I made a small 4 seasons greenhouse 8ft by 12ft (max I am allowed) but I didn't know to insulate the side walls. Then this year, I found videos of researchers on 4 seasons greenhouses at universities in the north central USA. Their greenhouse are at least twice the size and usually much bigger than that. They insulate their greenhouse sidewalls down 2 to 4 ft below the surface. They generally fill under the greenhouse with gravel or clay and then they blow air from the greenhouse into the gravel during the day to store its heat (and condense some of its water) . Heat still escapes downwards through the gravel by conduction. Why not use water as your fill material? Water has a higher heat capacity than gravel. And rainwater is free while gravel here is $70 or $80 per yard plus delivery. And if you had a meter or 4 ft deep water tank under the greenhouse, wouldn't the conduction of heat down be slowed down by convection moving heat up? I realize it's a bit more complicated to have a radiator and fan to cool greenhouse air rather than just a fan blowing the air under the soil but maybe it can be worth it? (I have a water tank under my greenhouse because we get drought in summer, so it was practical to store water in it). Is the risk of it freezing too great in a 4 seasons greenhouse in the central states? Anyway, I'd love to know how much convection slows heat flow down in still water compared to in soil or gravel. I looked online but found nothing then I asked some lake researchers and lecturers in emails a few months ago and I never got a reply. (Thermoclines are aparently a big deal in many lakes and these people would know). Any of you have the figueres or a formula? Thanks Brian
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