m Ransley wrote:

Cellular shades have up to R 3. Curtains with insulating liners help,
liners are sold separatly. 1" R7.2" foamboard popped in at night is easy
to do. Dont leave it in when the sun is out, I painted one dark and it
cracked the glass. Paint the foamboard with latex and store when not
used.

I remember using foil-covered foamboard on windows.

That reminds me: some of my windows have internal screens that
are easy to pop in and out. If I fastened foil-faced kraft
paper to the glass side of a screen, that would give me an air
space with an R value of about 3.2.