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 for the info guys.

I'll call Devon CC tomorrow and see if they're accepting Tri-Iso9 at the mo.

The cost of the upgrade includes materials, installation and VAT - they're
charging about £230+VAT for the celotex alone (seems reasonable from ebay
etc), and then 2 man-days labour (seemed a lot, but maybe if it's gotta be
cut careful with no gaps etc .... ??).

Anyway, any thoughts on how to work out a pay-back time for the celotex
addition? 10-20 yrs would do me, but not 30+

Cheers, Dave


Answer is to look at the area covered and start doing sums.

You can assess the heat loss from average inside/outside temps..

If the roof is say 30 sq meters, then heatloss at .15 a compared with
..25 goes from 7.5w per degree C to 4.5W per degree C.

At an overall - say - 10C average temp differential, that's a saving of
30W, or, over a year 262KWh...at at least 4p per Kwh on any energy,
thats about £11 a year :-)