the air space is there for a reason. once the material changes its normal lenght, it probably will need space. this is due to temperature, also know as specific heat capacity The air space causes the window to transfer less heat. Glass is a relatively good medium for conducting heat, while air is a poor conductor. With single paned windows, heat can easily flow from the inside of your house to the outside environment, wasting energy. However, since air is a poor conductor, heat does not transfer as well through thermopane windows.