window insultn. / blanket question

We have a cabin that we use about once a month during the winter.
I keep the propane heater in the bathroom set to about 60*F when we are gone and shut the door. There are a couple of large windows in there. Could I hang up a big insulated blanked over the windows or would this create condensation or frost on the inside of the glass. The windows are brand new so I sure don't want to damage them but if I can cut down on the amount of propane I use that would be great.

It is a direct vent propane heater on the wall.