Last year we had cavity wall insulation installed. This is the third house we have done this with, and for the third time it has been extremely effective at reducing the heating bill.
January 2008 had an average morning temperature (my measurement taken at whatever time in the early morning that I happen to have looked) of around 5 C. This winter has been the coldest for over a decade and the average temperature taken in the same way in January this year was -0.5 C. Despite this, the gas used was 281 cubic metres this January vs. 274 last January.
I didn't write down the temperatures for December 2007, but remember that the lowest temperature I cycled in was -6 C vs. -11 C this winter. However, our gas consumption was 396 m^2 in December 2007 vs. 298 m^2 in December 2008.
Overall this seems to have been a very effective measure once again. It looks like even though it cost us around 700 Euros to have the insulation installed, it will pay us back in about three years. It's much cheaper to have cavity insulation installed in the UK, so the payback period is even shorter.
We still have more to do here to get the heating bill as low as we'd like, though.
On the right, another photo of the frozen canal taken yesterday. It snowed a little. The snow was quickly cleared from the cycle path, but rested on top of the frozen canal. I wonder how much insulation the house boats have.