If you crawl into this space and shut the opening you see dozens of cracks and holes. Be sure you do this when the weather is very dry. Rain or moisture will swell the wood and seal the cracks temporarily. Some may even be in the roof. These are all areas that need to be sealed or insulated.
Windows are another area of great concern.
If you don't want to pay the high prices for storm windows, simply pick up a roll of plastic at the hardware store. Cut squares the size of your window frames and secure with masking tape. Then watch the electric bill drop. Using thin strips of foam around the edges of doors will stop most drafts. For the bottom of a door take a wide piece of cloth and sew it into a tube.