What makes a Greenhouse a 4 season greenhouse?

I found what looks like a really nice 9x12 greenhouse online that comes with 8mm twinwall poly. Could something like that be used 4 seasons in Seattle [zone 8b?] with heat added or is there more to the 4 season greenhouse thing than that??

What I want is to be able to grow cool weather greens,spinach, lettuce- that sort of thing, during the winter months..maybe not all of the time but most of the time.I would probably have to be able to maintain a temp of 45 at night.Possible or not?

To be a 4 season greenhouse, it has to be equipped to provide enough heat in the winter and enough cooling in the summer--if you're missing either one of those then it's only a 3 season GH. Twinwall poly is fine for you for winter, you shouldn't have any trouble keeping it at or above 45 at night with a heater. And since your summers are milder than most people's you won't have as many cooling challenges as a lot of people do, but you'll still need at least vent openers and fans if you want to use it in the summer too. I don't know how sunny your winters are, but you may need the vent openers and fans in the winter too--on a sunny winter day here when temps get into the mid 60's, my GH will get over 100 if it's not ventilated and that would fry your cool season veggies.

I agree with ecrane, with the additional comment that it depends on what you are willing to spend.

Yes, lots of money, and a big carbon footprint. This winter I'm going to experiment with a cold frame over one of my beds. Thinking of using a couple of those inferred heaters that are used in reptile tanks. Its on the drawing board and will post this fall. The Inuits use to use this double greenhouse method with great success.

Money is an object for sure.
Is 8mm double wall overkill for my climate? Would 4 or 6 be enough or even poly film??

I'm in 9a and I have the 6mm twinwall, but I have to spend a decent amount to keep it heated during the winter. Your choice is either spend more up front to get a thicker wall but less ongoing to keep it warm, or vice versa. Poly film will not be enough if you're planning to heat the greenhouse--if all you want is something to keep the frost off then it might work OK, but it'll be very energy inefficient to try and heat it. But if all you're doing is trying to grow veggies I'm not sure how warm you really need it to be, maybe you could do something with poly film and don't heat it.

Hi Robin ..........

I have had greenhouses in New York, Ohio, and Florida. The climates may vary - but one thing doesn't; it is much easier (and more cost effective) to build it right the first time .......... rather than having to modify and redo what you already have.
The idea of a four season greenhouse is to be able to control the interior climate throughout the year. This includes heating, ventilation (with exaust fan), misting system, exterior shade cloth to cut the sun's intensity, etc.. What may seem a bit expensive initially will more than "pay off" in the long run - when your maintenance is kept to a minimum!

I am in zone 9a and I have 8mm twin wall polycarb. I wouldn't change a thing - since I am prepared for whatever drastic swings the weather might throw our way. One thing which I have found extremely effective for heat control in this zone is the metalic shadecloth - which reflects most of the heat. If it's 95 degrees outside, I can often keep the interior temperature 10 or 15 degrees cooler by running the exhaust fan.

I "underbuilt" a previous greenhouse - and learned that I didn't save anything by the time I was finished "improving" it! Plan carefully - and save until you can have exactly what you want. It's well worth the effort!

John

I think I lucked out with my GH. Lean-to type design on cement slab and foundation. Small pic of construction phase showing design. Note partial window opening of 2nd floor to center right, and no side walls at all. Mulberry tree to the left (west side) barely noted growing just above roof line is now well grown in.

I have not had much trouble heating or cooling it, mostly because it is connected directly to the house at the gutter line of the 2nd floor. Faces true south and almost half shaded by the mature Mulberry tree on the west side. Floor footprint is 32 foot x 10 foot 2 inches. 320 Sq. Ft. I chose Mulberry, as it grows fast and loves to be hacked. Having too large a tree next to glass in a windstorm is not fun. Plus, the birds and I get a few weeks of treats around the 4th of July.

Roof is exactly 45º in pitch which is an important angle in trapping the heat in winter and reflecting it well in summer. The (2) upstairs rooms have large unobstructed openings in the wall (10-foot by 4-foot, with draw curtains) down to the GH. French doors open into the GH on the 1st. Circulate the GH air with a couple cheap box fans.

I want to invest in some aluminum mesh to cover the east 1/2 the roof for two months in the summer to relieve the a/c, which gets quite a work out on occasion. The Mulberry shades most of it from the west. Here, a/c is more to cut the humidity than used to cool. Usually don't use it much.

Each side of the GH does have a powerful exhaust fan in the top peak. Buffalo has fairly mild winters and I have not had the heating bills be at all excessive, as on the sunny days (yes, Buffalo has sunny days in winter) the house is sometimes 95º-100ºF (outside can be 20ºF or lower) and I actually have to open windows in the house. Needless to say, the furnace sometimes does not come on all day until after the sun goes down.

Creating Thermal Mass is another important key for the winter. Water (or rock) is the best for that. You can save a lot on night heating if you can get the sun to day heat your thermal mass. I am planning on adding stone siding to the south side of the house which is exposed to the inside the GH.

I still haven't made up my mind whether or not I want to close off the GH from the house. I love that crisp, fragrant smell throughout the house after the morning watering of the GH, esp. in the middle of winter. And the house air seems so clean (maybe that is just me in my mind, though) but people comment on the air quality. A lot of oxygen, perhaps.

I hope that wasn't too winded. :-)