Most efficient use of Celotex?

[David]

Planning the cuts to install Celotex between joists. As usual, the spacing doesn't really get the best use out of a 2400 * 1200 sheet.

So I'm trying to calculate how to minimise wastage and how many sheets to buy.

I'm now trying to match up individual joist runs so that the gaps between them add up to roughly 1200 - some are over 300mm and some under.

Ground floor, suspended with joists on brick mini-walls across the foundations, real 2" * 4" joists (not under sized as most metric ones are these days).

As I type this I realise that working the whole thing out is likely to waste more time than is saves in material.

So - cut crossways in 1200mm long strips so the wastage on each sheet is at maximum 1200mm long?

Still have the problem that unless I have matching (or lower) sizes in more than one joist the end of the 1200mm cut may be wasted anyway.

Oh, well - treat it like a builder. Time is more important than wastage.

I think this is why our builder for the extension at the back of the house got fed up one day with slow cutting and just ripped the sheets with a chain saw. Yes, it did look as though it had been snowing.

Sigh.

Any tips for getting the most out of Celotex (et al) and easy cutting?

Cheers

Dave R

[The Natural Philosopher]

yes, two things I found out about insulation.

(i) its cheap. (ii) if there is even the slightest air gap you have wasted your time.

What that means in effect is that you should cut to exactly fit, tape all joints and mastic (I used acrylic sealant) any insulation-to-something-else edges. ] And not worry about offcuts.

[Tim Watts]

A stiff spineless woodsaw seems to be the quickest for cutting, but makes a lot of mess. However, if you can sheet the floor, the dust is quite heavy and tends to fall on the spot in the absence of wind.

A breadknife makes less mess but is hard going for long cuts. Good for fiddly cuts.

Have you tried "score n snap" with a stanley knife? That depends on the thickness but might be worth a try.

[Tim Watts]

I think a foam gun is easier than mastic...

Taping across the rafters is good not only from the POV of reducing air gaps, but also by acting as a barrier against water vapour getting from the house into the wood then travelling through to the cold side and condensing (which risks rot).

[Dave Liquorice]

[fred]

In article , David writes

Your 120cm 'length' cutting sounds fine.

I have cut it with a coarse cut hand saw which results in marginally less dust.

Are you working from above or below?

Below:

Cut slightly undersize (say 5-10mm) and fix in place tight to the boards using the odd slant drilled nail (under, not through the foil. This can be very quick.

Uses a proper foam gun to absolutely seal every gap and every joint between sheets. The undersize cutting actually makes this easier as it's easier to apply foam into a gap. The set foam will hold the sheets permanently.

In addition, at the end of each run, add a little squirt of foam to the upper side of each board before pushing it up. That will stop any cross draught across the top of the insulation boards, negating the insulation.

From above I'd do pretty much the same but I'd make absolutely sure there was no way for draughts to get in at the ends as there is a possibility of a greater gap between the insulation boards and the flooring.

Don't forget to seal the gap between the flooring and the bottom of the skirting.

In summary:

Avoid draughts

Avoid draughts

Avoid draughts

[Dave Liquorice]

Ish, celotex isn't that cheap in decent thickness's.

Not worry unduely. But I'd probably measure each gap to the next cm and do some rough calculations to try not end up with loads of off cuts just a cm or so too narrow, Maybe end up with bits are just over half the required width that could be joined, taped and recut to make a full width ones.

Cutting an old handsaw but it's still messy.

[Andrew]

If you have a bosch jigsaw cutter, then you need a set of T313AW HCS blades. These have a sharp blade but with a wavy shape. Cuts up to 4 inch thick. Best things I have ever bought.

You can also make a diy version by sharpening the back of a hacksaw blade to create a nice sharp knife and make a handle.

[Andrew Gabriel]

I find a thin blade sharp kitchen knife works best (and you get no "sawdust"), but it does noticably blunten knives. Can also use a serrated bread knife.

The expansion gas released from newly cut edges can give a headache. It used to be a cyanide compound, although nowhere near as toxic as cyanide itself - don't know what they use nowadays. Probably want to ensure good ventilation.

