In winter in Massachusetts I doubt you would be comfortable in a short-sleeved shirt. There will be solar gain, but the loss of heat radiating through the glass, through convection due to wind, and through air leakage will be unlikely to allow the GH to stay at comfy temperatures. I'm only judging by my own experience with a glass GH in the PacificNW which is much milder (Zone 8) than your climate, although with grayer skies. Hopefully, someone in your area can provide a more encouraging assessment.

Yes it will be warm enough on days with clear skies. But it doesn't take much in the way of clouds to really cool things off. On a clear day in a sunny location one can get 50F or more in temperature increase. My sunroom hits 120F and higher when 60F outside. It's usually 80F by 9am in winter.

I love my sunroom in winter. Really cheers me up and warms the house massively. That's with 75% sunshine in winter.

I frequently need to ventillate my sunroom or put up 50% shade cloth. That's in addition to running heat into the house. On a 50-60F sunny day the house hits 85F. That's a 25-30F increase from an unheated night time low.

fruitnut-

But what kind of temps do you get in winter, compared to Massachusetts? And you have a sunroom which is part of your house, not an all glass, uninsulated, freestanding structure...

Kudzu:

You and I live in very different climates esp in winter. That explains our different perspective. We have 75% sunshine and you have what 20%?

I stated that it should be warm enough on sunny days even if it's 30F outside. In MA that might be 1/3 of the days. That assumes southern exposure and a tight structure. 30F + 50F equals 80F. Would work for me.

Mine unventillated would be deadly even when it's 40F outside. In fact I'd hate to be out there all day when it's 30F unvented.

The other thing is one won't need glass. It could be done with clear greenhouse poly on a southern facing wall of the house. That would be a cheap way of testing it out. Get it well sealed and it should show a heat gain of 50F or more on clear days.

fruitnut-

Not trying to pick a fight as I don't first-hand experience with greenhouses in Massachusetts. I also agree that our sun situations are different. I completely understand why you have such a comfy setup. However, I am still thinking that 100% of a glass greenhouse is heat loss surface, and Massachusetts can easily get to -10F in winter. Where I live it can be sunny and 40F outside, and my greenhouse isn't warm enough for short sleeves. Maybe if I had double wall glazing it would be better. But those of us in northern latitudes also don't have the same solar exposure that you do, even on sunny days.

My sunroom has full southern exposure and is on the south side of my house. That's what I'd want in MA. Not out in an exposed location where as you say all four walls are losing heat. Also the house wall absorbs the suns heat. I'd want something dark on that wall to absorb heat. Out in the open much of the heat just passes right thru. Even then it might not work on many days in Dec and Jan. But maybe in Nov, Febr, and March.

I started out doing this with clear poly in CA. There winter clear days are about 50%. It worked well on the full sun days but overall wasn't as helpful for house heating as here.

Your comment about sun angle is important. And perhaps I'm under estimating it's significance. MA is way closer to WA than TX in that regards.

You could also build a greenhouse with an insulated north wall painted black to absorb and retain the suns heat. Even with a low sun angle that would heat up nicely on clear days.

Part of the issue with SAD is also day length. I get outside in winter and visit greenhouses on inclement days, but also use a full spectrum lamp designed for SAD in early morning to convince my body that the day is longer.

This guy is in Mass.