Figure 1: Seasonal Variation of the Solar Path

For Barton Creek, Belize, approximately 17° North Latitude

Solar noon, approx. December 21, shortest day-^

Solar noon, approx.

Solar Incidence as a Design Element

The sun is the primary engine of heat gain in a tropical dwelling. It is not usually ambient air temperature that causes heat discomfort, but the radiant energy of sunlight, either directly or re-radiated in long wave infrared. The first line of defense against heat buildup in a building is to minimize the surfaces that sunlight can fall on.

It is obvious that the building's roof is going to be the main absorber of solar energy. If the roof is designed to block heat flow down into the dwelling, and made large enough to cover and shade the walls, the builder should be successful at reducing unwanted heat. This simple concept is more difficult to accomplish that it seems at first.

If the sun was always in the high-noon position, the job would be simple, but it's not. In the morning, it starts out shining low in the eastern sky. It can heat up a building's walls for many hours before it rises high enough for the roof's shadow to shield the east wall from radiant energy. In the afternoon, the sinking sun has the same effect on the western wall.

Orientation for Minimum Incidence

Something can be done at the design stage to reduce this wall heating. The very first effective step is to design and orient the structure on the building site so that the areas of the east and west walls are minimized. Long, unshaded walls on the east and west sides of a building can significantly contribute to the heating problem.

This problem is not as severe on the north and south walls. The sun will be lower in the southern sky in winter when wall heating is not as big a problem. But the sun will never be as low in the southern sky as it is near sunrise and sunset in the east \ and west, so engineering roof overhangs to block the southern sun is much easier.

Roof Overhangs

In Figure 2, angle A represents directly overhead. Angle B N has its pivot point at the base of the south wall. It is plotted at the local angle of north latitude. At that angle, the sun would appear

Figure 2: Angles of the Sun and Cast Shadows

Tilt of the Earth's axis in relation to the sun is 23° 27' (rounded off to 23° for our purposes)

Angle of the sun in the northern sky on the day of summer solstice

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