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Thread: JDW's DIY Greenhouse Plan

  1. #1
    JDW's Avatar
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    JDW's DIY Greenhouse Plan

    Of course we all want a greenhouse, but for some of us a prefab kit just won't do. I always try to DIY whatever I can to save money, because I can tailor it to suit my needs, and it usually costs a lot less. After reading a bunch of books and spending hours (days?) on the internet I've came up with a plan I want to show you all.

    This greenhouse is 12 feet wide and 16 feet long, with the peak running East-West. The top of the peak is 12 feet from the foundation. The two ends (West and East sides) are walls out of 2x6 pressure treated lumber, studs spaced 24" on center, with exterior grade plywood skin on the inside and outside. Although the East side would only have plywood 3 feet up, the upper part would be glazed to let in morning sun. The ribs along the North and South sides are either 3/4" EMT metal conduit, or 3/4" PVC pipe. The length of each rib is ~14' with a radius of ~14.5'. I think I could attach the pipe to the wood frame with angle irons bent to the right angle, with the pipe slipped over it? The South face would be skinned with a double layer of poly inflated with a fan. The North side I want to skin with something opaque and insulate. I also want to insulate the bottom 3 feet of the East and South sides. I don't know what I'd use to insulate the curved walls.

    First pic, view from the Southeast.


    The white rectangle above the door is the vent. I need to frame out a spot for the exhaust fan on the other side once I determine what size fan I need.

    Second pic, birds eye view, northish.


    The white space is all fixed benches, the red space is a movable bench, if I can figure out how to do that. The green box is an aquaponic system. And the orange cylinder is a water heater. I haven't ran the numbers to see if I could heat it with a household water heater, but if I can I could run hot water through the floor of the greenhouse.

    Third pic, South view.


    Fourth pic, East view.


    The big blue boxes are water storage tanks, maybe. Originally I made plans for a conventionally shaped greenhouse with gutters to collect rainwater, but with a greenhouse this shape I don't think that would work. My tap water is very hard so I would have to filter and treat it to make it suitable for the things I'm growing, or use RO or rainwater. Watering out of tanks inside the greenhouse would have the benefit of storing heat, and the water wouldn't be cold going on the plants. The benches will sit on top of 55 gallon drums filled with water, for heat storage.

    Areas: insulated - 433 ft2; double wall polyethylene - 160 ft2; twin wall polycarbonate - 59 ft2
    Volume: 1522 ft3
    Using a $2 price per gallon for LP gas, a minimum inside temp of 65F, and avg outside lows for Mishicot WI 54228, I calculated this greenhouse would cost around $500-800 to heat over the winter. That's not counting any heat storage, or night-time insulation over the glazed areas.
    The fixed benches equal roughly 70 ftw and the movable bench 24 ft2.
    If a 16 foot treated 2x6 costs $8.50 and a 10' long 3/4" conduit pipe costs $3.30, the greenhouse frame would cost just $200. Plywood would add about $125. Then gotta add glazing, insulation, heater, etc etc. I might be able to build this for less than a grand?

    Things I'm not sure about:
    Would I have to brace up the East and West walls?
    How to attach the poly to the wood frame?
    How to attach the exterior siding on the North side?
    What insulation would I use on the North side, and the bottom of the South side? Spray foam insulation would work best but cost effective?
    What to do for a foundation? Was thinking of a 12'x16' frame of 2x6's with 4x4 posts at each corner and every 4 feet on the sides.

    So what do you think? I know I still have a lot of work to do yet.

  2. #2
    scottychaos's Avatar
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    Looks cool! but..what do you want it for? what do you plan to grow in it?
    For the vast majority of Americans, greenhouses simply aren't practical..
    They are useful in March, where they can create an "early spring" as opposed to outdoors.
    and they could be useful in October, where they can extend the growing season by a month or so..
    For the remaining 10 months of the year, there isn't much you can do with it..
    its FAR too hot in the summer, and much too cold in the winter..

    Greenhouses are common in England, where the year-round climate is much more mild..
    a 30 degree frosty winter day outside the greenhouse becomes a 50 degree day inside..that is actually practical and useful.
    but when its 10 degrees outside, being 15 degrees inside the greenhouse isn't any benefit..its still cold.
    and in the summer, 80 degrees outside the greenhouse is *much* better than 100 degrees inside the greenhouse..

    If you can afford the heat in the winter, then more power to you! Could be good for overwintering VFT's and Sarrs I suppose,
    if you can keep it nicely above freezing..expensive, but certainty doable..(there are a lot of cheaper ways to do VFT and Sarr dormancy however!
    Probably wont be able to keep nepenthes in it all winter though..
    and you probably wont want anything in it in the summer..

    So, let us know what you plan to grow in it!
    thats makes a big difference..

    Im not saying you shouldnt do it..im sure it can be fun if you are really into it..
    but for most people, a greenhouse is not only impractical, its often actually *worse* for most plants than being outside..
    its just the nature of our climate..too hot in the summer, and too cold in the winter..

    Scot the realist..(sorry to rain on your parade)

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    JDW's Avatar
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    Quote Originally Posted by scottychaos View Post
    Scot the realist..(sorry to rain on your parade)
    Haha, no problem Scot. This greenhouse would be more for orchids than CPs. My small orchid collection is outgrowing it's 3x2x4' grow chamber I made a few months ago. I want to start breeding them and that's also going to take a lot more space. The CPs that I want to grow would mostly be lowland/intermediate neps, some sundews and a few others. I hope to produce a few plants to sell, hopefully enough to pay for most of the operating costs. I might also give a little space for fresh food during the winter. This greenhouse would still be a little small to go far with all that going on, but it's what I'm looking at for now.

