I wasn't sure which board to ask this on...

I am planning a greenhouse for year round growing in zone 5- with plenty of cloudy days in winter.
It will likely be cedar framed (or pt, something slow to decay)as there will be poly attached to both the outside and inside for insulation. I don't like PT but any wood in a damp airlock will rot and I figure 100% safe from veggie contamination behind a layer of poly.

Goal is no electricity, just using alternative methods- including hardy winter plantings to extend growing season as far as possible.

Plan is approx 25' x 40', more or less. East to west.

The pond question is... any thoughts on whether and approx 3-3.5' deep pond, maybe 4' x 8' located centrally in the GH help significantly at all as a heat sink? A larger pond is of course possible so long as I can keep it productive enough to produce a similar amount of food as the row crop displacement which I'm sure shouldn't be a problem.

Any ideas appreciated! Thanks.

Certainly. A 3.5 x 4 x 8 pond is 3200 liters... That's over 13 megajoules of energy per degree Celsius. The question is... Do you get enough solar gain to more than offset your heat losses at your design temperature?

All the thermal mass in the world won't help if the goes-outs are greater than the goes-ins.