When Blooie first posted the concept of brooding with a heating pad, my imagination went wild. (See "Heating pad for the Brooder" on this forum) I imagined a heating pad brooder cave to brood a couple dozen chicks could be fashioned out of a camping foam pad with Mylar heat reflective material glued to the inside surfaces, the principle being that the body heat of the many chicks would be reflected back at them from the top and sides, requiring only a minimal heat source. I did go on to glue the Mylar "space blanket" material to the inside, and it worked beautifully. The chicks had the option of spending time on top or under the heating pad. As long as the thin Mylar was glued down, the chicks left it alone. I didn't put it on the roof, but I see no problem with placing it on the rack in the area not covered by the heating pad. It would help in reflecting heat back downward, thus maximizing the heat being delivered by the heating pad. I don't know the wattage, but it shouldn't be much more than a 100 watt bulb. Maybe less.
