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Simple dual wall greenhouse idea with climate battery

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Hi, I had an idea for a energy efficient greenhouse that doesn’t cost an arm and a leg to build. I have attached a diagram to demonstrate my ideas. The first idea is to make a dual wall poly covering greenhouse without an inflation system by using two sets of hoops with a 4”x4” lumber spacer on the floor to create a roughly 3.5” air space to insulate the greenhouse.

The second idea is to use 4” corrugated perforated pipe run along the length of the greenhouse at 4’ deep and 2’ deep and circulate air underground and back into the greenhouse with 4” inline fans rated to 195cfm. My understanding is that this will raise the temperature in the greenhouse using less energy than a traditional heater and I’m thinking less than a heat pump as well. I believe it will also dehumidify as air condensates underground. If I have access to a trencher is this something I can install one or two pipes at a time to spread out the cost and still receive some benefit or is the climate battery an all or nothing deal?

Combined with the air layer for insulation I am hoping I can raise the temperatures enough in the greenhouse here in zone 8b (15-20F min temps, 35F avg low during Jan-Feb, but very cloudy) to maintain temps at least in the mid 40’s and hopefully higher. I think combined with some bottom heat this would allow me to grow perennials for my organic nursery through the winter so I have a significant crop ready in early-mid spring when most people are buying plants. Without a heated greenhouse I would not be able to have much available until late spring when nursery sales begin to slow down but I hate the idea of using an excessive amount of energy to heat it as I am attempting to be as sustainable as possible with my nursery.

Any feedback and input is appreciated. Thanks!
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Posts: 5050
Location: Northern WI (zone 4)
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Hi A, welcome to Permies!  I don't have experience greenhousing in your climate but it seems like you'd be in decent shape with that idea.  If you can get earthmoving equipment to excavate for the piping before you build the greenhouse, I think it would be quicker/easier than a trencher.  

Do you get frozen ground in winter?  If so, a little perimeter insulation underground may be worthwhile.

Orienting the greenhouse with the long axis E/W would allow you to insulate part of the North wall during winter without reducing solar gain.

I believe PVC reacts with greenhouse poly and reduces it's lifespan so you need to keep them from touching by painting the PVC or wrapping it with tape or something.

My greenhouse has 1.5" spacers on ribs with 4' centers so that I could avoid the inflation blower.  Over time the poly stretches and now the outer layer sags and touches the inner layer quite often.  So if you have hoops that are farther apart than 4' (likely), you may need more than 4" to keep the layers from touching once the poly gets droopy in the sun.  
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