Question: Skylights or "roof windows" are commonly used in homes and manufacturing facilities since they let natural light in during day time and thus reduce the lighting costs However, they offer little resistance to heat transfer, and large amounts of energy are lost through them in winter unless they are equipped with a motorized insulating cover that can be used in cold weather and at nights to reduce heat losses. Consider a 1-m-wide and 2.5-m-long horizontal skylight on the roof of a house that is kept at 20°C. The glazing of the skylight is made of a single layer of 0.5-cm-thick glass (k = 0.78 W/m.K and ε = 0.9). Determine the rate of heat loss through the skylight when the air temperature outside is -10°C and the effective sky temperature is -30°C. Compare your result with the rate of heat loss through an equivalent surface area of the roof that has a common R-5.34 construction in SI units (i.e., a thickness-to-effective-thermal-conductivity ratio of 5.34 m2.K/W). Evaluate air properties at a film temperature of -7°C and 1 atm pressure. Is this a good assumption?
