Decades ago I read a book about winter gardening in Maine. That author had really big, stand-up hoop tunnels with heavy plastic film,
Under that, floating row covers.
Under that sometimes, cloches.
He figured that each layer gave him some number of degrees more warmth, at least in the coldest part of the night. But it was important not to let the heat out, especially in the late afternoon and at night.
I really like the idea of bubble wrap, a blanket or even polyfilm over seedling trays and heat mats. Anything that keeps air from floating up and away keeps a lot of heat from escaping, and heat mats don't put out very much heat!
Looking around online, I see heat mats for home growers rated at 17, 22 or 25 Watts per tray. One commercial brand had models rated at 40 or 50 Watts per tray - thermostats must REALLY be necessary with those!
The "hobby" brands suggested that they could raise the soil temperature by 10-15 F, but never specified whether that was with or without insulation and a heat dome or tent.
I always have drywall (also called plasterboard, wallboard, gypsum board or sheetrock) under my heat pad. Capturing the heated air is so much harder that I usually don't try, except for trays that still have a humidity dome on.
But I really like the idea of draping something over the lights, the trays, the heating pads and everything.
