What insulation for new loft, and how to calculate U value?

[antsbradley]

Hi,

Fitting out a new 2 story extension, and the building specs say roof needs a U of .16, and mentions 300mm of mineral wool.

The joists are only 70mm deep, so 300mm would need some serious packing before I could put any flooring down (just for storage), and its not a very high pithch to begin with anyway.

So what other insulation products shall I be looking at that offer the required rating with less thickness at reasonable value? I guess celotex / kingspan is one option, but I know its not cheap.

Also how do I calculate the U value, all the insulation I have seen lists its R value. Does the insulatining properties of the plasterboard come into the U calculation also?

Many thanks

Anthony

 

[chappers]

even if you could cram 300mm of insulation into 70mm it wouldnt have the same u value as the u value is obtained due to the entrapped air.
Every element you add has a u-value including flooring , plasterboard etc these are available on the net.
from a practical point of view something like tri-iso 10 with rockwool between the rafters and 70mm mineral wool between the joists should be more than enough.