In reply to gingerkate:
> Is there a rating system for how insulating an insulating fabric is?
As Alicia says, there's the R-value (commonly used for building insulation; have a look when you're not under the floorboards...)
For clothing, the
clo is commonly used. Then there's TOG.
You might ask the nice people at your local university; the outdoor clothing lot. I think they might be able to help.
I suspect that, for curtains, you'll want to stop radiant heat loss, whereas clothing also needs to consider evaporation, convection and conduction losses.