Solar orientation of a greenhouse
In order to obtain maximum benefit from the sun,
the glazing (window wall) of the greenhouse should face nearly due
south. In adding a greenhouse onto an existing house, the wall can
be up to 30 degrees off true south without losing too much benefit
from the sun. A wall between 30 to 45 degrees off true south will
benefit by using the triangular design described at the end of Low-Cost Sunroom, as might those marginal ones
between 20 and 30 degrees.
There are two ways of finding solar orientation.
One is to use a magnetic compass. Check with your
library or high
school science teacher to find out how far off true south magnetic
south is where you live (a figure known as declination). Or
visit
http://www.ngdc.noaa.gov/geomag-web/ to find
declination for your location online. If your declination
consists
of only a few degrees, and if the wall you want to put the
greenhouse on is close to true magnetic south, you have nothing to
worry about. If, on the other hand, the compass shows your
wall is
10 to 15 degrees off true south, make sure the difference between
true and magnetic south is not great enough to throw you beyond 20
degrees off true south.
True south is where the sun is at solar noon. The
other way of finding out how close to south your building is facing,
is to
find out when solar noon is in your location and to use a post or
perpendicular stick to cast a shadow on the wall at solar noon. If
the wall is facing due south, the shadow will stand straight up on
the wall, and will be perpendicular to the wall on the ground in front.
The distance that the shadow is off from this description shows the
number of degrees the wall varies from true south. You can use a
protractor where the shadow on the ground meets the wall to measure the angle.
When this booklet was first written (1981), Scott
County, Virginia, had a declination of only 3 degrees, so the
compass reading could be used without correction. However,
declination changes in each location over time, and the area's
declination is now 6.75 degrees.
To read more about building a greenhouse add-on for less than $15 per square foot, download the 99 cent ebook Low-Cost Sunroom.
| This post is part of our Low-Cost Sunroom lunchtime series.
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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.
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It needs one small addition: "on the northern hemisphere".
Alternatively, using "should face the equator" would hold true everywhere.
Although people living within say 20 degrees of latitude from the equator are unlikely to be needing greenhouses.