m Ransley errs again:

Skylights are like a hole in your ceiling letting out heat. Since heat
rises no glass made is considered energy efficient enough for ceilings
in high heat Zones.

Nonsense. InsUlation only slows heat loss. If each layer of glazing
blocks 10% sun and adds R1, the net loss for an N layer skylight on
an average Dec day in Chicago is 0.9^Nx460-24h(65-26.7)/N Btu/ft^2,
ie any skylight with more than 2 layers will have a net gain, vs a loss
for mere insulation.

Nick