Insulating the attic with rockwool batts. In the US the products and company is called Roxul.
To keep the heat in a house, it is very important to prevent heat slips out through the roof.
My PlayHouse is a channel where i will show, what i am working on. I have this house, it is 168 Square Meters / 1808.3ft² and it is full, of half-finished projects.
I love working with heating, insulation, green power, alternative energy, solar, wind and more. It all costs, but I'm trying to get the most out of my money, and my time.