Most efficient use of Celotex?

[Roger Chapman]

I wouldn't call a 2 metre head of water next to nothing. That is the sort of pressure you get on your bath taps if they are fed from a storage tank in the loft just above.

[GMM]

I would agree that cutting in 1200mm 'lengths' is the most effective way. You may find that the wastage is a bit less than you fear as few of your dimensions are likely to be exact multiples of 1200mm, so you can use at least some of the offcuts to top and tail the 1200mm lengths (placing them at 90 degrees to the long pieces, so to speak).

[Roger Chapman]

Silly me. I was so keen to sign off my previous reply so I could eat my supper that I didn't acknowledge that I too live in the North. Of course that doesn't have to be the same North as Fred. He could be in the North of Scotland while I am in the more balmy North of England.

I have considerable doubts about whether my weather station gives realistic readings for wind speed. It is often reading next to nothing when I can see the trees moving. However it being dark I can't look out at the trees and try and estimate wind speed from the Beaufort Scale criteria. ;-(. My wind speed has been generally averaging 8 - 10 mph since noon, gusting 20 - 25 mph and peaked so far at 27*. Got an isolated 40 mph gust yesterday morning though which is as high as I ever get with any regularity.

*Ha, just as I was about to post it downloaded another batch of data. The peak is now 30 mph.

[Dave Liquorice]

Gaps *will* have air flow through them.

Don't you believe it. OK we are another 50 mph gust place this afternoon, with a 1 minute mean speeds over 40 mph, that don't half find the gaps. Air pushes its way in so you get a pressure difference across the building, high on the exposed side, low on the lee. The old windows would have 3" high fountains of rain water being blown through the gaps by a gale.

I'd probably fit it slightly (mm or two) proud with the abilty to move when the plasterboard is fitted. The joints along the joists being taped, a) to stop draughts b) stop warm moist air that filters through non-foilbacked PB getting to the cold side.

[fred]

Let's start with the fact that the o/p is insulating a suspended floor (plasterboard?), move onto previous advice here that it doesn't really matter if the insulation boards are tight to the underside of the flooring and then leave the ends beside an outside airbrick open. How much good do you think 4" of celotex will do then when it is blowing a hoolie?

My advice to the o/p has been to mount the insulation tight to the floorboards and to make sure that gaps are sealed, particularly at the ends.

Do you think that is good advice or bad advice?

Why spend lots of money on 4" of celotex and then bypass it with a free flow of air?

[Roger Chapman]

I think we have been talking at cross purposes. I wouldn't argue with your advice since it is much more difficult to windproof floorboards than it is plasterboard and in any event I wasn't suggesting leaving large gaps. Sorry for the confusion.

[fred]

In article , Roger Chapman writes

Ok :-)

[Big Les Wade]

David posted

Cut the celotex slightly too wide and use a surform to shave it into a tight fit.

Bear in mind that your joists are unlikely to be exactly parallel, so you can't expect to cut the celotex exactly square either.

Wear a mask.

The whole thing is a pain. Horrible job.

[David]

I think I'll be using the Celotex clips and a foam gun, thanks - surforming Celotex sounds a horrible and slow job.

Just got the clips, and they look quite reasonable (subject to testing.

Cheers

Dave R