Originally Posted by
Duncan74
The stated figures are for the particular product. So if it's supplied in 100mm and stated R value of 5 then that's the same as another product of 300mm and R value of 5. However, if you put two layers of the 100mm in then it would be about twice as insulating as the 300mm version (unlikely to be exactly twice as insulating in reality, but as near as damn it.)
This method is used below and between joists or between joist/rafters and above and it helps greatly with thermal bridging.
David