Fully spray foamed between the hull and the lining, everywhere, under the floor, in the sides, and the roof. And only tiny portholes instead of huge thermally inefficient windows.
And I have 2 hot water tanks, one heated off the engine, the other heated off the vaporising diesel stove, which also heats the central heating.
And if I CBA with all that, I simply switch on the diesel generator, and put a few electric heaters on.
I can easily heat the inside of the boat to 45C if I want to, irrespective of outside temperatures.
Good deal more comfortable than the mud hut that you live in!