We're having a loft conversion done - work starts tomorrow - and I'm
wondering about upgrading the roof insulation they've specified. They use
TriIso9 as standard, and it could be upgraded by adding 50mm of cellotex
between the rafters . This would reduce the U-value of the roof from 0.25 to
0.15 according to the engineer. Cost of the upgrade would be £630 - ouch!

Can anyone hazard an educated guess as to what the likely pay-back time will
be? It's a 3-bed semi, and we're miserly with our heating!
Or is a compromise worth considering? Eg. only upgrading over the new
landing, or only the north-facing side of the roof.

Thanks, Dave