Celotex GA4000 as loft insulation? Foil problems?

NewtoDIY

I'm still looking at insulating our loft space as a cold roof and in order to keep some storage space, am thinking that I can lay sheepswool in the 60mm deep joists and then on top of this across the joists, Celotex GA4000 100mm boards (and then chip boards over these)

The only thing that I'm unsure about is that these are foil facing boards and I don't know whether these will cause a problem and also, which way they should face?

The rooms underneath are heated so through the lathe & plaster ceiling, the the first bit of insulation would be the wool and then the celotex (or equivalent). Is it going to cause huge dampness problems by having the foil face downwards onto the sheep's wool? Or should it face upwards into the cold loft space?

Any ideas would be appreciated and also cheaper alternatives to the celotex?

Thanks!

muckybutt

If you intend to use celotex on top of the joists you will need to raise the level of the joists to the same level as the celotex, laying them across the joists then putting flooring on top, all you will do there is crush the insulation.

You would be better off using the celotex in the rafters.

There isn't a wrong and right way of putting them in, both the foil sides are the same. As for damp, provided there is still a source of fresh air in your attic you should be fine.

NewtoDIY

muckybutt wrote: »

If you intend to use celotex on top of the joists you will need to raise the level of the joists to the same level as the celotex, laying them across the joists then putting flooring on top, all you will do there is crush the insulation.

You would be better off using the celotex in the rafters.

There isn't a wrong and right way of putting them in, both the foil sides are the same. As for damp, provided there is still a source of fresh air in your attic you should be fine.

Thanks for your quick reply. I thought that Celotex PIR boards can take weight if they are then covered by loft boards? It's more of a crawl space as I#ve only got 80cm head height! I was doing a costing last night for 200mm space roll insulation and realised that 100mm of celotex is about equal for the m2 I need.


It's not possible to increase joist height because of the lack of head space.

NewtoDIY

Just spoke to Celotex tech support who said that it is no problem to place Celotex over the joists but, that there should be a layer of boarding first, then the celotex, then another layer of boards. They said it doesn't matter which way the foil faces also.

Captain_Flack.

muckybutt wrote: »

If you intend to use celotex on top of the joists you will need to raise the level of the joists to the same level as the celotex, laying them across the joists then putting flooring on top, all you will do there is crush the insulation.

You would be better off using the celotex in the rafters.

There isn't a wrong and right way of putting them in, both the foil sides are the same. As for damp, provided there is still a source of fresh air in your attic you should be fine.

Not if the joists are insulated and the loft is unheated.

NewtoDIY

Which is my plan - a cold roof space.