The Owner Built Home & Homestead
(Page 7 of 7)
July/August 1972
By Ken Kern
Greenhouse experts, like gardening experts, are never in agreement as as to proper direction to orient the structure or to plant the crops. A southwest exposure may provide more light, but in the afternoon the energy of the plant has started to wane. So actually a southeast exposure is best, as the morning hours for a plant are most productive. Of greater importance than orientation is the slope of greenhouse walls; the amount of fight transmitted or reflected depends upon the angle that the light beans makes with the greenhouse wall. The angle of incidence should not be less than 70 degrees. Some solar heating engineers use the formula "Latitude + 13 degrees = angle of glass to horizon" to get maximum winter penetration and maximum summer reflection.
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About 50% of the total sunlight striking a greenhouse is dissipated. This loss can be reduced considerably by reflecting the light from the southern half of the sky against a north-facing wall. This north wall should have a smooth white surface for maximum reflection. And of course this north wall should be properly insulated. From my accompanying drawings it becomes clear that the ideal greenhouse should have a dome shape, cut vertically along an east-west line.
My half-dome sun pit greenhouse was designed to meet theoretical solar conditions—both from the standpoint of maximum mid-winter absorption and mid-summer reflection of the sun's rays. The accompanying drawing illustrates some of these solar considerations, and how they might influence greenhouse design. As shown, the noontime altitude of the sun gives only a minor part of necessary design criteria; among other things one needs to know the azimuth angles. And of course these sun angles vary according to latitude, north or south of the equator.
So following theoretical considerations, the practical approach to homestead greenhouse design is to build a scale model and investigate the yearly sun path with a heliodon. A heliodon is simply a simulated sun machine. It gives an accurate solar account for any time of the day, at any season of the year, for any specific latitude.
The heliodon that I have used in the past (mostly in conjunction with architectural models) was recently revised so large-size three-dimensional homestead layouts could be designed and analyzed. I also simplified the fabrication, to make it feasible for any homesteader-builder to have his very own. With this machine, one can determine optimum building location and orientation, roof overhang and window placement. It is especially valuable for locating new trees-shade tree size and positioning, in particular. The solar effect on every homestead building can be immediately perceived, as with garden, fields, wood lot, and general land topography. All of this contributes to that all-important, number-one factor of homestead planning: make your mistakes on paper.
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