    A greenhouse, even one this small, is a very expensive endeavor, but I think it's the most cost effective solution for me. To grow the same amount and types of plants indoors, under artificial light, would cost about the same, take up room I don't have, and create problems with water and humidity. My greenhouse would have more surface area that is heavily insulated compared to the area that is glazed, so would lose much less heat in the winter than a conventional greenhouse, like the prefab kits on the market. This, together with lots of water volume for storing heat from the day and releasing it at night, should cut heating costs to a level that's acceptable for me.

    As for cooling, since the South face glazing is polyethlyene plastic, I will remove it during the summer months. I can replace it with a shade cloth, which will cut heat gain and light levels that would be too much for the plants anyway. Plus misting and fans for evaporative cooling and I should be able to keeps temps in the safe zone.

    Someone pm'ed me and suggested I build it pit-style. Since I'd be insulating the bottom three feet of the wall, I could dig the floor three feet down. The earth down there (well below the frost line) stays around 45-55F so would provide heat in winter, and cooling in summer. But I'm not sure if I want to dig a big hole in the yard. It's likely I'll move in 2 years or so, and that would create problems when I take down the greenhouse.

    Thanks for your input Scot! I appreciate any help I can get with this crazy project.

  4. #4
    JDW's Avatar
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    RL7836 pm'ed me with some good questions, he said I could share this:
    Quote Originally Posted by RL7836
    Very interesting design - I'll be following your thread closely. Most people skimp to save on upfront costs and then get killed with the real money areas - operating costs. You seem to be considering those as well but I would try to get as much input from people who run greenhouses in similar climates as possible (Tony P. being number one - he's in upstate NY).

    Some thoughts

    - you use 65*F for winter operating minimum? Is that realistic? Is it necessary for the plants you'll be growing? I'd consider running the numbers for 50 & 45*F
    - consider what plants you'll really want to grow - HL, LL or intermediate. Trying to grow both HL & LL in same space can be tough.
    - you've considered winter heating - what about summer cooling. Depending on the types of plants - exhaust fans may not be enough (swamp cooler?)
    - back up systems or at least alarms that will page you if your systems crater. Depending what you have inside, this could save a lot... (Michael Catalani has some good info on his setup on his site).
    - have you considered a pit greenhouse (depending on your site)? Having the lower ~3' sunk into the ground can help a lot in both summer & winter.
    - have you looked into misting & sprinkler systems?

    Good luck with your endeavor...
    Just so I don't have to type it all again I'm just going to copy my reply:

    "1. I will actually be growing orchids mainly, and 65 is a safe bet. I might be able to get away with as low as 55.
    2. For neps I'm mostly looking at LL/Int. I have a small grow chamber in the basement for HL if I really want them.
    3. During the summer I was planning on removing the polyethylene covering on the South face, and replacing it with a shade cloth. Also plan to put mist nozzles on the HAF fan for a little evap cooling. Not sure if this will be enough but adding more shouldn't be too hard.
    4. I will get a controller/monitoring system of some sort, possibly build my own Arduino based controller. Not sure if this is feasible, but I thought of putting a webcam or two in the GH and hooking up some sprinklers to solenoids that I could control over the internet. Then I could view and water my GH remotely!
    5. Actually sinking the GH 3' was my original plan, but since I'll likely be moving within 2 yrs I wanted to build something I could take with me and didn't leave too much of an impact on the land here, it's still an option though.
    6. No but I plan to have some mist/sprinkler thing going on."

  5. #5
    Rocketcaver's Avatar
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    I think I might be worried about wind loading on those tall sides.

  6. #6
    JDW's Avatar
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    Quote Originally Posted by Rocketcaver View Post
    I think I might be worried about wind loading on those tall sides.
    I am. I'm worried that the structure won't be stiff enough, and would fall over to the East or West. Currently the East and West end walls would only be held up by the ridge beam, the glazing, and the attachment to the foundation. I'm not sure this is enough, or what I would need to add to fix it...

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    scottychaos's Avatar
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    Quote Originally Posted by JDW View Post
    Haha, no problem Scot.
    *snip*
    Thanks for your input Scot! I appreciate any help I can get with this crazy project.
    ok! cool..sounds like you know what you are doing, so thats all good then..
    (sometimes you cant tell if someone is a newbie who wants a greenhouse just because they think it would be cool, but doesn't really understand the ramifications..
    or if someone is a more advanced hobbyist who knows the pros and cons, and is choosing to build a greenhouse because it will actually work for them..)
    we can see now you are in the latter group..so, carry on then!

    Scot

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    JDW's Avatar
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    Because of all the problems/unknowns in the gothic GH plan, I'm drawing up some different options and see if I can come up with something better and less worrisome. The main reasons: A. want more growing space/ better utilization of space, B. doesn't seem strong enough, must resist high winds and snow, C. hard to insulate curved wall as much as I need, D. not enough water storage. So now I'm leaning toward a more permanent structure of wood, maybe sunken 3 feet with a cinder block foundation.

    Can you change a threads title?

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