What's your roof situation like?  Is it flat, and barren of snow?  If so, can you get snow on it?  I'm imagining it's flat.  Maybe that's where you are losing most of your heat.

I have a poorly insulated roof on the lean-to type extension on my house, which is where the kitchen and bathroom are.  I notice a big difference when there's a foot or so of snow on it, as opposed to when it's bare. 

Unfortunately, because the temperatures don't stay cold here, I have to remove the snow from the roof from time to time to eliminate the ice dam at the eaves trough, which, if neglected, will actually cause leaks in the roof as the melt water builds up.