- 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.