Author Topic: An idea for portable greenhouse that can survive a real winter.  (Read 13678 times)

Offline “Mark”

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An idea for portable greenhouse that can survive a real winter.
« on: January 16, 2010, 02:16:06 PM »
In winter, you have two elements to consider: wind and snow.

I'll start with snow, as it's actually the easier one to deal with: simply design your greenhouse in such a way that the snow doesn't stay on it. That would be something like an A-frame roof (leave lots of height between the eves and the ground for the sloughed off snow.

The second trick to snow is building your roof out of something slippery, to encourage it to slide off. Metal (which works great for houses) would be a bad choice for the greenhouse for obvious reasons, so that pretty much leaves plastic. With plastic, the best material is polycarbonate, which is hard, won't yellow, and will last for years; however, it's very expensive. You can also get corrugated plastic which they used to use, but it doesn't last long and is brittle and cracks easily, so I would discard that option. The last is option is 6 or 8 mil poly, usually used as vapor barrier.

I would go with the 8 mil for durability. The 6 mil rips too easily, especially if you're in any kind of an area with wind or if you're holding back snow. You'll get a couple years out of it, but it's so cheap that you can easily replace it as needed (and patch it with tape). A staple gun is all you need to attach it to the wood.

If you build your roof out of poly, you must make sure that at no point will any heavy load of snow on the roof encounter anything it can get hung up on when sliding off. So all your roof supports must go top to bottom instead of across. As poly will flex under the weight of snow, even the top of the wall where the roof meets could possibly catch the snow. To reduce the bowing of the poly, you'll need to space your supports pretty closely together.

If you built an 8 foot wide greenhouse, with a 45° roof angel (thus a 5'6" peak to wall roof), you could probably get away with 1x2's every 8 inches. I would also recommend tying the opposite walls into each other at 4 foot interval, as the roof will exert a lot of outwards pressure at the tops of the walls when loaded with snow (might be worth using some metal brackets there).

Why does the thin plastic work particularly well? After a good dump of snow, the next time it's sunny, the greenhouse will warm up, and will melt the bottom layer of snow. Water on poly is very slippery, and at a given moment, the whole load of snow on one roof will go flying off. This sudden change in load is another reason why you should tie your walls into each other, to keep the forces balanced. As it is, the other side of the roof will quickly  shed its snow due to the warming or the vibration of the snow coming off the first side.

For your walls, the limiting factor will again be the snow load. They need to be strong enough to hold a roof covered in snow (think tons of weight if you get a good dump), and they also need to be strong enough to hold back snow resting against the wall. 2x2's every 16 inches ought to work. 1x2's aren't strong enough. Use the same plastic on your walls.

For your roof ends, I would use plywood for structural support. Speaking of which, cross brace all your walls, and your two roof halves too. You can also build vents into your roof ends for the warm weather.

If you're clever in your design, you can make the 8 major pieces bolt together: your 4 walls, your 2 roof halves, and your 2 roof ends. If you built no piece taller than 8 feet, everything will fit on a flat deck trailer and you can take it to your next place. I would use small bricks for a foundation, and don't forget to put some poly between the bricks and the wood to stop the moisture wicked up by the bricks from rotting your wood. Again, very portable.

And other thing about the poly: if you want to extend your season, poly the inside of your greenhouse, too. The air pocket created will insulate the greenhouse from cold nights.

The price of wood will very from place to place, but an 8x8 greenhouse should easily cost under $500.

Offline JGreene

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #1 on: January 19, 2010, 07:07:33 PM »
I like the A-Frame idea.  I wouldn't use staples though, I'd use wooden strips (1/2 x 1 3/4) pre drilled with 1 1/4" deck screws.  That would facilitate changing the plastic. 

I'd like to use PVC, but it makes it more difficult to 'attach' the plastic sheeting.   Huh... why not glue? 

Offline Cool Blue

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #2 on: January 19, 2010, 07:55:14 PM »
I was thinking of a rectangular wood frame with a curved dome PVC roof.  It would look like an upside down "U" from the ends.

The poly would be stapled to the wood on one side, then tightly stretched over the PVC, maybe attached by duct tape in a few places, and again stapled to the wood frame on the other side.

I like the idea of attaching the poly with the wood straping.  That would make it more tear resistant.

Offline “Mark”

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #3 on: January 19, 2010, 08:04:59 PM »
You'll have to be careful how you orient such a roof. The dome shape will act like a giant wing in the wind. That gives you a couple complications. First, it will try to lift the roof off your greenhouse. Second, it will pull and stretch the poly. Once it's loose, it will flap violently and pull itself loose, especially if you don't attach it tightly to those roof ribs (perhaps glue?)

The PVC can also become quite brittle in cold weather. This is not a good situation in cold winter winds, nor is it good if the roof loading from snow suddenly shifts.

That being said, if your greenhouse is in a sheltered area and you don't experience heavy freezing rain or now, such a domed design with PVC may work well.

Offline AZCeltic formerly occeltic

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #4 on: January 19, 2010, 09:02:43 PM »
I really like your suggestions Mark. Years ago I used a similar design that worked out really well, but it was in an area of the country that didn't have to deal with heavy snow. The property that I will build my next one on is in the high country, and will have to handle below freezing temps as well as snow, so I need to put much thought into making it right the first time.
In the area I am in now, the local growers use a "Quonset" style long greenhouse that seems to have either a pvc framework or perhaps metal, and they are using a heavy gage poly-vinyl as well as ventilation systems of some sort. They are probably typical of large growers of nursery plants. I just wonder how well they would do in my situation. I'm still leaning towards an A-frame or half an A-frame. What would one call a half and A-frame?  :)

Offline “Mark”

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #5 on: January 20, 2010, 10:04:48 AM »
Yeah, the quonset style works, but again, they're dealing with much heavier and stronger skin material.

