Oh yea,...those windows that allow heat to come in so well during the day,..act almost like a heat sink to pull heat out of your house at night,..a lot of it is lost thru radiant heat loss,...(ever see an infrared picture of a house? look at the windows! esp ones with no covering,..) so if you can,..cover those windows with "something" best is something called WINDOW QUILT and another (a kit) is WARM WINDOWS both can be seen with a search from GOOGLE or similar,..if you can't swing that,..hotel liquidatators (.com?) can sell you those THICK hotel drapes with the hardware really cheaply,..(they pull em out of good hotels and often they look like new for a fraction of the price) if you stand naked in a glass box and its 80 inside the box and zero outside,..you will start to shiver in a very short time,.lining just a sheet of paper on the inside of the box (still 80 degrees inside ) and you can be in that box for a very long time before you start to shiver,(and YES you WILL start to shiver after a while at 80 degrees if you are naked and not moving)...cause the radiant energy isn't being pulled from your body like an anode,..and enough bounces back to feel warm,. hope it helps in what you decide to do. BTW you can do a "poor man's infrared scan" by taking a portable stove and on a very cold day boiling a lot of water as quickly as you can,..note WHERE the water condenses,..(generally the windows,..even the double pane low E argon ones,...often you can find where they skipped insulation and even see the studs in the wall,..(use a small fan to stir the moisture around a bit,.) The gasman was right,..all combustion produces water,...every ten gallons of gasoline your car burns produces about a gallon of water,..an unvented heater I think it would be a mess in that room! not to mention poor air in your home.