General
Sunlight is made up of visible light and non-visible radiation such as ultraviolet (UV) and infrared (IR). Depending on the color and surface characteristics of an object, various wavelengths of solar radiation may be absorbed, reflected, and emitted (i.e., “released”). The more light absorbed and the less heat capacity (i.e., thermal mass), the greater the object’s ability to be heated by sunlight. For example, a dark driveway becomes much hotter on a sunny day than a light colored concrete sidewalk. Thus, the sun produces two significant effects that attack materials and shorten their life expectancy:
- Chemical reaction (i.e., breakdown) from ultraviolet radiation and heat
- Physical reaction (i.e., expansion and contraction) from daily temperature cycles caused by objects absorbing and emitting heat gained from sunlight
The chemical and physical reactions caused by sunlight can cause colors to fade and materials to become brittle, warp, or crack. Deterioration can happen relatively quickly (a year or less) or over longer periods of time depending on the characteristics of a material and its chemical composition. In some cases, materials like plastics that are vulnerable to UV degradation can be made resistant by adding UV inhibitors to the chemical formulation. A prime example is vinyl siding. As an alternative approach, materials can be protected from sunlight by matter of design (e.g., providing shading or using reflective coatings).
UV light is not always bad. For example, there are portable UV light wands available for use to sterilize surfaces or materials, UV is used to disinfect water and wastewater, and older style blueprints were created with UV light and special paper that “developed” into the blue line drawing.
However, almost everyone has witnessed or experienced the painful effects of UV radiation on skin, which causes sunburn. Consider that the exterior of a house is like its skin. Therefore, the proper selection of materials determines to what degree the building exterior will be able to withstand the damaging effects of UV radiation similar to the sun protection factor (SPF) for skin. The amount of solar radiation also varies by geography (Figure 6–1), with the number of cloudless days strongly affecting the dose of UV radiation over the lifetime of a product. As you see in Figure 6–1, even though Florida is a very warm and humid state by Climate Zone characteristics, its solar radiation is significantly impacted by the amount of cloud cover, as opposed to the Southwest, where sunshine is abundant and clouds are few.
The following section presents a few measures that can help to manage the effects of solar radiation on building materials and systems. For homes, some of the primary problems associated with solar radiation are color fading, premature asphalt roof shingle failure, and vinyl siding warping. Excessive exposure to sunlight will also cause caulk joints to fail quickly if the proper caulk is not used. In addition, when shining through windows, sunlight can cause interior colors to fade and possibly unwanted heat gain at certain times of the year.