with foam.

Note that I'm planning to fix it underneath the joists, not between. So none of the timber will be below the dewpoint, as it will all be above the Celotex.

Cheers Richard

Reply to
geraldthehamster

011

Elaboration forthcoming - yes, I'll have to cut the sheets, as I won't get them under there whole. As for draughts I'll seal all round with low expansion foam. Yes, I plan to screw them up into the bottoms of the joists.

Cheers RichaRD

Reply to
geraldthehamster

Thanks. I'll have to see which works best. I reckon I can get 800mm wide strips under there, which would span two joists. That would leave me a 400 offcut on each board, so you're right that it does then become a moot point whether it's easier to go under or go between.

This first, unrenovated room is a dry run for three other downstairs rooms that I have already renovated, in the sense that the skirtings and radiators are in, and various other things I don't want to disturb in those rooms mean I will only be able to take up maybe 3/4 of the boards. So I'll think carefully which approach might work better in that situation.

Cheers Richard

Reply to
geraldthehamster

I thought about this again - as I understand it, the point of the under-floor draughts (from the air bricks) is to enable any dampness that occurs to dry out. In an uninsulated floor the joists may already be below the dew point. FOil-backed Celotex also provides a vapour barrier.

Concerning heat loss, its true that if you insulate between joists only, the joist can theoretically act as a cold bridge, making the floor colder. I;d say that's another reason to insulate underneath them.

Cheers Richard

Reply to
geraldthehamster

No. Celotex far better in windy spaces than space blanket etc.

Or the first time you walk over your unboarded celotex floor and put a foot through it :-)

Reply to
The Natural Philosopher

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> if you don't fancy home-brew. You could even lay fibrous stuff in the

Fair point, though the electrics can go under the whole shebang.

Reply to
The Natural Philosopher

Oh?

I see

You could put UFH in the void as well. and cook the joist slowly as well..:-)

Reply to
The Natural Philosopher

geraldthehamster ( snipped-for-privacy@hotmail.co.uk) wibbled on Sunday 09 January 2011

10:27:

You could just dispense with the screws on the last panel - foam it in, support it (with duct tape or summat) while the foam sets.

But, yes, it seems a solid plan.

Reply to
Tim Watts

The Natural Philosopher ( snipped-for-privacy@invalid.invalid) wibbled on Sunday 09 January

2011 12:58:

Can't see why that wouldn't work. *Might* need a little more pipe surface area, but the pipe will be cheaper and easier to thread about.

Reply to
Tim Watts

Reply to
fred

In article , geraldthehamster writes

Bear in mind that the slightest draught in the space above the insulation will negate the benefit. I'd suggest intimate contact between the boards and the insulation and heavy duty sealing at the ends and any other possible gap. Despite the bridging and other issues, I think this is the best way.

I did mine ages ago using fibreglass (before the days of celotex) supported on hardboard trays resting on battens nailed side on into the side of the joists. I felt that was better than risk drooping that might occur with netting and it helped create a barrier against draughts.

Reply to
fred

I meant more as a technique than a product. Sling membrane under boards and fill with fibre.

Reply to
Skipweasel

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