insulating a loft with kingspan/celotex

[Stephen]

Hello,

It has been suggested here before, I think by the late Andy Hall, to use celotex sheets to insulate your loft.

How is the best way to do this? I would seem to me that the easiest way would be to butt the boards up to each other, above the joists; that way you would not have to trim them to fit between the joists. Also if the celotex was put between the joists, you would have some heat loss through the joists as the wood would not insulate as well as the celotex would. This way, there is a continuous surface of celotex.

But... what happens when you get into the eaves?

I tried pushing an off cut into the eaves but the sheet hits the rafters, so there is a large gap between the rafters and the eaves/sarking felt, which defeats the object if the heat can just convect around the edge.

Are you supposed to cut out sections for the joists? I do not have a ventilated soffit, though that said, I'm sure it's not airtight either, would it cause problems if the celotex was butted right up to the sarking?

Should I just use the traditional rolls of fibre glass instead?

Thanks, Stephen.

[Another John]

My inexpert, but experienced, views are:

- Put fibre glass between the joists.

- *And* put good thick Celotex across the joists.

- Don't worry about a gap at the eaves: you need ventilation through a loft, or you will cause trouble.

Bonus: if you get good thick celotex, and walk carefully, you have almost as good as boarded out your loft.

Cheers J.

[Brian Gaff]

What about using bubblewrap to do the awkward areas where you don't walk. It seems cheap almost too good to be true. Brian

[Nightjar]

Use one of the methods detailed in the Celotex guide. Roof insulation starts at page 44:

[Stephen]

What size is considered thick?

Thanks.

[Stephen]

Thanks, I never thought to see what the manufacturer said.

It seems most of it is about fitting celotex over the rafters rather than the joists. The pictures are not very good. In fact the text says to have the non-printed side facing out and the photos show the printed side facing out! I never realised a little red ink would make so much difference.

I get the impression that I was looking at doing this the wrong way; rather than insulate over the joists, it seems I should insulate over or between the rafters (or both) but leaving an air gap from the eaves to the apex to ventilate the rafters.

It seems the traditional fibre glass roll still has a place between the joists.

Thanks, Stephen.

[Martin Brown]

I hate to be a party pooper but exactly how do you get large thick celotex boards into your loft?

Loft access is typically something like a 3' diagonal if you are lucky.

[Martin Brown]

The aluminised plastic wrapped glass fibre is what I used. Much less messy than the old stuff. Still need dust mask and gloves but relatively easy to get it up there and deploy.

[RJH]

That's the method recommended in the Celotex guide - link elsewhere in this thread.

Thickness would depend on the insulation levels you'd like - I'd have thought 50mm on top of existing rockwool would be enough before diminishing returns start to set it.

Cut lengthways, rejoin with tape?

[RobertL]

AIUI the foil on one side is deliberately perforated; on the other side it is a an impermeable vapour barrier. I'm not sure which side has the printing on.

Robert

[harry]

I have done this. It's not possible to accurately butt joint. The slightest gap defeats all your work The best way is to buy the foam sheets the same thickness as the joists. Cut around an inch smaller than the gap between the joists. Put in place & fill the gap with expanding fixing foam. Before the foam has gone off, put more sheets on top. The expanding foam from lower gaps sticks to top layer. (Leaves gaps on upper layer as before & fill with fixing foam) You need bricks/concrete blocks to put on top to stop the sheets from being displaced as the foam expands.

The foam is twice the insulation value of mineral wool.

You can lay chipboard on top to provide walk way.

It is closed cell and waterproof.

You can get the insulating boards around half price with slight damages. (Easy to fix with the expanding foam) Google:- insulating+boards+seconds

Don't bury electric wiring, especially heating related, it may overheat. It's good to thread wiring that passes through the insulation through a bit of plastic conduit. (Keeps it cool and removeable.)

Well worth doing IMO.

[The Natural Philosopher]

2" min.

[The Natural Philosopher]

Yes. IF you are doing a 'warm' roof, with stay 6" rafters, use 4" celotex between and say 2" celotex over, with a 2" gap from eaves to apex and roof vents.

tape the first layr to the rafters as a vapour barrier.

Its OK. If that's where you want to insulate, esp. if boarded over.

[John Rumm]

Yup, you can then lay floorboards directly on the foam, and they will take the weight.

use normal rockwool or similar between the joists.

Just rely on the rockwool etc.

With the insulation underfill that should be negligible.

Generally as long as air can reach the roof timbers, they should be fine. You may also have a breathable sarking depending on the age of the property.

As well, rather than instead. They are cheap and easy for between joist installation, but far from ideal for anything deeper. The rigid boards and then ideal for the next layer.

[John Rumm]

That's nonsense.

It may be removable, but plastic (or metal) conduit is not going to make the insulating effect of the insulation go away.

A flat T&E cable in >= 100mm of thermal insulation is treated as installation method 103. Method 103 reduces the current carrying capacity of a cable to HALF its clipped direct value. The addition of conduit makes no difference.

[Roger Hayter]

How long does a piece of well-insulated conduit have to be before the cable through it is fully derated? Common sense suggests that an inch long piece of 20mm conduit will have negligible effect. What about six inches, such as proposed *through* the loft insulation? I can't believe that the equilibrium temperature rise at the centre is the same as in, say, ten feet?

[RJH]

Is this really the case? So long as there's no air movement between the warm and cold areas, I'd have thought the impact would be small. And down to negligible if neatly cut board was joined with tape. Perhaps a thin line of expanding foam at the join for good measure?

[John Rumm]

500mm

(note its not the conduit itself that is the problem - its when its in insulation - with or without conduit)

Indeed, you can ignore anything up to 50mm.

There is a sliding scale such that the de-rating factor increases with the length that is insulated.

So from table 52.2 of the OSG you get factors of:

0.88 for >= 50mm, 0.78 for >= 100mm, 0.63 for >= 200mm, 0.51 for >= 400mm, 0.50 for >= 500mm

Note that there may be an additional factor to apply due to the ambient temperature - since its common for many lofts to get very hot.

[Another John]

Good point Martin (that was me above btw). I've just been up the loft to check: evidently, I sawed them in half, lengthways. I didn't tape them back together: (1) a PITA and (2) I have rockwool between the joists anyway.

I disagree with the point made in another post that being unable to butt-joint tightly is a problem -- "The slightest gap defeats all your work": a gross exaggeration.

To Stephen: I used the 50mm (2") thickness -- mainly because i didn't want to spring for the 3" thickness.

J.

[Andrew]

I used Knauf loft boards which are 50mm thick EPS, the closed cell varient which is pink. Back in 2009 they were being sold at a very subsidised priced courtesy of the energy cos.