celotex, kingspan, xtratherm

[John Whitworth]

You can certainly put T&G floorboards directly onto PIR when that PIR is on a flat surface. However, I am not sure how it would perform if the PIR was sitting on joists itself.

JW

[Jim K]

messagenews: [e-mail]...

given that it's a loft and shurely the boards would be screwed through into the joists -- how much "action" is it likely to get/suffer from? any point loads spread by chip and PIR onto several joists - maybe a bit of compression on joists in well used hatch areas possibly? worth a test tho - it's firm stuff (kingspan partic - Xtratherm seems softer but not a lot)

Jim K

[John Rumm]

Yup. While the foam can be damaged by point loads - spread over a joist with a board over, you can screw the board down through the foam and it hardly crushes at all. The same construction technique is used for "warm deck" roofs these days.

[John Rumm]

Its fine - try it. If you stand on a bit, your foot does not sink in.

[<me9]

I've put mine (actually T&G chipboard) exactly like John's ascii art above. No problem. It may be a problem below a floor in a habitable room, but for usual loft storage it's fine.

[Fred]

Thanks for all the replies.

I was going to ask whether you needed something between the joists and the insulation boards but other people have beaten me to that. I had always thought (wrongly) that it would crush where the weight was applied over the joists and it's too expensive to tread on some to see what happens! I have seen spaceboard in the shops and always thought it was an expensive way of buying small blocks of PIR but according to their web site, I was surprised to see it is a type of polystyrene:

only disadvantage with the arrangement above is that I still lose some headroom, albeit less than if I used rolls. I think we've agreed that PIR requires half the thickness of a roll.

I'm still curious whether I could fill with celotex between the joists to maximise headroom or would the heat losses through the joists be too much? Do they really need to be covered?

I'm afraid that whatever I do, I will have to "tit about" because I've got one of those roofs with "W" shaped trusses on every joist ;(

[Fred]

I suppose it should be no surprise that the marketing department use the best figures ;(

I'm surprised that a small gap can half the effect of the insulation though. Ouch! That's a big drop. Does the usual advice apply: spray expanding foam into any gap > 1cm?

I suppose the same is true for whatever insulation you use though? Surely if you used rolls and left a gap the air space would reduce the effect of the insulation there too.

[Fred]

Sorry but I've thought about this some more, isn't there a problem when using celotex between joists that some gaps around the sides are inevitable?

I'm thinking that the spacing between the joists may vary along its length by a few mm so you would have to measure carefully and cut trapezoids to fit. And wouldn't you need an occasional hole to pass the lighting circuit through? Or do you fill around the cable with caulk to make airtight? I presume T&E and PIR is ok, unlike T&E and polystyrene?

TIA

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Fred saying something like:

It just requires a bit of care in cutting and trimming down - the foam gun is handy, as you say, when there's a gap.

I've seen foam, wads of f/glass, r/wool stuffed in holes - not perfect, but apparently good enough. On some jobs the gaps were left bare - I wonder if the people who were paying for those jobs knew quite how much better it could have been.

[Fred]

I've fitted it in my wall now and will think about doing the roof another time. It has occurred to me that if you did the whole loft insulation in celotex you would need ~150mm, so you would need two layers any way, so whether I use celotex/celotex or rockwool/celotex, there are still going to be two layers and the joists will be covered either way, so I guess I don't need to worry about losses through the joists.

I did have some cut-offs of 50mm celotex and trod on them; you are right they do not crush. The foam is tougher than it looks.

BTW at what point does a sheet become a second? There were some marks on the foil of mine but isn't that inevitable? I did reject one sheet of 25mm which was squashed where it had been tied to the pallet and was completely bent along its length. I did get a couple of "you won't see it when it's in the wall" comments before they finally agreed to swap it. I think some of the 50mm sheets look wider at the edges than in the middle. Is that a side-effect of the manufacturing process, is there always some curl at the edge? They were from a subsidiary of Travis Perkins so it wasn't some fly by night shop I bought them from.

I didn't have snap-off knife so I used a kitchen knife (don't know what type but it was a long one!) and it worked ok. Next day I bought a snap-off knife. I see you can get 9mm, 18mm, 20mm blades. I guess this is the height of the blade? What is the advantage of a taller blade? Is it more rigid and therefore stronger?

TIA

[John Rumm]

It might be knocked about a bit, or have a void somewhere. Sometimes it can just be because its an odd shape it seems.

Might be down to how they are stored. Many boards come with instructions to keep under cover - yet many places store them outside.

yup

[Fred]

They were taped onto a pallet to stop them blowing away. The 25mm one got the tape wrapped around it, and this crushed it to less than 25mm! That was definitely a second imho but I did have someone on the phone and the driver grumble about swapping it!

They were saying you won't see it when it's in the wall but my line of thought is that if I get a second, I should pay for a second, not for a "first" and surely it being crushed to less than 25mm would affect its performance.

Never mind, it was swapped the next day.

I got an 18mm knife. Is that a good size for general DIY use? Is there ever a time that a smaller or bigger one is better?

I don't know why I never thought to use a saw. Only after I had finished did I use a saw on an off cut and it went through it like a hot knife through butter; much easier than the kitchen knife. If only I had done that before!

How many tpi does the official celotex saw have?

[John Rumm]

I doubt a bit of crushing makes that much difference...

Yup the larger ones tend to be more generally useful. The small one is handy for opening stuff I suppose, but I don't bother buying them these days.

Never seen an official one. A standard hardpoint jacksaw seems to work fast enough.

[Jón Fairbairn]

I've cut xtratherm with two types of jigsaw blades, a long wood-cutting toothed blade, and the a Bosch T313AW "special for soft material" with a scalloped cutting edge. They both cut fast enough, but the amount of dust produced by the wood blade was enough to be a nuisance, (it gets electrostatically charged and sticks to everything). The scallop-edged blade does produce dust, but *much* less.

[Fred]

It must eventually make a difference because if crushed enough. This really was a bent and squashed all over the place. Even if the damage had not affected its u-value, it certainly was not "first" grade. Since I was paying for "firsts", I didn't want to be sent a second! ;)

I was gluing it into the window reveal so I wanted it to be as straight as possible.

There is one but I have been too lazy to register on the celotex site to read the spec. It's also listed on the toolstation web site but they don't go into details. I wasn't going to buy one especially but I thought if I knew the tpi, I could match it to the nearest saw I had next time.