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Solar orientation of a greenhouse

Variable degrees of south-facing buildingsIn 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 Declination10 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.  Read all of the entries:





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In order to obtain maximum benefit from the sun, the glazing (window wall) of the greenhouse should face nearly due south.

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. :-)

Comment by Roland_Smith Wed Nov 20 15:37:32 2013
I always go back and forth about whether to add that note in. It seems to confuse Americans, while people in the southern hemisphere often get enough of their literature from the northern hemisphere that they know to make the mental switch. (I guess what I'm saying is that Americans are stupid and Australians are smart? Maybe I should rethink this.... :-) )
Comment by anna Wed Nov 20 16:34:54 2013

One very unique homestead, $1,500 per acre, the opportunity of a lifetime