Conduction and convective heat transfer base on heat conductivity of air between glasses. There are 2 principal methods to reduce the conductivity: A. Add thickness of air. Conduction conductivity α=λ/δ(λ= conductivity of air, δ=thickness of air), so you can see that add the thickness of air could reduce conduction conductivity. Experience shows while thickness under 10mm, heat lost through conduction transfer more. So add the thickness to 12mm is the best way. Moreover, manufactory adds space sometimes means multi insulated glass or instead normal glass with ply glass. B. Change the air. Filling low conductivity gas such as Krypton or Argon to reduce heat transfer between glazing layers. But this method is too expensive.

   All technical above could save energy effective, but it limited by weather, area environm.