Peter Gedeon

The honest answer for zones 4–6 is that 365-day growing is possible, but it's usually a heating-bill decision more than a gardening-skill decision. Once you're keeping a greenhouse above ~4C through December–February, you're paying for every degree, and the size of that bill decides whether a head-start-only greenhouse is the better choice.

I would run my actual greenhouse size and glazing through a heating cost calculator. This one estimates the seasonal cost for any size/zip combo: https://comparegreenhouse.com/tools/heating-cost-calculator If the winter number makes you wince, the Eliot Coleman approach Paul and others mention above — hardy crops + row cover instead of heat — is usually the better permie answer anyway.

22F above ambient in a 12×7 at 6200 ft is genuinely good — nice work documenting it.

Two thoughts for "what next": for the bigger greenhouse, the size question is worth doing backwards from what you want to grow (headroom for trellised crops + path width changes the floor area fast). When you get to shopping, I use a tool that tracks spec-level data when I mess with bigger Backyard kits and other polycarbonate kits it helps me with sizes, panel thickness, snow/wind ratings, etc. Here: https://comparegreenhouse.com/tools/comparison-builder (free, no signup).

And for squeezing more out of the current one before then: a fan sized to the actual air volume makes a surprising difference to both daytime temps and disease pressure in small kits. This sizes it for you: https://comparegreenhouse.com/tools/fan-cfm-calculator

That site actually has a ton of useful tools.

The 20C differential question is mostly a heat-loss math problem, and you can nail it down before building anything.

I use a free heater BTU calculator online that does this math for any size/glazing combo (it works in C too): https://comparegreenhouse.com/tools/heater-btu-calculator — and this one compares glazing options side by side: https://comparegreenhouse.com/tools/glazing-chooser