Again, breakage resistance values represent probabilities. The maximum tensile stress acceptable depends on the application, tension duration, breakage consequences, etc. As with U-factor ratings, these are valuable for approximate comparisons of the relative performance of products and for establishing code compliance in some jurisdictions. People should not assume that relative differences in SHGC are proportional to differences in real world performance. At normal incidence, heat gain from high transmittance glazing systems is much greater than that of more reflective glazing compositions.
Reflectance varies with angle of incidence.
Above the critical angle, both systems provide total reflectance. Most solar energy is incident upon vertical glazing systems at incidence angles more nearly parallel than normal to glass. Normal incidence of solar beam radiation is a rare and extreme condition. If the solar beam always remained perpendicular to the plane of the glass, the SHGC would be more valid. By overestimating transmittance, and underestimating reflectance, the SHGC tends to overstate the relative benefits derived from better-rated systems.