# Celotex Calculation

Discussion in 'Builders' Talk' started by snappyfish, Jul 12, 2017.

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1. ### snappyfishActive Member

Wanting to insulate log cabin floor its 5.5x4

Anyone know how many sheets id need of 50mm celotex?

Thanks

2. ### Dr BodgitSuper Member

8

2.88 m2 per sheet

5.5 x 4 = 22
22/2.88 = 7+

3. ### Dr BodgitSuper Member

Except my cabin at 5.5 x 3.5 only needs 5 sheets. Area goes down with joists

4. ### Dr BodgitSuper Member

I'll try and be more accurate for you, as I drew out a sketch of my subframe including joist and noggins.

Cabin is 5.5 x 4 except the inside walls are less. Take off 100mm each end plus 65mm bearer so insulated floor area is more like 5170 x 3670

Say there are 8 intermediate bearers at 50mm each, that reduces the length by 400mm so 4770 x 3670

So that's 6.07 sheets. Pays to work it out accurately on a drawing. Looks like you'll need 7 but might get away with 6.

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5. ### PollowickScrewfix Select

I would not use a single layer of 50mm but two layers of 25mm.

It makes it easier to cut - you can have a small few mm gap and not worry. Cut the first one, and drop into place, push across to teh left and use a couple of small offcut slivers to hold it there. Then, cut the second, place on top and push to the right, again with a couple of slivers to hold it. Yes, you may have some small parts with just 25mm, however that will be way better that all trough gaps.

It also makes it easier to use the offcuts - full lengths on the bottom layer, and cuts on the second.

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6. ### Dr BodgitSuper Member

Celotex is easy enough to cut, using a saw or sharp knife. Using 25mm means twice the measuring and cutting, more work.

And I assume this celotex is going on concrete, as the bearers are 50mm also, that's not ideal as any dampness that gets in will not have air to evaporate.

7. ### PollowickScrewfix Select

Based on the 4770 - there will be two lengths needed at 2400 long and for the 3670, nominally 3 and a bit widths at 1200 ! So, yes, 6 might just do it depending on actual joist and noggin sizes.. OR using, as I posted above, double layer at half thickness 12/13 sheets at 25mm thick.

The OP needs to sketch it out will all joist and noggin thicknesses allowed for - they could be 52 or as low as 40 ...

8. ### PollowickScrewfix Select

I know how easy it is and having recently done a large roof with it, putting a double layer in, and cutting to within 3 or 4mm twice, is a lot quicker than cutting to within 0.5mm and forcing it in

9. ### snappyfishActive Member

What about 40mm Celotex? would that be enough? Is it vital to leave a gap?

10. ### Dr BodgitSuper Member

I wouldn't want the celotex to be up against the concrete, leaving even a 10mm gap would be better. You then just need to find a wee batten to hold the celotex off the concrete or some other way of fixing it in place.

May be another way to is get some 2x2 treated timbers and make a shallow sub-frame on which the provided bearers can sit. That way the whole lot is above the concrete.

11. ### snappyfishActive Member

Too late it's off and up now, started today.

12. ### Dr BodgitSuper Member

Ooh, you going to post piccies?

13. ### snappyfishActive Member

I may well do so, damn I can't think straight now about the Celotex...

14. ### snappyfishActive Member

Right so you reckon 10mm gap minimum? Definitely don't just leave against concrete floor?

15. ### Dr BodgitSuper Member

Yes...think about it. If celotex is against the concrete, any moisture will be trapped. Any kind of gap is better than none. I don't know in practice how much moisture will get in there, may be none.

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16. ### Dr BodgitSuper Member

Another way to do it if your cabin is already being erected and hence the perimeter timbers are down, is to put the intermediate bearers on 10mm packing/shims, and pack out 10mm on top of the perimeter bearers where the floor is going to sit. Then you can put celotex in between the bearers which sit on the shims which extend either side of the bearers.

17. ### snappyfishActive Member

Thanks, intermediate bearers are also in. Think it should be ok to drop down to 35mm or 40mm Celotex, Hopefully that should keep it nice and warm???

18. ### Dr BodgitSuper Member

Not much difference between those thicknesses, some is better than none.

19. ### snappyfishActive Member

Bloody shortage everywhere, wickes have some in stock but 25mm should I hang on for 40mm or even 35mm or get cracking with 25mm?

20. ### PollowickScrewfix Select

Where in the country are you ? West London, out into parts of Berks, Bucks and Surrey - I can suggest a wholesaler who will be way cheaper than a DIY shed.

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