I'm looking for suggesting for upping the r-value on top of my radiator. Every window in my apartment has a radiator underneath it. I'd like to have some plants by those windows ('cause y'know, they need light and all) but I'm concerned about heat from below drying & killing the plants. A couple of the radiators have covers, but they're metal, so no insulation there. Ceramic pots may provide some insulation, but I'd really like to lay down a layer of some kind of building insulation proper just to be safe. The question is, what kind? Batting won't work, and the foam insulation panels I've seen at the hardware store I know are a fire hazard... plus not too fond of plastic-type materials. My dad suggested something he called "thermal block" which I believe is aerated concrete panels, but which I can't find a source to purchase. I'd like to spend less than $20, preferably less than $10 on this. Please help!