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Greenhouse Solar Wall Advice

 
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Hello all! I'm building a small greenhouse here in Missoula and was wanting some advice on finishing off the inside of the north wall. I plan to make a solar wall that works similar to a solar dehydrator by heating up a small airspace and venting that air through perforated drainage pipe under the raised bed built into the greenhouse. Air would be drawn through the system by a small solar-powered fan at the far end of the pipe. The wall currently has reflectix foil bubble insulation over the studs and foamboard insulation between them, and I plan to put one of two material options on top of that (with risers/furring strips between the material and the foil bubble insulation to create the airspace). During summer I would be covering this with a white shadecloth (as well as another shadecloth over the whole greenhouse if need be) to make it not work against me. I may also divert the drainage tubing somehow during the summer to make it cool the soil instead of heating it if I can find a suitable method for that.

The first option (and my initial thought) was to use some leftover sheet metal from the shed I tore down to convert to a greenhouse and use metal spray paint to paint it black. The second option is some black/grey-tinted corrugated polycarbonate I saw at Home Depot recently (https://www.homedepot.com/p/Suntuf-26-in-x-8-ft-Solar-Gray-Polycarbonate-Corrugated-Roof-Panel-101929/100062380), which I'm thinking might work better at heating the airspace as well as allowing the foil insulation to reflect a bit of light back at the plants. I'm not sure if anyone has built a similar thing before or has recommendations on which would work better, but my thoughts of the pros/cons of both are listed below.

Sheet Metal
Pros: already have material (though I can find other ways to repurpose it), I've seen another commercial scale build where black metal was their choice as well
Cons: material is a little banged up and possibly not long enough for the peak so it may require some significant stitching, painting metal and having it look good/last can be hard especially as it gets colder

Tinted Corrugated Polycarbonate
Pros: Inexpensive-ish, slightly easier to work with, small potential light benefit to plants, consistent look/no need to paint, slightly harder to damage
Cons: Unknown effectiveness in comparison (but possibly better?)

I appreciate everyone's input on this and will try to post pics when it's finished!
 
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Welcome to Permies Lexi!

This seems like great project.
Of those two options, I would recommend using the scrap metal, and darkening it with fire.
The plastic is expensive and will probably offgas something unpleasant.

The BuilditSolar website has quite a bit of information about solar thermal air heaters.

This page shows a comparison of aluminum cans and window screen as heat absorbing /exchanging materials.

https://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm

The screen comes out ahead, if I remember correctly.
 
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