Q. How big does an air space need to be before it stops working as insulation and starts working as a convective loop?

A.Engineer Joe Lstiburek, a principal with Building Science Corporation in Westford, Mass., responds: The answer is 1/2 inch. Still air is an insulator. The thicker the layer of still air, the greater the reduction in conductive heat transfer. However, the greater the gap, the easier it is for buoyancy forces (the stack effect) to overcome boundary layer friction effects and create fluid convection flow. If the width of an air space measures 1/2 inch or more, the heat transfer convective losses become greater than conductive heat transfer reductions.

The situation changes if the air is replaced with argon or krypton, since those gases have a much lower conductivity and a much higher density, making-them less prone to convection. With argon and krypton, the bigger the space, the better. The only limitations are cost (argon and krypton are expensive) and the technical difficulties of making an effective seal.