Loft insualtion with celotex/kingspan

[p1esk]

worsy said:

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.

Thanks for that, but I don't really want to use more than about 100 mm of Celotex for practical and cost reasons. Incidentally, our joists are 100 x 36 mm at about 450 mm centres.

[rb5er]

Vapour barrier can be just taped plastic sheeting,foil backed plasterboard or PL celotex (insulated plasterboard) as long as its taped and jointed or skimmed.

Dont stand on it, it will break and you will go through the ceiling below.

[worsy]

p1esk said:

worsy said:

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.

Thanks for that, but I don't really want to use more than about 100 mm of Celotex for practical and cost reasons. Incidentally, our joists are 100 x 36 mm at about 450 mm centres.

100 at 450's gets you 0.27.

rb5er said:

Vapour barrier can be just taped plastic sheeting,foil backed plasterboard or PL celotex (insulated plasterboard) as long as its taped and jointed or skimmed.

Dont stand on it, it will break and you will go through the ceiling below.

All these calcs assume

Polythene, 1000 gauge, VCL + Air Leakage Barrier
Knauf Wallboard

[eniacs]

Mojoo;

I've done the celotex in the loft thing, slightly differently to you however. I replaced the ceilings as you did and ditched all the old itchy stuff along with the vast amounts of plasterboard and dust.

I put up a layer of insulated plasterboard - the thickest type available, I think it was 60mm celotex/kingspan with 12.5mm plasterboard glued to it. Then insulated between the joists above with 90mm boards. I've foamed all the gaps and taped over with aluminium tape above as well.


It is a costly, time consuming and fiddly/complicated way of insulating the loft... I still worry about putting things directly on the rafters/boards and have left a load of planks up there to place heavy items on. (empty boxes can easily sit on the boards).


Next time I will be putting down new beams accross the rafters, stuff extra rockwool between them, and screw a new floor above that. Cheap and easy!!!

[227bhp]

The general rule of thumb when comparing glass wool type insulation to the rigid type like Kingspan span is 50%.

If glasswool loft insulation is supposed to be 260 thick then Kingspan will be 140 because it has twice the insulation value for the same thickness.

Why is a vapour barrier needed in a ceiling?

[Mojooo]

my understanding is that the silver foil on the underside of the celotex between the celotex and the plasterboard ceiling can cause condensation.

I havent got round to doign this yet but I was thinking of just using mineral wool stuff and putting soemsort of breathable roofing membrane over it to keep the nastiness away and using bits fo timber to increase the floor before board it out.

[akaAlso]

227bhp said:

The general rule of thumb when comparing glass wool type insulation to the rigid type like Kingspan span is 50%.

If glasswool loft insulation is supposed to be 260 thick then Kingspan will be 140 because it has twice the insulation value for the same thickness.

Confirmed... served me well as a rule of thumb for many years

227bhp said:

Why is a vapour barrier needed in a ceiling?

To prevent humid air getting through to the cold surfaces above where it will condensate.
I'm sure an approved document used to advise having a vcl (Vapour control layer) in just bathrooms and other high humidity areas, but I can't find it at the moment. I get asked frequently whether foil or polythene backed plasterboard works, but it's recommended polythene be used with a 50mm taped overlap, so unless the boards have a selvedge overlap they won't be as effective. It also recommended to seal services passing through the ceilings in the humid areas and loft hatches as well. Effective mechanical ventilation also helps to get rid of humid air.

[p1esk]

This VCL business has got me a bit worried.

Our bungalow, built round about 1960, appears to have plain plasterboard ceilings (roughly 12 mm thick by the look of it) and about 100/150 mm of fibreglass insulation - the thickness varies in different areas - which was put in about 15 years ago, but there is no VCL.

Given that we introduced a good system of eaves ventilation when we had some roof work done earlier this year, I'm still wondering if we actually need a VCL. Does the need for a VCL increase if Celotex replaces fibreglass insulation?

[akaAlso]

It's only as houses become more airtight and highly insulated that these problems come to light.
If you don't have a condensation problem in your roofspace then you obviously don't need a vcl.

[Pheo]

p1esk said:

This VCL business has got me a bit worried.

Our bungalow, built round about 1960, appears to have plain plasterboard ceilings (roughly 12 mm thick by the look of it) and about 100/150 mm of fibreglass insulation - the thickness varies in different areas - which was put in about 15 years ago, but there is no VCL.

Given that we introduced a good system of eaves ventilation when we had some roof work done earlier this year, I'm still wondering if we actually need a VCL. Does the need for a VCL increase if Celotex replaces fibreglass insulation?

Think also celotex is a particular problem because of the silver foil. Most houses have plasterboard and fibreglass with no issues...

[p1esk]

akaAlso said:

It's only as houses become more airtight and highly insulated that these problems come to light.
If you don't have a condensation problem in your roofspace then you obviously don't need a vcl.

Thank you.

We did have a condensation problem, all over the place actually and not just in the loft, but we have overcome that by getting plenty of air circulation. I like the idea of having good insulation, but I'm not a believer in this fashion for airtight houses; my preference is to have a good breeze through the place when outside conditions are suitable.