I am hoping some one who is more experienced than I am can answer just a few questions. When it comes to water( im planning on full passive solar)....Calculating how much water I need to store heat, do I just go by floor area, or do I count all area of the interior?.I understand that thete Asia re varying views on whether water works effectively or not,and I prefer to find out by trial and error what works and what doesnt. If you could build your green house differently, what would you do and why???

Hopefully you just want to use the greenhouse in the spring, right, and not the winter?

The double poly layer is only R1.3 - it just doesn't hold much heat. Retractable insulation is what I would build. You can roll an insulated tarp up and down, like a pool cover, or build insulated shutters that open and close. But for all the work of opening and closing them, you might as well just take your plants inside your house on cold nights.

You would need a minimum of 4 gallons of water storage per sq, ft. of floor area to make even a small difference. However, in your zone, shortly after sundown in the winter, the heat will begin to be released into the greenhouse. By early morning, the inside temperature will be close to outside temp.
As noted by cole robbie, trying to go with "full passive solar" in a zone 3 winter would be quite a challenge.

So insulating the non south facing walls wont help??
Thank you for your replies, and yes, I was hoping to be able to use it during winter.

Well, then I guess that I will gauge it this year, keep good records and see what happens. My "experimental plants" will be items from the herb garden. If they freeze and go dormant, its ok, since they are perennials, and will come back come spring, no harm no foul.

Irene, I wouldn't say insulating the N and non-S facing walls won't help. It just won't be enough in your climate to prevent freezing at the coldest.

As CR has suggested, there are other things you could do, such as movable insulation on all sides and the roof. Also additional protection inside the GH, such as ag fabric or plastic tunnels over some plants. All these things will help, and extend the time your GH stays above freezing. They just won't likely keep it above freezing all winter.

This post was edited by steve333 on Sat, Oct 18, 14 at 23:38

Has anyone tried using water heater insulating jacket material for insulating or making heater curtains/heat barriers?

I put a link below that should help you do calculations. You'll need the surface area of each wall and the roof, then plug in the R-value of each wall's insulation. Estimate the outside temperature, and the calculator will tell you the BTUs required to keep the structure at whatever temperature you choose.

Be prepared to be let down :( Any clear building material that makes a good greenhouse will also have a low r-value. The most expensive double-pane glass is only R2, and I think all the poly coverings come in less than that.

BUT if you had shutters with R12 fiberglass in them that you could open and close, and a small electric heater, I think you'd do fine.

Part of the problem with passive ideas is the weather, especially in the winter the sun likes to not come out for a week or two at a time, and some of those days the high is below freezing. With no sun, the water barrels don't heat up like they should. When they turn into chunks of ice, they become a cool sink, and not a hot sink. If it gets cold enough, mass can work against you, instead of for you.

Here is a link that might be useful: heat loss calculator

Thank you. I appreciate the helpful link. I can clearly see that this is going to be a challenge at best.