Designing Sun Control

[Published in AIA East Bay's ARCHnews May 2008]

Until the energy glut of the 1900s, architects designed the architectural shell to reduce loads. Reducing the cooling load has a positive ripple effect through all the systems of a building. Cooling equipment can be downsized, perhaps crossing a threshold to a less intensive cooling system. Fan power and duct sizes can be reduced. Less space is needed for mechanical equipment.

One of the easiest ways we can do this again is by keeping the sun out of windows. Properly sized overhangs are a great place to start with sun control. A fixed overhang will admit low winter sun when the radiation might be useful for heating, and exclude high summer sun when the indoor climate is too hot already.

How do we properly size overhangs? First, we have to know what seasons need shading. For this, a good tool is Climate Consultant 3 software developed at UCLA. The newest version works on both Mac and PC systems, so there is no excuse for not using it. A free copy can be downloaded from: http://www2.aud.ucla.edu/energy-design-tools/.
The program’s Sun Shading Chart will show graphically which times of the day and year that solar heat is useful for the indoor climate. At my office, we start each project by printing charts from Climate Consultant to guide our schematic design.

Next, we have to design geometry that shades in the hours and seasons we’ve identified. Until the advent of an easy 3D program with solar shading like SketchUp, we had to use mystical tools like the Pilkington Sun Angle Calculator and design in plan and section using a protractor. (SketchUp is available from: http://www.sketchup.com.)

In SketchUp, I can quickly draw a test overhang and then turn on the shadows. Using the sun shading controls, I can move the sun through the seasons and watch the overhang’s shadow move up and down the window below it. Then I resize the overhang to shade farther into spring and fall, if that’s what my Climate Consultant chart indicated.

Using shadows in SketchUp, it’s easy to see that some orientations have no overhang solution. Even vertical fins cannot keep low sun out of windows that face due east or west. On a current project, we designed operable, louvered shutters that the occupants will close at night before leaving the building, and open in late morning during a coffee break. We cut the simulated cooling load by 50% by shading these windows between sunrise and 10:30 am, when the sun is too low for overhangs to work. More synergies can come from such a layering of the window openings: the shutter design creates the opportunity for a lockable security system over the east windows, and the potential for leaving the windows open at night for night-ventilation cooling. We can pay for the shutters using savings from the mechanical system.

Sun control is usually one of the most cost effective measures to squash the loads on mechanical equipment, which is the key to having a low-energy building. Using Climate Consultant 3 and SketchUp, shading design becomes very straightforward. The next step is simulating the shading design in an energy model to see the ripple effects throughout the building system.