As I understand it, you get "free" insulation, but all the reductions in fuel bills as a result of the insulation are channeled off to pay the for the insulation.

The loan for the insulation is against the house, not against you, so if you sell the property, then the new owner's fuel savings are still being channeled away to pay the outstanding loan on the insulation.

Presumably at some time in the future, the insulation will be paid for and some future owner of the house will start getting the benefit of the fuel savings.

You would also want to take a good look at restrictions on who can do the work. The laudable aim to make sure the work is done to good standards sometimes means that you're tied to large national firms and you can't use the local firm who you know and trust.

I've seen some suggestion that future payments for people to instal solar panels will only be given if the house has already been insulated to approved standards.