Insulation, insulation, insulation

They say the three most important things about buying a house are Location, Location and Location. Well the three most important things about keeping your house warm are Insulation, Insulation and Insulation (although I guess you need a boiler too!).
I was doing some more insulation yesterday, I put glass wool between the joists on my ground floor. Why? Well I have a vaulted floor with about 450mm of space below it, and this space, as with all houses, is vented to the outside via some vent bricks to allow air to flow and to keep the space dry. Well at the moment I don't have carpet down so two things happened when it was cold or windy. Firstly, you get drafts coming through any gaps in the floorboards (the boards are tongue-and-groove but there are still spaces in some places). The other is simply that heat from the living room is conducted downwards into the cold void. Even carpet does not insulate that well, although it can reduce drafts, otherwise we would just use carpet to insulate our lofts.
To fill the cavities, I made a hole in the floor and crawled under the void. I pushed the glass wool into the gaps and then covered the underside with a large tarpaulin which I nailed into the joists to hold the glass wool in place. Note that my floor joists are only 100mm because they are supported by a small wall in the middle and I assumed that they would be 200mm because they were floor joists so I had to cram the glass wool in a bit. Better check the size and get the correct insulation because if you crush too much air out of the wool, it doesn't insulate as well.
Another area I insulated was the floor void between ground and first floor, particularly the part next to the outside wall where it is common to have gaps between the inside of the void and the external wall cavity - another place for drafts and cold. I stuffed some more glass wool into these gaps and it should make a bit of difference.
I also bought a couple of curtains to cover my front and back doors which although U-PVC do leak drafts around the edge. The curtains help to remove the drafts and slow convection of the warm air inside the house onto the door which then conducts outside.
Once the loft is insulated, none of these other fixes make a big difference by themselves but once added up, they do make a noticeable difference.