Loft insualtion with celotex/kingspan

[Mojooo]

Hi

A simple issue seems to have become a complex one

Situation before - loft was insulated with mineral fibre type stuff. Probably had about 15cm of it.

Situation now - the loft has been cleared of all insualtion and the plasterboard ceilings below have been taken out.

I want to re-insualte with rigid celotex or kingspan boards primarily because I want to use the loft for storage and I don't want the nasty mineral wool.

My roof 'joists' are 10cm.

I contacted Celotex and the advised I get a vapour barrier, then use 9cm of celotex between the joists (the reason seems to be because they vary in size) and then put a further 6cm of celotext on top of the 9cm between the joists.

My questions are aroudn the sue of a vapour barrier.

1 - is there any particular type to use (vapour barrier)?

2 - I presume you just tack it to the underside of the loft (i.e basically where the plasterboard goes and you just plasterboard over the vapour barrier?

Obviously where I want to use the loft as storage what I don't want is when I walk upstairs it doesn;t push the celotex down and break the ceiling plasterboard so I need to ensure that the first layer of celotex stays within the 10cm joist which is why it makes sense to use 9cm - that could then cause a 1cm airgap between the 2 layers of insualtion - will this be a problem?

Celotex also said I can store/walk stuff directly ontp the top level of celotex - is that OK or is it safer to put chipboard on top to spread the weight?

Finally, I have a breathable membrane - I wonder if I shoudl add additional veintilation like soffit vents or ventilation tiles to be sure the roo is ventilation enough.

[Spudler]

At 60mm it's boarder line. Ideally it'd need some flat support, preferably chipboard on top. Could always try without first. Anything other than boxes etc can pierce the sheets.
Additional ventilation won't do any harm.

[roofer]

Should of just boarded over the mineral wool, or put new back and board over. Celotex will get damaged by walking on it, or putting something down too hard.

Vapour would be visquenne tacked to underside of joists, preferably with taped joints.

[wolfracesonic]

Celotex said you can walk on the 90mm stuff between the joists? I wouldn't try that if I were you. Put the 90mm celotex between your joists, board it over then on the underside of the joists use something like this insulated plasterboard It'll give you some more insulation, your vapour barrier and plasterboard all in one

[Mojooo]

No they said I can store stuff on the 60mm which goes over the 90mm.

They didnt say walk per se but they pretty much said it acts as a floor.

The old stuff had to be taken out for non related reasons anyway.

[BristolRich]

Walking on Coletex is a big no no. Not only will it compress and reduce its efficiency I wouldnt trust it! It might hold your weight but will it hold your weight plus the christmas tree/box of old LPs you are shifting around? Besides which, retrofitting it around any existing wiring and light fixtures beneath (assuming these are still in?), and maintaining performance will be a PITA.

I'd just use rockwool to the apropriate thickness and board out as normal making sure not to compress the wool.

Edited by BristolRich on Monday 29th June 07:27

[Little Lofty]

I'd be more inclined to do as if you were insulating a loft ceiling. 90/100mm in between the ceiling joists, 40/50 mm to the underside of the ceiling, tape the joints with foil tape to create a vapour barrier, plasterboard then a Weyrock floor in the loft. Or just build up the joists in the loft and fill with two layers of Rockwool and Weyrock over.

[TA14]

wolfracesonic said:

Celotex said you can walk on the 90mm stuff between the joists? I wouldn't try that if I were you. Put the 90mm celotex between your joists, board it over then on the underside of the joists use something like this insulated plasterboard It'll give you some more insulation, your vapour barrier and plasterboard all in one

I'd go with this. Be sure to pack/fill any gaps around the Celotex/Kingspan.

(In your (the OP) original proposal it doesn't matter whether you walk on the 90 or 60 depth, the force will be transferred to the plasterboard...)

[guindilias]

Kingspan is pretty robust stuff- on my last UFH installation we had trades walking over it for 2 weeks before the screed was poured, with just sheets of hardboard to protect it.
Agree that I wouldn't be walking between the joists though, unless you want to be re-boarding the ceiling below before long. I'd just put loose board (ply, etc) over the top of the whole lot and not worry about fixing them down unless you are anal about it looking nice and "finished".

[Jamp]

TA14 said:

I'd go with this. Be sure to pack/fill any gaps around the Celotex/Kingspan.

(In your (the OP) original proposal it doesn't matter whether you walk on the 90 or 60 depth, the force will be transferred to the plasterboard...)

Not if the upper 60mm layer is resting on the joists, which is what the 90mm layer being less than 100mm is intended to ensure, I assume.

