Greenhouse for vegetable plants

[trigger]

I made a small greenhouse, it only has one layer of plastic on it, should I have a second layer on it. also, at what minimum temperature would be okay to leave greenbeans, tomatoes, onions, etc in the greenhouse overnight? it is still a bit colder at night here in Michigan.

[hendi_alex]

Yes, second layer. Warm weather crops like tomatoes, bell pepper, beans?, not much colder than sixty degrees. Cool weather crops like onions, cabbage, broccoli, etc., down to 40 or lower would be o.k. If I were you, for this year would consider starting cool weather plants in the greenhouse, and heat it in a minimal way. Get a light arrangement as described in threads on this site, and start your heat loving plants inside the house. Use a heat mat and have the room over 60 degrees.

[Gary350]

I do a lot of solar research and 2 layers of glass is the best. 1 layer looses a lot of heat and 3 layers only adds another 4% in heat. The 3rd layer of glass is a waste of money.

Plastic is a different story because it is flexable. When the wind blows the plastic sucks in and out. The plastic acts like a hydraulic pump it sucks the air inside the green house in and out, it also sucks the air in between 2 layers of plastic in and out too. 80% of the heat inside the green house can be lost by the movement of the plastic caused by the wind if everything is not 100% air tight.

Polyethylene plastic usually will not last more than 2 years UV rays will distroy it.

[hendi_alex]

I made a silly mistake on my greenhouse. Put twin wall polycarbonate on the walls, but then used the single layer, Lowes grade, of polycarbonate on the roof. Didn't realize that about 70-80% of heat loss is through the ceiling/roof. This year I put UV stabilized film on the underside of the rafters and let is drape over half way down the walls. Before next season will completely seal the walls with the inside layer.

The biggest improvement that I made for heat efficiency was in putting an adequate bank of fans to blow the air back down from the ten or twelve foot peak. The inner skin was the second biggest improvement. Of course caulking every seam that could be caulked helped a bunch as well.

Also, a couple seasons ago, I installed a used hot water tank and put heating coils on top of foil backed foam board and covered that with rocks. That warm water circulates via a circulating induction pump, gently heating the surrounding rocks, and the heat stays in place longer, and keeps the bottoms of the planters about 70 degrees. That heating system has worked very well.

Here is a photo of the second layer of plastic on the inside of the wooden frame. I retrofitted an old barn for my greenhouse.

[img]https://farm4.static.flickr.com/3250/2915141357_15ba529558.jpg[/img]

Renovated barn and greenhouse. Barn was about to fall down before much needed overhaul. Everything has been replaced except the joists and rafters overhead.

[img]https://farm3.static.flickr.com/2044/1690953145_079aedc3b1.jpg[/img]

[trigger]

the green house is a ice shanty project that never got finished, I could put another layer on like an inch away from the other, its just what iam working with, the plastic is quite taut, at least the best I could get it.