[David]

In this case it is a suspended floor with a ventilated void under, so thankfully that should not be an issue (I hope).

[David]

Thanks.

Working from above because I'm not a midget ;-)

I asked a few days back about using the Celotex joist clips (as shown in the Celotex PDF) and I note that nobody has mentioned these.

You say "using the odd slant drilled nail (under, not through the foil)".

Now my joist are a true 4" (so very marginally deeper than the 100mm sheets but not really enough to securely fix a nail and rest the Celotex on top.

So is it O.K. to fix a nail then press the Celotex down onto it so it is embedded in the foam? Or is this likely to result in all sorts of problems like pushing too hard then not being able to get under the board to push it back up again?

Celotex clips are around £18 per 100 so not a vast amount to risk, but still...

Cheers

Dave R

[fred]

Luxury :-!

You get to measure up and cut in the room then adjust if needed before fitting _and_ you get easy placement for nails. Sweep the dust into the underfloor as it is inert. It's heavy enough not to get airborne dust but if in doubt, hang an old curtain on a batten nailed above the door on the outside. IME this works better than polythene as a self closing drape seems to get closed more readily.

You'll get a feel for this as you go along but I would start nailing very close to the bottom corner of the joist, driving in at an upwards slant to avoid breaking out the wood by too thin a grip. Then either ding the nail up to get a good height or subsequently place the nails slightly higher.

Pushing the celotex down onto the nails is no bother as it has no elasticity so will stay where it put (as long as this is for a temporary fix only)..

I'd still cut undersize and then foam it as it makes it such an easy job, you can cut back any excess foam when it has set. I don't see the need to foil it if you seal it properly with foam as I've not seen any evidence that flooring really needs a vapour barrier (walls and ceilings a different matter altogether). A cheap 18quid 'pro' gun makes foaming sooo easy and is easily controlled.

I can't state too strongly how important it will be to seal the gap between the flooring and insulation boards at the ends of your joist runs as these often run to outside walls where vents could easily force cold air over the top of the insulation if there is the tiniest of gaps, particularly in bad weather. It needs to be something flexible/compliant and a spare bit of fibreglass of 300mm wide or so and well compressed would do the job here, it will be enough to encourage the cold air to take an easier alternative route.

That's a good deal, well done on finding that. Probably cheap enough not to worry about offcuts but if you feel the need you can drop a hardboard 'floor' onto those nail at the bottom of the joists, drop in your offcuts on a best fit basis and fill the gaps with foam. Probably not the best option but if you get to the end of a job and it saves you buying an extra sheet then it saves a few bob.

If you fancy, drop us a pic for the wiki when insulated and before the floor goes back down as this is a frequent question but I have yet to see any pics that capture it at that stage so it may help the next guy.

Good luck!

[Roger Chapman]

big snip

I don't think most builders care about the end result. As long as it is buried no one even gets the chance to question their (sometimes very) questionable practices.

I don't buy the notion that it is imperative not to have any gaps otherwise the insulation ks ineffective. What matters is not to provide a path for drafts between the insulated interior and the other side of the insulation. If the heat loss is going to be just from the convection currents within the joints then the additional heat loss is going to be proportional to the ratio insulation to air gap and very minor. For the situation to be any worse you need an air flow through the gap, not just convection currents within it. If you plasterboard your ceiling where is the air flow coming from?

Neither did I find PIR insulation cheap even after my builder got a good discount on a less known brand.

FWIW I have been using a wood saw after careful measurement and cutting on a bench, not in mid air but the dust has still been a nuisance. I cut marginally oversize and rely on friction to keep the insulation in place. With care even the off cuts can be trimmed and wedged in placer reducing the wastage considerably. Might not have been perfect, it is extremely difficult to get every measurement exact when none of the corners are square and none of the sides parallel. However it was good enough the pass the BCO inspection. Now all the ceilings need is plaster boarding.

[fred]

In article , Roger Chapman writes

If you don't seal all gaps then how can you be sure that some of that air is not coming from the outside and therefore circumventing your insulation?

