Windows, even new ones, can lose a lot of heat around the sides of the window through gaps in the trim,. You can fix that easily and cheaply with a tube of paintable silicone caulking. Simply seal all the gaps in the window trim around the windows of your house (ie; between the trim and the wall, the trim and the window) to dramatically improve the air tightness of any home. The cost of the caulk will easily be recovered in a winter month with lower heating costs.