Studies show that windows account for anywhere between 15 – 20% of a home’s heat loss to over 30%, depending on the source and the data modeling being used. Despite the discrepancy in the figures, it is clear that windows are a significant source of potential loss of energy efficiency in our homes. Before an actual discussion of how windows affect your energy efficiency, however, it is important to discuss the terms that will be used to discuss the features that make a window energy efficient, or not, as the case may be.
An item’s U-value is the measure of how much heat the item allows to flow through it; specifically, it is the rate of heat transmission through a square foot of the material in question when there is a degree of difference in the temperature of the air on either side. The lower the U-factor, the less heat it allows to escape, and the more energy efficient the item is.
Directly related is an item’s R-value; where the U-value measures the amount of heat transmission through an object, the R-value measures an object’s resistance to heat flow. The higher the R-value, the better an insulator the item is.
A window’s Solar Heat Gain Coefficient, or SHGC, refers to the amount of radiant heat the window admits in the form of solar radiation. Given as a value between zero and one, the lower the SHGC, the less heat a window structure allows through.
Visible Transmittance, or VT, measures how much sunlight passes through a window. Again given as a value between 0 and 1, the higher the value, the more sunlight the window allows to shine through.
Air leakage is mostly self-explanatory; it is the measure, expressed in cubic feet per square foot, of the air that passes through a window’s structure. The lower the value, the less air leaks through, and thus the lower heat loss or gain via heated or cooled air through the window.
Except for the air leakage value, all of the above values are typically listed on a window; if either U-value or R-value is not included, they can easily be calculated, as one is the inverse of the other.
Stay tuned for Part II, coming soon, which discusses how the different window frame types and structures affect a windows’ energy efficiency.