If you lived in the wind blown north I think you would have fewer doubts about the effectiveness of draughts in negating insulation. We are experiencing 50mph gusts here at the mo and I am left in no doubt that some of the insulation that I so carefully installed is being bypassed and therefore negated by draughts.

The heat loss from 100mm of celotex is so low that it takes the tiniest amount of air change on its warm side to negate at least part of its effect.

Convection behind is trivial by comparison (particularly in a horizontal flooring gap).

Until recently they have been passing fraudulent baco foil coated bubble wrap as effective so I don't view their passing of an installation as a true indication of its effectiveness. If you want truly effective insulation then IMV you need to look at first principles instead of box ticking.

[RobertL]

If you cut at a slant, so the cut edge is not exactly at 90 degrees to the surface, then it's easier to wedge in place and more tolerant of errors.

There is a special smooth-bladed saw you can buy for cutting it. You don't get that snow of little bits.

Trivia: Chickens like to eat the little bits but I'm sure it's bad for them.

Robert

[David]

I'll try and remember to do a start to finish record so people can see the steps.

If I feel really adventurous I may try different fixing methods and record which works best.

I've ordered the Celotex clips from Toolsatan and they should arrive tomorrow but I'm prepared to write them off if they prove in any way problematic. I still can't quite see how they can lie above the joist and not interfere with the flatness of the floor.

Anyway, next big hurdle is to find out if I cleaned the foam gun properly after I last used it :-( I have a can of cleaner but if it is bunged up I'm not expecting miracles.

Oh, and I saw a Japanese Saw in Lidl which is a bit like a carving knife with exchangeable blades so I bought that to see how it did at cutting Celotex.

Hopefully all the buggering about before hand will result in an easy install.

Cheers

Dave R

[RJH]

Gaps will compromise the insulation, of course, but not negate it. If there's no path inside-to-outside to allow convection or draughts, I'd have thought the loss is small.

Of course, if you're getting draughts, yes, a problem. I thought I'd got it sorted, but somehow a draught gets through a small piece of celotex I'd left open at one end. The air somehow travels about a metre between the insulation board and the floor, and pops out at a floorboard join.

Well, tests with a temperature 'gun' suggest that the surface temperatures around the draughty part are pretty similar. It's the draught that lowers the in-room temperature.

Just for the OP - bread knife to cut. Once you get the knack, easy and no mess, although some of my cuts have been a bit wavy. Might try the jigsaw 'knife' blades suggested up-thread.

[Roger Chapman]

Given what I said above I am not sure what you are getting at. Are you for instance suggesting that there will be a void between the warm side of the insulation and the containing plasterboard and that the wind will whistle in though one gap and whistle out through another with whatever heat it could accumulate in its passage through the void? I did consider that but I think it inherently unlikely since the void above the insulation should have next to no pressure difference from end to end and thus no motive force for the draught. Add to that my insulation is mostly flush with the bottom edge of the ceiling joists which I assume is the normal practice.

Depends what you mean by the North. I haven't driven up the full length of the A1 for a number of years but it certainly used to be the case that the 'To the North' notices started appearing even before the Midland but however far North you got the signs were always indicating further North. Probably also the case if you continued up on the A9 but my memory of that road has faded to nothing.

Unless you have a heat recovery system you will lose significant amount of heat in the necessary air changes. Far more than can escape through small gaps shielded from the actual house interior by a solid layer of plasterboard.

Unfortunately I am working from first principles. However what I have written is primarily aimed at ceilings. I have solid floors so don't need to consider suspended flooring. The construction might be rather different but the principles don't really change.

[DICEGEORGE]

a house with a window open is warmer than one with all windows open, but I' d seal all air gaps and control your ventilation

Cut celotex at a slant, then wedge it in, then foam

I just got a van load of kingspan offcuts for £60 cash from secondsandco presteigne, (it was a bit wet though.)

Cut with an old blunt woodsaw, and an old blunt tenon saw.

Collect the dust and put it between floorboards.

I used to use pinkgrip foam, but then burnt some as an experiment, quite fl ammable, so now I use firefoam.

[George]

[The Natural Philosopher]

The pressure difference between the eye of a hurricane and the rest of te earth is next to nothing, too. 0.2 bar at most....