Boarding loft 'Floor' - Rigid Insulation Question

[headworx]

Hiya peeps,

Had a look around but dont think this question has been asked before so here we go:

I've ripped out the 30yr old stuff and laid insulation along the rafters (the space blanket stuff) 200mm.

Now I want to place the rigid PIR insulation board up and then board it.

Basically I've seen the Knauff Space Board (50mm)
http://www.wickes.co.uk/red-plastic-wallplugs/invt/161280/

But have also seen the 50mm Celotex 1200x450mm which is foil both sides.
http://www.wickes.co.uk/red-plastic-wallplugs/invt/190546/

As they are both the same price, can I use the celotex stuff instead of the space board (they seem to be the same rigid strength) and then board on top? I just need someone to confirm they will be able to withstand the weight of the boarding and some boxes on top.

Am I right in presuming if the Celotex is as strong as the spaceboard then it will be better as its foil backed both sides? Prob be a better U value too?

Or will the foil cause vapour issues?

Thanks in advance my good fellows

 

[freddiemercurystwin]

I'd go for the Celotex, God knows what the other stuff is.

That said you should bear in mind that 50mm of PIR (celotex) is only equivalent to about 75mm of mineral wool.

So if you were to attempt to fit the government recommended thickness of 270mm mineral wool (3 layers which is the standard upgrade and fitted by the energy companies) you would need around 180mm of celotex.

 

[headworx]

Cheers Freddy - Very helpful as usual.

I have already put 200mm space blanket down so with this extra 50mm celotex then were looking at 250mm insulation (plus the fact that the celotex as you say is better) So were then looking about 270mm total.

As the boards are gonna be on top that will add a bit too I imagine.

I reckon that should do. Just dont want to end up with a loft with all insulation and no space to store stuff.



So just to confirm, you would think the celotex would be strong enough to walk on after the boards are on top as I'm laying them across the joists ( I dont mind if they sink a bit on the joists as long as its safe)

I've read the celotex has a compression strength of around 17pounds per square inch so I thought it would be alright.

Hopefully I can get the celotex a bit cheaper too