Loft flooring & Kingspan

[ironsidebod]

Hi,

Sorry another loft floor question!

I'm looking at putting down 18 or 22 mm P5 flooring in my loft, done the job in previous houses but in this one the roof height is a bit lower so no cross battening. The setup of the loft would easily take the T&G screwed down onto the joists but I would loose some insulation.

Is it possible to use the sheets of celotex/kingspan on the joists and drop the P5 on the top (screwed down obviously)? Is it sturdy enough to take the weight of storage items and walking used like this?

Thanks

 

[foxhole]

Yes very strong under compression.

 

[ironsidebod]

Yes very strong under compression.

Thanks, you think this is a go-er then?

 

[oldbutnotdead]

You might change your mind when you see the price of the stuff!

 

[ironsidebod]

You might change your mind when you see the price of the stuff!

Well yes I know it's not cheap!! Just trying to find the best solution...been so easy in the past and subsidised insulation last time I did it.

 

[oldbutnotdead]

Mmmm. Do your sums & see what works out most cost-effective- the price of Celotex is not a linear relationship (ie 50mm is not half the price of 100mm). Even non-subsidised rockwool is about 1/3rd the price to achieve any given U value- if you have 6" joists up there you could have 150mm wool between the joists & 80mm Celotex above.

 

[ironsidebod]

Mmmm. Do your sums & see what works out most cost-effective- the price of Celotex is not a linear relationship (ie 50mm is not half the price of 100mm). Even non-subsidised rockwool is about 1/3rd the price to achieve any given U value- if you have 6" joists up there you could have 150mm wool between the joists & 80mm Celotex above.

That's very helpful thankyou.

 

[ironsidebod]

Doing some sums, there's a couple of options. The existing joists are 100mm with what appears to be 270mm mineral wool giving a U of ~0.164. If I keep 100mm of mineral wool, drop some 80mm Celotex over the top it gives a U of ~0.17. 100mm Celotex gives a U of ~ 0.148. As I understand the target would be 0.15 ideally?

The width of loft to board is 2.44m, doing this means no cutting of the PIR, no cutting of the flooring sheets. Perhaps a few noggins if the existing joists don't run on nice centres.

Anything else loses more height and/or requires cross battening increasing the cost and messing about cutting sheets I think.

 

[oldbutnotdead]

Sounds like a plan- go with the 100mm while you're at it. And lucky you, cutting PIR is a rubbish job.By the way, how are you going to get 2400 x 1200 sheets into the loft

 

[JohnD]

Is it possible to use the sheets of celotex/kingspan on the joists and drop the P5 on the top (screwed down obviously)?

It used to be a recommended method, and I had an installation brochure from a German manufacturer aimed at the UK market.

Since Grenfell, all such recommendations have been withdrawn.

Glasfoam is still suitable, but seldom seen in UK.

 

[ironsidebod]

Sounds like a plan- go with the 100mm while you're at it. And lucky you, cutting PIR is a rubbish job.By the way, how are you going to get 2400 x 1200 sheets into the loft

Thanks. Yeah the very small bit of work I've done with stuff didn't fill me with joy at the prospect of cutting it into strips.

Just fitted a nice big hatch/ladder so fingers crossed they'll go up in one