Double Wall Hoop House

By adding a second layer of  plastic with and air space between, I hope to ameliorate the extreme temperature fluctuations in my small hoop house.

Background:

Hoop houses, or poly tunnels are structures farmers use to extend their growing season. By erecting a simple tunnel of pvc or wire and covering it with clear plastic, growers can create an area where their crops are somewhat protected from the elements, allowing the Gardener to start growing much earlier in the spring and grow crops much later in the fall than would be possible otherwise.

The problems with hoop houses, particularly small ones, is that due to their massive glazing and their lack of insulation they actually accentuate temperature fluctuations, often to the point of being unusable.

Case in point: Several years ago I tried building one of these structures. As I had a number of cattle panels lying around I opted for a design that used these panels.

The resulting structure was simple to erect, incredibly strong, and functioned well as a temporary shelter for plant starts in spring. Unfortunately, I found that with the sun shinning, the interior temperature was A full thirty degrees warmer than the outside temp. On a sixty degree day, inside it was ninety degrees. At night, however, the temperature dropped precipitously to whatever the outside temp was. If the night time temp was forty degrees, so was the inside temp. In essence, I had created an environment where the temp. fluctuations were greater than if I had simply put the plants outside.

Needless to say, with a daily temperature difference of fifty degrees the plants I put in the hoop house were not happy campers.

I have been pondering this problem for some time. That is, how to build a small hoop house so that it won’t cook your plants in summer and freeze them in winter. I tried adding thermal mass – concrete blocks and jugs of water – and increasing air circulation and venting. Despite my best efforts the small hoop houses I built remained stubbornly hot in summer and dangerously cold in the winter. It would be relatively simple to ameliorate temperature swings through the application of fossil fuel energy. But in my mind the whole point of using a hoop house is it’s simple “off grid” design. Thus, I was convinced that any changes would     have to be passive solar design.

The Project:

My latest attempt at making my hoop houses usable revolves around adding a second layer of plastic with an air space in between. The idea is to add a small amount of insulation (R – value) while still allowing a reasonable amount of solar gain.

I considered several strategies for attaching a second layer of plastic. In the end I settled on simply adding some 2×2 sleepers, replicating the wire mesh and tacking on another layer of plastic. The result looks like this:

Sleepers added.Second layer of plastic.Close up.

The only change I made from the original wire frame was to use concrete re-mesh instead of cattle panels. I found they are much lighter and easier to handle.

I have finished the second skin and now am in the process of testing temperatures to see if the double skin has any effect. Conveniently, the temperature has dropped (in November) to below freezing so I should get some results fairly quickly.

Updates: Tuesday Nov. 7

temp this morning in hoop house was the same as outside. 32 degrees. Duscouraging. Read how air infiltration has huge negative effect on temperature. Realized I hadn’t done a very good job sealing cracks. Worked for two hours sealing up cracks where plastic sheets joined. Will try painting three 55 gallon drums of water black tomorrow to see if that helps to hold in heat. Also, will paint ceramic counter top tiles black to see what effect that has.


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