I wonder what the U value of 18mm T&G chipboard is? Surely trapping that 10mm air gap below the boards must be worth a fair bit. In my own place I'm inclined to go for just 90mm celotex then board it over and call it quits, as I don't want to loose the height by putting insulation above the joists.

[TA14]

Jamp said:

Not if the upper 60mm layer is resting on the joists, which is what the 90mm layer being less than 100mm is intended to ensure, I assume.

I just had a quick look at the Celotex website. There is no allowable bending stress or load table that I can see. If you fancy walking on a 60mm deep sheet spanning up to 600mm then you're a braver man than I am

[worsy]

Celotex is incredibly expensive though. I'd have left the lagging there and boarded over.

[Mojooo]

I presume you dotn need a vapur barrier with mineral wool as I am gussing no one has it

What if you put celotex over the mineral wool.

An alternative is that I use mineral wool but i put some sort of breathable roofing membrane on top as a barrier between me and the mineral wool.

[guindilias]

YOu should lay a membrane over mineral wool/fibreglass/whatever anyway - otherwise the light breeze you get in a properly ventilated loft blows across the top of the insulation and "sucks" some of the warm air trapped in it out, reducing it's effectiveness. I'll try and find a page somewhere, it's a bit difficult to explain!

[dxg]

guindilias said:

YOu should lay a membrane over mineral wool/fibreglass/whatever anyway - otherwise the light breeze you get in a properly ventilated loft blows across the top of the insulation and "sucks" some of the warm air trapped in it out, reducing it's effectiveness. I'll try and find a page somewhere, it's a bit difficult to explain!

wicking...

[guindilias]

That's the word!

[p1esk]

I've been attracted to the idea of removing our old loft insulation and putting celotex in instead. I know celotex is expensive, but the existing fibreglass stuff is messy and the untidy wiring system is mixed up amongst it, so I'd prefer to have a fresh start, insulation, electrics, the lot. Using celotex would also mean that the boarded area could sit directly on the joists, avoiding the need for spacers to accommodate a thick layer of insulation, so that looks like a plus point to me.

Incidentally, how does 90 or 100 mm of celotex compare with the often recommended 270 mm of fibreglass in terms of insulating performance?

Also, why is a vapour barrier needed? I haven't heard of such a thing being needed at loft floor level. Certainly there doesn't seem to be one in our loft at present.

[Stig]

Celotex (or similar) is great.... until you need to rewire a ceiling rose or suchlike!

I had a similar issue, wanting a proper walkable floor in the loft but only 100mm trusses to work with. Solution was 150mm fibre between joists, boarded on top with T&G (compressing fibre), then foil insulation to inside of rafters and all foil taped to up the U value.

[p1esk]

Stig said:

Celotex (or similar) is great.... until you need to rewire a ceiling rose or suchlike!

I had a similar issue, wanting a proper walkable floor in the loft but only 100mm trusses to work with. Solution was 150mm fibre between joists, boarded on top with T&G (compressing fibre), then foil insulation to inside of rafters and all foil taped to up the U value.

I see what you mean about Celotex in relation to the wiring, but I'm planning on running the wiring along the purlins and rafters, and keeping it away from loft floor level except where it needs to drop down for a ceiling rose or switch etc. The plan is to have the Celotex panel joints at the points where cables need to pass through the ceiling.

This is an unusual scheme, but I really don't like the tangled wiring arrangement we have at present, and it is due for re-wiring.

With regard to your 150 mm of insulation being compressed down to 100 mm, does this not reduce the insulating properties? I've heard that it does, but I don't know by how much.

[worsy]

p1esk said:

I've been attracted to the idea of removing our old loft insulation and putting celotex in instead. I know celotex is expensive, but the existing fibreglass stuff is messy and the untidy wiring system is mixed up amongst it, so I'd prefer to have a fresh start, insulation, electrics, the lot. Using celotex would also mean that the boarded area could sit directly on the joists, avoiding the need for spacers to accommodate a thick layer of insulation, so that looks like a plus point to me.

Incidentally, how does 90 or 100 mm of celotex compare with the often recommended 270 mm of fibreglass in terms of insulating performance?

Also, why is a vapour barrier needed? I haven't heard of such a thing being needed at loft floor level. Certainly there doesn't seem to be one in our loft at present.

I had a ceiling calc done by Knauf for 200mm of loftroll between 600 spaced joists. That came in at 0.22 W/m²K

I've just done a calc for you on the same with 100mm Celotex between 600 spaced joists and it comes in as 0.25. To get to 0.22 you'd need 120mm.

You have insulation over the joists so I expect your u value will be much lower than simply working out 270/200. This (http://www.vesma.com/tutorial/uvalue01/uvalue01.htm) suggests 270mm of rockwall and 12mm plasterboard is 0.14 which would require 200mm of Celotex.