Thursday, July 23, 2009

Greenhouse in the heat: greenhouse operation update

It was 100 degrees today in Missoula, summer is here, and the greenhouse is fairing well. I installed a shade cloth a couple of days ago (see photo above), to help moderate temperatures, but mainly to reduce the intense, glaring sun, rather than for cooling. Actually, despite the high temperatures, the greenhouse is staying relatively cool. By relative I mean less than 120 degrees, and this maximum has not changed in a few months.

I was curious how the greenhouse would do in the summer, and in May we were already getting temperature in the 110's (yes, I do monitor the temperature daily, hourly in fact, with a wireless thermometer that uploads data to my computer). These temperatures are at about 7', most plants are actually cooler, growing from the floor of the greenhouse.

I designed the greenhouse for the winter, and for the low winter sun. The roof angle is pretty steep to intercept the low sun, and the west wall is not fully glazed. The north wall, and north facing roof is conventionally framed and insulated (click here for more information).

Because of the steep roof angle, and glazing only on one face, thermal gain in the greenhouse in the summer (when the sun is higher in the sky), is not as much as you'd expect and a lot less than with a lower pitched roof, or with a conventionally designed greenhouse or commercially available or kit greenhouse.

Since windows on the west and east sides are set pretty low, in the afternoon in the summer the west side provides shade. In the winter, because of the low sun angle and the sun's position further south, the sun won't even hit much of the west wall and only for a brief amount of time.

Cooling greenhouses in the summer is normally a bigger issue than getting it hot enough. As a result, I planned on venting the greenhouse with a solar louver vents on the gale ends, a solar window opener on the east end, and a thermostatically controlled exhaust fan. After the results for far, I am not sure the exhaust fan will be necessary, though I will have to look a little more at my temperature data as the summer goes on.
So far so good. I have been really happy with my design, but we'll see what happens in the winter and how well the compost furnace works. Although I am really into the design, operation and construction of the greenhouse, my wife is more interested in using it and enjoying it.

1 comment:

  1. Hi from Germany
    Glad that I found your page! While planning my own little greenhouse I found your page and I am going to built a similar one. After I got some nice old windows for free, the plan grew in my mind. How exciting - construction starts next week. I think I am going to steal the idea of your sliding door. Thanks :-)