Chinese Design for Passive Solar Greenhouse

Courtesy of James K., Virtually Green, San Francisco, CA

In northern China, solar passive commercial greenhouses cover over 1.8 million acres. These greenhouses typically use no supplemental lighting, and little or no heating, to produce vegetables from Fall through Spring. It’s a highly evolved and very commercially successful design, and virtually unknown in North America.

One of the central design precepts of the greenhouse is a high volume to surface area ratio, which ensures a large interior thermal mass in the soil, air and back walls relative to the surface area of the glazing.  At the end of a day of Winter sunlight you close the thermal blankets and the internal large volume of thermal mass efficiently retains the sun’s heat.

The Chinese designs are typically from 26′ to 46′ in width.  The 26′ width though is considered less than optimal, with most of the greenhouses closer to the 46′ width:  these wider greenhouses have much more favorable volume to surface area ratios than the narrower ones and they maintain higher and more uniform soil and air interior temperatures.

In a Calgary winter a 10′ x 10′ design won’t give you enough volume to surface area to maintain an adequately warm and stable interior soil and air temperature for optimal, or even adequate, plant growth.  And a 50′ x 14′ design is suboptimal because it’s so narrow:  You should consider widening it to at least 30′, and even wider if you can.

I work with cities and counties in the USA regarding zoning and building codes routinely.  I definitely don’t recommend building a greenhouse in a city where one grumpy neighbor can bring the codes officials down on your head and perhaps hit you with a hefty fine or shut you down.  Plus, of course I think the solar passive design won’t work well at 10′ x 10′ anyway.

So I recommend building your Chinese solar passive greenhouse outside of town and widening it to at least 30′, or wider if you can.

Vapor Barrier for Greenhouse Construction

Courtesy of Scott W., Growing Gardeners Calgary Urban Farms , Calgary AB
I am setting up a 50’x12′ hoop house this. I was going to order greenhouse poly from Johnny’s, but a few people have recommended using vapor barrier as it stands up to hail better, is cheaper, easier to work with and can be dealt with in sections. Apparently it also helps to not burn your plants as well. I have a fair bit of wind and lots of snow where I am, so they also suggested using an elongated “portable car shelter” instead of a hoop house because of the added strength.
The vapour barrier I would be using is heavy duty 6 mil, comes in 8′ sheets and is UV stabilized. I am growing in a yard that has tree and house shelter on two sides and, according to our average wind speed and directions, that would buffer the wind we get.
The cost of building a strong, wooden frame greenhouse and using this heavy duty vapour barrier is $350. If I anchor it down, attach the plastic to the frame and also bury the plastic around the outside I will be golden. I have several friends who have done this here and it works excellent.                                                                                                                         I will be holding it down with wiggle wire as I have heard that is the best and allows for some rolling up of the sides. The main thing that changed my mind to spending a fair bit of money on this was the look of it. This is going to be in one of my property owner’s yards. I have a great relationship with him (he is also a restaurant owner/chef so I sell to him), but I wanted to keep it that way by  making sure the hoop house doesn’t have ugly vapor barrier writing all over it. The shape will be straight up walls for 5′ and then an arch. This has worked well for my friends and will give me some headroom to work. If I put cross beams I could even hang some pots in there and really maximize the space, but we’ll see how it goes. This is a learning experience, and it really isn’t going to cost that much for me to try it this way. If it goes well I can’t wait to spend like $3k and get a really nice professional greenhouse (they are so fancy!).

Greenhouse Construction

Courtesy of Brenda S., Thompson Street Farm, Glastonbury CT
*We built our 14 x 24 greenhouse last year and here are some books that helped us. Because of my  husband’s limited knowledge and experience in building structures, we needed a very simple design. I ended up buying the basic frame kit without all the bells and whistles from Farm Tek (www.farmtek.com). I think the total cost was around $1,000, which included the plastic and hardware. Could we have done it cheaper? Probably but  again, my husband was unsure of his skills. In addition to the kit, I also
researched books and designs and found some books that were helpful.

Books I recommend:

• The Winter Harvest and The Four Season Harvest – Eliot Coleman. He lacks some of the details we needed to construct a greenhouse. Having said that, I loved his idea of using plastic electrical conduit pipes for quick hoops and it works well. These are different from the white PVC pipes they are grey in color and very flexible. We found ¾ inch pipes worked well. Eliot recommends ½ inch to ¾ inch pipes depending on what you are using them for. They’re cheap and hold up well in Connecticut winters. I paid 0.95 per pipe at Home Depot.

• The Polytunnel Handbook by Andy McKee & Mark Gatter. They are from the UK and have great ideas for greenhouse layout and making quick doors.

• The Hoophouse Handbook: Growing Produce and Flowers in Hoophouses and High Tunnels by Lynn Byczynski. This book is not long. I liked the drawings on the construction and we were able to adapt somethings to our project.

Between these resources, and what I found online, we constructed a beautiful green house that survived the big snow of 2010. Now I hope it survives Hurricane Irene this weekend. Yikes!