i just wanted to share another way to use thermal mass that I've recently found to be quite effective. My homemade greenhouse has single pane tempered glass on the roof and as you might expect it doesn't hold heat very well overnight. So instead of trying to heat the whole 12x16 foot greenhouse I made a rectangle on the ground using 5 gallon vegetable oil jugs full of water then I put my seed flats and dahlia pots inside the rectangle. At night I cover the whole thing with old blankets and deck chair cushions for insulation.

I've just been doing this for about 4 days but it seems to be working quite well. There was one night where the outdoor temperature dipped to around 20F after a sunny day. The temperature inside the greenhouse dropped to around 30F but under the blankets the temperature stayed over 50F. Two days ago was a sunny day and the water warmed up nicely. The outdoor overnight low was 27F but under the blankets it was 54F. Yesterday was rainy and overcast so I didn't uncover the pots at all. This morning the temp in the protected area was 49F after two cold nights and a full day with no solar gain at all. (The overnight low last night was only 36 so that helped slow heat loss too.) Today is forecast to be 54 and partly sunny so I have the jugs uncovered hoping for a solar recharge.

Obviously this won't work for tall plants but for getting an early start with seedlings or potted dahlias that can fit under the 14 inch height limit of the jugs it works pretty good.

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I use a long 4x4 to keep the blankets from sagging onto the pots.
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All bundled up for the night.
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Older picture of my greenhouse
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