Masonry Magazine June 1981 Page. 19
Transmittance
The glazing material used should have a high transmittance, low absorptance and low reflectance of solar radiation. The high transmittance is important so that the maximum amount of solar energy may be transmitted to the interior of the passive solar building where it can be absorbed and stored in the interior brick masonry. Generally, the higher the transmittance the lower the absorptance and reflectance. When the glazing is desired to provide specific daylighting requirements, it may be preferred to use a glazing material which diffuses the direct solar radiation.
Absorptance
Depending on the glazing material and the frame assembly, absorptance should be a factor in selecting the glazing material or a proper framing assembly. The higher the absorptance, the lower the amount of energy transmitted to the interior. Typically, the absorptance of most glazing materials is low. The amount of solar absorptance may be important because of thermal stress that may occur within the glazing material itself or between the glazing material and framing assembly. The framing assembly should be such that it allows for thermal expansion of the glazing material being used to avoid structural failure of the collector component.
Reflectance
The reflectance of the glazing material should be kept to a minimum, so that the maximum amount of solar radiation may be transmitted to the interior. In addition to the consideration of reflectance, as related to transmittance and maximum solar performance, reflectance might also require consideration of exterior glare which may be annoying or even hazardous because of impairing vision.
Thermal Conductivity
Typical values for overall coefficient of heat transmission of glass are given in Table 3. Table 3 also provides solar transmittance correction factors for glass. The solar transmittance correction factors are based on double glass having a solar transmittance value of 1. Information regarding plastics or other glazing materials must be obtained from the material manufacturer.
The basic material properties for single, double and triple glass given in Table 3 may be used as a means of selecting the appropriate glazing material by considering the trade-offs between the transmittance and U value. Single glass has about a 21% increase in transmittance and 124% increase in heat loss, over double glass, and thus should only be used in lieu of double glass when night insulation is used. However, triple glass has approximately an 18% reduction in transmittance and a 37% reduction in heat loss over double.
TABLE 3
Material Properties of Glass"
| Material | U Value | R Value | Transmittance (Solar Radiation) Correction Factor |
|---------------|---------|---------|----------------------------------------------------|
| Single Glass | 1.10 | 0.91 | 1.212 |
| Double Glass | 0.49 | 2.04 | 1.000 |
| Triple Glass | 0.31 | 3.22 | 0.825 |
*Values may vary with material, see manufacturers recommenda-
tions.
glass and may be used in lieu of double glass with night insulation, to increase economic feasibility at only a slight reduction in overall performance. When considering such trade-offs it is extremely important to consider the specific environmental factors at the site. Triple glass in lieu of double glass may be excellent in areas of high solar radiation and cold exterior temperatures, but not effective in areas of low solar radiation and moderate exterior temperatures. The effectiveness should be determined by steady-state heat loss calculations and passive solar performance analysis.
Durability
The durability of the glazing material must be such that it resists failure due to thermal stresses and that it does not degrade or discolor when exposed to solar radiation for extended periods of time. Of course these considerations will vary with economics. If a material degrades frequently but is inexpensive to replace, it may be more economical than a more expensive, more durable glazing material. Additional considerations in selection of a glazing material may be resistance to impact, and ease of replacement due to breakage.
No matter what glazing material is selected, the designer should be assured it will maintain an expected condition that is not detrimental to the performance of the passive solar energy system for its intended period of use. Materials that discolor or degrade rapidly and have significant reduction of solar radiation transmittance should not be used.
METRIC CONVERSION
Because of the possible confusion inherent in showing dual unit systems in the calculations, the metric (SI) units are not given in the examples. Table 13 in Technical Notes 4G provides metric (SI) conversion factors for the more commonly used heat transmission units.
SUMMARY
This Technical Notes provides information on the component materials for passive solar energy system applications. This offers a designer or owner sufficient information regarding the material properties of brick to assist in the design and use of brick masonry in