I have a 900 sq/ft cottage in Kenora Ontario, it sits mostly empty from Nov - Apr each yr with the heat on. It would be impossible to get the water system and toilets running when we go out every 2nd or 3rd weekend during the winter (to check on it and enjoy some ice fishing etc) if we turned off the heat and let it freeze.so the heat stays on at the minimum setting of 45 f. Last yr that cost me $500 in heating oil, this year I suspect it'll be WAY MORE since oil went from 80 cents/liter to 93 cents/liter and the Wx has been brutal here....we are in week 2 of -30 to -40 and colder, today it was -33 c and expecting -36 tonight. So as I was strolling through Home Despot I wondered about making an insulator blanket of sorts for the windows. There are several and they are BIG. All triple pane low E argon but hey its COLD! What I want to do is take 1" Styrofoam Sm (the blue stuff) and glue some heavy duty aluminum foil to one side and trim the edges with the flexible metal tape and custom fit these to each window, completely filling the frame with the foil on the inside. This would give me R5 for the Styrofoam and I think the alum foil would reflect heat back into the cottage. The windows are always the biggest heat sinks in a house! The walls are currently R20 the roof has R60. I'll post pics of the MBR window (about 10ft wide X 5 ft high) and the living room patio door (5 ft) both face east so no solar gain to be had. What do you think? I had the thin plastic film on the windows last yr and I don't think it adds much in terms of R value. I would take these out of the window openings when we are there and using the place.