Cutting and placing Celotex

[wwwebber]

Hi,

Will soon be insulating between the rafters in our loft with celotex and will be doing this DIY. I'll be putting some insulation stoppers to ensure an airgap but have a couple of questions regarding cutting and sizing etc :-

What is the best way of cutting it to size ?, with a hand saw ? a circular saw ?

Also, for placing, I'm guessing I just slightly oversize it and squeeze it in between the rafters to ensure it stays put ? or do I need to fix it in with something ?.

Cheers.

 

[Ian H]

Cut it with a saw and to the exact size. If it's oversize it won't go in.

You don't really need stoppers, the friction will stop it. I would recommend you get an expanding foam gun and gun grade foam from Toolstation to seal any small gaps and get some 100mm foil tape off e-bay.

Also a white paper suit and mask/giggles. It's messy stuff.

 

[Ian H]

If your doing a full layer under the rafters I'd recommend these:

Wedi Marmox Metal Fixing Washers Insulation Board 36mm x (100 No.) https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/142644259370

I used a bag then couldn't find any locally.

 

[wwwebber]

Cut it with a saw and to the exact size. If it's oversize it won't go in.

You don't really need stoppers, the friction will stop it. I would recommend you get an expanding foam gun and gun grade foam from Toolstation to seal any small gaps and get some 100mm foil tape off e-bay.

Also a white paper suit and mask/giggles. It's messy stuff.

Thank you. I guess it needs to be sized carefully then as it would simply fall out even if cut even slightly too small ?. Noted regarding tape & foam - thanks.

 

[Ian H]

Pretty carefully.

I measured mine along the rafters, some were 5mm +/-, then I put a good sized timber offcut onto them and used that as a guide for the saw. I only had one that didn't hold itself up, I just wedged in an offcut the foamed the edge. Just make sure they stay flush with the undersides of the rafters.

You will soon get the hang of it.

You can see a bit of everything here, foam, tape, rafters...

 

[wwwebber]

Very useful - thanks again

 

[geraldthehamster]

You can't squidge it in if it's oversize, it will crumble and break.

In the past I've cut slightly undersize, held in place temporarily with screws, then foamed all the gaps. Once the foam has gone off, the Celotex won't be going anywhere.

You can trim excess cured foam flush, with a serrated breadknife ...

 

[garyo]

After two or three roofs I've given up cutting it to size. Rafter timber is never perfctly straight. I go 10mm under then wedge it in with little pir offcuts if required. The foam then goes in easier because I'm invariably trying to fill a 4mm gap rather than a 1mm gap. I find it makes for quicker job because I spend less time trying to measure & cut so accurately.

 

[wwwebber]

Do I need to use foam ? can I not just use tape everywhere ? or is this incorrect or too costly ?

Thanks for your comments so far.

 

[geraldthehamster]

You don't need to use foam, it's just a quick and easy way of fixing it into place without having to cut it exactly right for gaps of varying width and having it fall on top of you

 

[Ian H]

For the £22 it cost for the foam and gun I would definitely use it.

 

[John D v2.0]

Whatever you do use a foam gun at extra cost rather than the disposable straw cans. Especially overhead. The cans are either on full blast or off.
We are doing a floor and using foam around the edges and tape on the joists, because foam is less easy to control than tape, but you can't tape to the brick wall and the gaps are quite irregular and big.

 

[Doggit]

If you're just insulating between the rafters, and you've got an airgap underneath, then I'd be inclined not to worry about the tape, but as others have said, the foam will work wonders. I cut to size, and find it fits in, and the foam then holds it in place, as it only needs a small squirt, but you could try adhesive foam instead for such small gaps.

 

[John D v2.0]

Heads up to anyone reading, that I just bought some diall brand "aluminium foil" tape from screwfix and I kid you not it's silver parcel tape and doesn't stick to anything. It's going back not fit for purpose. Used to sell decent stuff. Wickes sell the same size for about the same cost and is the proper stuff with the backing paper. So beware!

 

[foxhole]

Cut to size if a loose fit a few nails in the timber just short of where you need it to stop then push in , the nails will rip into the edge and hold it.