As you say high country with freezing temperatures, I would try to keep it sheltered from the wind. Wind cools off a greenhouse much more than a lack of direct sunlight.

Offline JGreene

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #6 on: January 20, 2010, 10:26:26 AM »
What about a more flat design?   Lower to the ground would help with the wind problem.   Depending on how much clearance you needed between the soil and the top of the shelter.

I guess it depends where you are and what you're growing.  I'm in PA, so snow loads aren't as big of a problem as in some areas.  The rounded top of a quanset won't be as good as the A-Frame as far as snow loads, but I'd think it would be less of a sail in windy conditions as the A would be. 

I'd go with the quanset, U shape and add as much strength as needed on the top for the snow.  Again, it depends on where you are and how permenent this needs to be.

Chesty5nc

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #7 on: January 28, 2010, 06:47:53 PM »
Great idea, thanks.

Offline TOWcritter

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #8 on: February 01, 2010, 02:35:20 PM »
If you use the snow to your advantage, it acts as an insulator.  Sounds crazy, I know, but with the right design it could work well.

Offline “Mark”

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #9 on: February 01, 2010, 08:11:45 PM »
If you use the snow to your advantage, it acts as an insulator.  Sounds crazy, I know, but with the right design it could work well.

Indeed. It's a good wind break.

Offline mobilus

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #10 on: February 07, 2010, 08:17:32 PM »
Mark, this is what I've built this winter.  Forgive the quality of some of the pix...



Bad photos, but you can see its portability.

Steel frame and tin on the north and east sides.

Then I doubled the steel up with wood, put 6 mil plastic on it and then used little 1/4" thick strips to anchor the plastic.

I used 2X4s and 2X6s (scrounged) for the flooring.  I debated on flooring it, but I'm glad I did.

I ran some fabric tape stuff I had in the opposite direction of the "rafters" to give it a little more support.

I'm working on shelving inside it now...should have plants in it by next weekend!

Offline Cool Blue

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #11 on: February 27, 2010, 06:00:28 AM »
Nice!

Offline mobilus

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #12 on: February 27, 2010, 07:12:01 PM »
Thanks!  Just moved all the plants we've started from the house to the greenhouse today.

Offline Illini Warrior

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #13 on: March 02, 2010, 05:58:05 PM »
Peaceful Valley Farm & Garden Supply ...... www.GrowOrganic.com  .......  has all the PVC fittings for the framework ...... use conventional 1/2" thru 1 1/2" dia PVC piping for the frame runs......

"rowcrop snap clamps" (looks like cut lengthwise sections of PVC pipe) for holding poly sheeting on framework

they also sell "super strong woven greenhouse poly"

Offline Cool Blue

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #14 on: March 06, 2010, 05:39:39 AM »
If you paint that metal wall flat black I bet it will really collect heat from the sun.

Offline “Mark”

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #15 on: March 09, 2010, 07:52:37 AM »
If you paint that metal wall flat black I bet it will really collect heat from the sun.

I might advise against that. Black also radiates the heat the best at night, plus you may end up making your greenhouse too warm in the summer. On the other hand, some black barrels filled with water are a fantastic idea. Similarly, if the wall were made out of cinder blocks or some other material with lots of heat storage, painting it black wouldn't be so bad. But with nothing to hold the heat in the metal wall, it's going to go straight into the air.

Offline mobilus

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #16 on: March 09, 2010, 08:24:30 PM »
I agree, Mark.  I have a couple of 55 gal black plastic barrels filled with water in there now, and it seems that they do act as good heat sinks.

Offline Fatchumang

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #17 on: March 12, 2010, 10:57:59 PM »
Why would black radiate heat more at night than another or no color?

Offline Cool Blue

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #18 on: March 13, 2010, 05:09:35 AM »
Why would black radiate heat more at night than another or no color?

I'm don't think it's the black, so much as it is the metal.

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #19 on: March 13, 2010, 05:31:31 AM »
Mobilus- is that a Kubota tractor?  If so, are you as impressed with it as all the other owners I've talked to? We've heard good things about them.  There is a dealer about 20 min from me who just recently included Kubota on his lot.  We are thinking about a small Kubota to do what the 8n can't, just curious.  Thanks!

Offline “Mark”

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #20 on: March 13, 2010, 09:01:00 PM »
Why would black radiate heat more at night than another or no color?

The same physics that let black absorb more energy also work in reverse when radiating heat.

I'm don't think it's the black, so much as it is the metal.

Metal is an excellent thermal conductor. It will keep the black paint hot as the paint discharges the thermal energy. Metal painted black will cool faster than metal painted white.

Offline mobilus

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #21 on: March 14, 2010, 08:38:42 PM »
That's right, Mark.  And that's the reason that the barrels are black, they absorb heat faster but they also dissipate that heat at night when you want them too.

Gunz, I love my Kubota!  To think of it, I've never met a Kubota owner that wasn't satisfied with the brand.  Maybe wanting a bigger tractor than the one they purchased, like me (now wanting 40 hp instead of 30), but no qualms with quality.  If you get a loader, make sure you get four wheel drive.

Gunz

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Re: An idea for portable greenhouse that can survive a real winter.
« Reply #22 on: March 16, 2010, 11:36:57 AM »
Thanks for the info Mobilus, I am hoping I can use an atv as partial trade at the dealer.  I'll get way more use out of the tractor than the atv!
Thanks