Mark wrote:

I feel better about my south facing windows now.. (here in PA in the winter)

They can more house heat on an isolated sunspace that gets cold at night.

If you add in wind chill factor and radiation cooling losses and the
fact that most skylights or windows are not aligned directly to the
sun, they may end up being a net loss but not a hugh loss.

Where I live near Phila, 1000 Btu/ft^2 of sun falls on a south wall on a 30 F
average January day, so a square foot of R2 window with 80% solar transmission
will gain 800 Btu and lose 24h(65-30)1ft^2/R2 = 420. On a sunspace, it might
only lose 6h(65-30)1ft^2/R2 = 105.

Nick