My house was built with a room in the loft. On either side of the room there's a low wall (dwarf wall is the technical term). This forms voids with triangular cross-section that I've filled with insulation.
The voids are ventilated with air from outside, so I also sealed the small access-doors in the dwarf wall to prevent cold air getting into the top room.
The most difficult part of the whole thing was getting into the space. I put down planks and bought knee-pads, and put up a shiny board to prevent myself scraping the existing insulation from the back of the dwarf wall. My dad helped me as well.
As well as keeping the house warmer in winter, I'm hoping that the insulation will also help keep the house cooler in summer. Do you think it will?