insulation board: is one thick the same as two thin!

[neilw]

Hello,
Would 100mm of rafter insulation board (kingspan tp10, celotex ga4000, etc) result in a better, worse or just the same thermal value than 2x50mm boards?

I'm converting a loft and I'm now getting round to ordering the insulation. After contacting all the suppliers (kingspan, celotex, etc) they all calculate than 100mm between rafters (50mm air gap plus two lots of 50mm insulationt) and 20mm over rafters plus the wall board will give a u-value of about 0.23, which is under the 0.25 requirement.

However, after pricing I found 100mm is a fair bit cheaper but I've contacted them that many times I'm embarrassed to email again

Not to bore* with the details I only got them to provide values for 2x50mm boards, but looking at the prices I can get 100mm a lot cheaper.

Thanks.


*If you really want to know, it's because I'm doubling up the current 100x50 rafters with 150x50 and so 100mm would need to be notched.

 

[freddiemercurystwin]

Using the Celotex 'U value calculator' (which you can also use yourself)

the results are as follows

400mm rafter spacings: 0.24
450mm rafter spacings: 0.23
600mm rafter spacings: 0.23

 

[mointainwalker]

The u value is the number of Watts lost through the insulation, so a result of 0.23 is better than 0.25, so 100 mm board exceeds the requirement.

2 x 50 is the same as 1 x 100 unless you make a sealed air-gap which improves the insulation.