Shading
And Cooling Periods
Solar control
is a critical requirement for both cooling-load dominated and passively
solar-heated buildings. High-performance glazing systems can significantly
reduce solar heat gain while still admitting daylight. Exterior shading
devices can further reduce solar heat gain, control glare and permit
a wider choice of glazing options. The crucial design issue is to
achieve a balance between solar control and daylighting.
Awnings and overhangs
are the most effective means of solar control since they prevent sunlight
from striking the windows. Movable systems are adjustable according
to season, but are more prone to failure or misuse. Fixed overhangs
are more dependable, but their design must account for daily and seasonal
variation of the sun’s path. A properly sized overhang on southwest
and southeast oriented windows can reduce energy use by up to 6%.
A further 20% can be gained by combining overhangs with daylight controls.
There are no simple
rules of thumb for appropriately sizing overhangs. Each project should
be evaluated depending on its relative cooling and heating needs.
- Design overhangs
appropriately for window size and orientation. The effectiveness
of fixed horizontal overhangs on south-facing windows depends on
the projection from the window plane and distance above the window.
- Extend the
overhang beyond the sides of the window to prevent solar gain from
the side.
- Use slatted
or louvered shades to allow more daylight to enter, while shading
windows from direct sunlight. Tightly spaced louvers near the window
shade high summer sun, and loosely spaced louvers further away shade
low winter sun.
- Reduce solar
heat gain by recessing windows into the wall. The reduction depends
on the size of the window and tends to be modest for recesses less
than 6 in. Deeper recesses provide additional shade and control
glare, but may reduce daylighting.
Vertical fins
at right angles from windows on southwest and southeast orientations
are typically not effective in Santa Monica. Designs should be carefully
evaluated using shading studies to ensure effective shading is provided
without compromising daylight.
Cautions
- Carefully examine
possible reductions in daylighting caused by fixed overhangs or
fins.
- Peak cooling
and heating loads trail summer and winter solstices by up to a month.
- Shading devices
should not block openings for natural ventilation.
