U-value of expanding foam

[Frogeye]

I've got quite a few small pockets of space to try and insulate within a pitched roof. I've got Celotex between the rafters and more to come under them, but I've also got some really awkward corners where different roof angles meet.

I've got the option of perhaps shoving a fistful of rockwool in them or filling up with some expanding foam

....which leads to me to wonder what is the general consensus of the U-value of expanding foam?

Edited May 24, 2016 by Frogeye

[jack]

At least some Soudal foams give you the specific insulation value.  

This is one of the products that does so: http://www.screwfix.com/p/soudal-genius-insulation-foam-hand-held-750ml/25943

There may be others.