Joyce I have a couple Hydrofarm mats and all heat unevenly. The only mat I have that has even heating is my 5' x 3' and that's made from a different material. Instead of the thin black plastic it's a thick heavy duty rubber. I bought it from a farm supply store years ago. I even out the heat on the smaller mats by placing a 1" thick piece of wood over the hot spot and setting the pot on that. That doesn't help when using the heat mats for a flat but it works well with pots. I've never tried a thermostat to see if that makes a difference but don't think that it would.

I will double check which type of thermostat our tile man plans to use to be sure that it can operate with the floor sensor alone. There will be a heat vent in the closet, so I don't want that small room's temperature to interfere with controlling the floor heat in the adjoining bathroom. Re two sensors: We asked about getting a second floor sensor as back-up in case one fails. The tile man said he had never heard of a need for that, but he is willing to do that for us and he included the second sensor in his bid.

Weight isn't always the decision to buy. The amount of true "down" is what will determine warmth and weight. Plus the cover material. I bought two down comforters 15 years ago and they are still in great condition. Couldn't find them to buy another until recently, but they are called FairyDown from the UK. You can get them online now and still at a very good price. I also bought a sheep wool mattress cover, they are wonderful to sleep on. With this company you will know also where the down came from.

"I was definitely planning to use heat mats for my hot flowers (zinnia, celosia)." Absolutely. They'll do well at anything from room temperature to warmer, and prefer the warmer. Post sprout, however, I still relegate these to the cellar and the 60-65 regime; slow growth is fine for the development phase and tends to lead to bushier, stronger plants as I can't recreate full sunlight (I manage about 2500 lumen/sq ft, which is a terrible way to measure plant lights...but about 75% of the spectrum is usable under modern definitions). 17 hours a day. "Would you recommend grow lights in the 60 degree room before germination? I know that a lot of these need light to germinate and I didn't know if the ambient light in barn would be enough. So far the plan now is to set them up on shelves, NO heat mat, and let them do their thing naturally!" They probably won't need it; light doesn't mean much light, just enough to set off the seed. If it's bright enough that you don't turn on a light to see by to walk around in there, it's bright enough. If it's so dark a human can't see, they'll need some light...but not a vast amount before sprouting. Post sprout, almost everything you listed needs the brightest illumination you can give it from an LED, fluorescent, or HID source (I am NOT in love with the HID spectrum, which is not optimal for plants, but was historically used because it was the best thing going at the time; today, any cheap LED is better). People will argue spectrum. It doesn't matter; warm white, cool white, for that period of time, go for intensity over a favored output in your price range. Plants use red AND blue, and young plants use both very efficiently; yellows and greens less so (although they do use them). And don't forget you'll need a hardening-off period when they go outside; they'll never have seen a day under 63, probably a day much over 70, rain, full sun, or a wind. Plan on a week to ten days for that.