I'm planning to build a large (25' x 40') poly covered, wooden greenhouse next summer.

Main use will be spring, fall and some winter growing- without added electricity if possible (planning on a black lined pond, east/west placement for longest solar gain here in the north, row covers plus 'maybe' a small woodstove when really cold as required.

Due to heavy snowload that takes down unmaintained steel hoophouses I'm hoping 2x4" construction will do the trick.

As it won't be heated for the most part I was thinking about having the poly on both outside, then another layer attached on the inside. So there'd be a 3.5" airlock of likely wet stagnant air. I figure PT or cedar would be ideal here- if affordable in quantity.

Has anyone tried anything like this? Any issues with condensation build up in the airlock effecting light transmission?

Any thoughts good or bad appreciated.

Even if you do use wood put both layers of poly on the outside and inflate. Your wood will just rot between layers. That inside layer would be poorly supported and not add anything to strength of the covering re heavy snowload.

When snow builds up on mine, which isn't much here, I turn on the heater and melt it off.

I have a greenhouse now that uses a blower for the inflated double layer of poly. I would guess that it only works because moving the air through it dries up condensation before it can turn to mold and algae. That same moisture would also rot your lumber if it wasn't treated. You can use treated lumber, but the fun part is that the arsenate in the new stuff dissolves greenhouse plastic if it comes in direct contact for a long time. I use treated, but I paint it with multiple coats of latex and try to also put down a layer of white duct tape where the plastic hits the lumber.

A steep roof is actually easier to make than the typical quonset hoophouse. Check out the link to "kee clamps" below. You could build a lumber north wall and insulate that, then make the roof and sides out of straight pipe. I would stick to one layer of poly if you don't have some way to power an inflation blower. fwiw, I have seen a pic of an Amish design that is like a wind vane, except the vane is a big cone that pivots to catch the wind. It's funneled into dryer hose and used to inflate a double poly layer.

You can also use the double-wall rigid poly panels, but I know they're expensive.

Thanks, I will check into the feasibility of a metal framed of that size. Locally I have seen at least two hoop style coldframes collapse under heavy snows.
Actually, two main suppliers would not guarantee their product for this area- and suggested shoveling around perimeter.

Those clamps are a neat idea.

CR- are you sure about the PT breaking down the poly? Any idea the approx longevity? Poly does need replacing every 5 years or so. That is a lot of painting.

I should get some scraps and see how long it takes to dissolve the plastic if I clamp the two together. I have noticed a big difference in treated lumber from Lowe's, and I think it's because their lumberyard is indoors. The lumber seems to weigh twice as much and ooze out the arsenate chemical like crazy. It squirts out when I sink a screw. Within a day or two of being painted white, it starts to turn green from the arsenate bleeding into the paint.

A standard hoop house isn't designed for snow. You need a real greenhouse frame designed for 30 lbs per sq foot snow load or whatever is appropriate. Either that or remove the covering in winter.

At least with a steel frame rated 30 lbs you know what your getting. How many 2x4 does it take to handle that? No way of knowing. Dollar for dollar I think steel would be stronger and allow in more light.

Let me add something else. I had a wooden porch built of treated lumber. After a few years it is still bending and twisting. Maybe cedar would be better but it's not solid and stable like steel. I built my 1700 sqft greenhouse to stand 90 mph winds, our main hazard here. It has 15,000 lbs concrete holding down each side. When the wind is howling that doesn't seem like too much.

I'd bet a really well built steel frame will outlast wood by 2-3 times and without changing shape every year.

Growers Supply a division of Farmtek has some cheaper models but they can be beefed up. Gothic Arch, The Greenhouse Megastore, and Stuppy's are possible suppliers.

Do a google search for Commercial greenhouse manufactures. And search their offerings for structures rated 30 lb per sq ft snow load. A structure the size you are talking is commercial not hobby. The cost per sq ft of covered area is less on the bigger units than on the hobby and they are usually much stronger.