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PVC hoop house -- with a twist.

 
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Location: Rocky Mountains, USA
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Looking for comments/suggestions on a greenhouse idea I've been toying with.

The idea is to utilize the presently junk-collecting space under a second story deck/balcony.  Not only would this provide a greenhouse space, but at night plants would benefit from waste heat from house, and house would presumably radiate less heat (lower temperature differential).  Win-win!

The general principle is to run a short knee-wall to prevent snow buildup from distorting the structure, then do a half a PVC hoop house.

The main thing I'm fishing for comments on is, well, you know how commercial greenhouses often use two layers of plastic and inflate the space between for insulation?  I wanted to marry that to the typical PVC frame.  My idea is to have two staggered rows of pipe so that the inflated plastic would form a sort of zigzag (red lines).

1) I already have a blower motor from a microwave I scrapped, and the house is right there for power, so there couldn't be an easier time to try.
2) Plus under the balcony just happens to be where my rainbarrel lives, so, hopefully no more freezing, AND it's right there by the plants!  Make that win-win +win!
greenhouse.png
[Thumbnail for greenhouse.png]
 
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I think that seems like a great place for a greenhouse.  Assuming it faces south

The zig zaged poly looks neat too.  One challenge would be fishing the poly between the pvc.  Another would be attaching it at the top and bottom in an air-tight manner so the fan can inflate it.
 
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Have you considered a greenhouse within a greenhouse?  A greenhouse nestled within another wouldn't need a blower to separate the layers of poly.  Although if using the blower in that situation, the largest benefit I can think of would be having the ability to pull heated air from inside the house or greenhouse to fill the gap and slow the rate of temperature loss at night and while cloudy.

Something else that can help hold heat is insulating around the exterior perimeter of the greenhouse down into the soil.  Optimally, you want to get the insulation to at least frost depth.  That layer of insulation in the soil prevents the frozen ground outside from cooling down the soil in your greenhouse.  All of the interior soil becomes a thermal battery too.  Angling the insulation out instead of having it vertical is also said to be better because you will collect more of the rising ground warmth.

You may know this already but I'll mention it just in case.  PVC will eat through greenhouse film, so the pvc needs to be painted.
 
K Eilander
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Location: Rocky Mountains, USA
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Joshua Parke wrote:You may know this already but I'll mention it just in case.  PVC will eat through greenhouse film, so the pvc needs to be painted.



Thanks! I did not know that!  Is the gray plastic electrical conduit better, or does that need paint also?
 
Mike Jay
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Joshua Parke wrote:Although if using the blower in that situation, the largest benefit I can think of would be having the ability to pull heated air from inside the house or greenhouse to fill the gap and slow the rate of temperature loss at night and while cloudy.


I've heard that if you do this, the humid air from the inside of the greenhouse will condense in the air gap and cause water to collect.  The condensation might also block some sunlight?

I believe that whether you're pumping hot air between the gaps to fight off the outside air temps, or if you pump outside air into the gap to fight with the inside air, there is always going to be a boundary between hot and cold to lose heat across.

I also think there isn't much air moving into the gap so weather it's hot or cold doesn't matter that much.
 
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