Dave Rowell wrote:
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

Ask the architect/builder how many 8' x 4' sheets of Xtratherm or
Kingspan or Celotex (all same product near enough) are required. To do
this properly you need 6" of flexible/on a roll type insulation between
the rafters, then 50 mm celotex under the rafters. You may not have the
depth of rafters or headroom to do this so you will have to go 50mm
celotex between the rafters and 35mm under the rafters. I would advise
to get a breathable roof membrane installed as then you can get away
with a 50mm gap between the insulation and this membrane.
I get 8' x 4' sheets of 50mm celotex for £20 incl vat incl del from
my local builders merchant - had to haggle a bit. Or, look on ebay
(search for insulation).
It the conversion has flat roofing near the roof apex (i.e. a small
loft area) then fill this void with flexible insulation to save money,
or rigid if you want a storage area.