> Can be a 20, 30, 40 year payback on insulation. For the builder?

In the UK, houses have energy ratings, which are largely not that useful, but they do allow estimated annual running charge.

The house that I live in we moved in and were spending ~1.7k on gas a year.

We needed to re-render the place, because it has a few missing pieces. we spent the extra £4 to put in 90mm of external wall insulation. We also had to replace the glazing. It was cheaper to get triple glazing (for some reason), however the results of that was that it was 6degrees warmer in winter, and 10 degrees (celcius) cooler in summer. Even with gas prices doubling, we spend about £70 on hotwater and heating.

There's also the simple reality that houses with better energy ratings are worth more when they are sold. If you own a house, it's a good way to lower your bills and increase the value of your house. The only thing you know for sure when you pay hundreds per month for gas is that it goes out of the chimney. Over ten years, that adds up. Most houses you can probably do some sensible things that definitely earn themselves back in that kind of period while also increasing the value of your property. The inconvenience and financing tend to be the big obstacle. Add incentives to the mix and it becomes an easy choice in a lot of places.