Optimizing Glazing Angles
This simple process identifies the angle of glazing (in either the walls or roof) that will maximize light transmission for a particular time of year. You can also find the range of angles that will allow for sufficient light transmission, taking into account that multiple angles yield nearly equivalent light transmission levels (per discussion in Chapter 5, “Calculating the Angle of Glazing”).
First, determine the angle of the sun during the season you want to grow. Ideally, this would be the average angle over the whole season. For instance, you may aim to maximize heat and light during the winter, rather than at winter solstice specifically. You can find the average solar altitude angle for a season by using online tools such as suncalc.org, or see the sidebar in Chapter 4, “Tools for Siting the Greenhouse.” Here we’ll use an example angle of 30 to indicate the average angle during the winter season at our greenhouse site.
1. First draw your ground plane and sketch the angle of the sun.
2. Draw a line (#1) at a right angle to the angle of the sun. This is your “perfect roof pitch,” the perpendicular plane that would receive the maximum light transmission — although it’s one that would probably be difficult to build. Keep in mind that this is a representation of a 0-degree angle of incidence, or “normal.”
3. Now, draw a second line (#2) 45 degrees from line #1. This represents how far the angle of incidence can deviate from the ideal and yet still yield sufficient light levels; this can be much more practical to build. (It is based on the fact that up to an angle of incidence of 45 degrees, there is little change in light transmission, as shown in Fig. 5.9.) Keep in mind that building to line #2 reduces the area that receives incident solar radiation (the view window) during the winter, so you would want to augment with enough vertical or near-vertical glazing on the walls of the greenhouse. Anywhere between line #1 and line #2 should be a suitable roof slope to ensure enough light during the winter.
4. Finally, you can calculate for the angle of the glazing relative to the ground simply by knowing that all the angles shown add up to 180 degrees. Since three of them are given, simply subtract the sum of the three from 180. In this case the result is 15 degrees for the minimum glazing angle, also considered the roof slope.