This article by Clarke Snell was originally published in the New Life Journal.
If you ask 10 kids to draw a picture of a house, I can almost guarantee that they’ll all include a door and at least one big window. Ask those same kids 25 years later to describe their dream houses and I predict they will all be crammed full of windows. What I’m saying here is that in my experience, we all love windows. What’s wrong with that? Well, if our goal is to create an energy efficient building, typical glass-filled openings are actually a real pain in the astragal because compared to modern wall systems they perform horribly. In this month’s column, I’ll explain the basics of why this is true. Then next month, I’ll tell you what you can do about it.
Sidebar: R-value vs. U-value
Resistance to heat flow in building materials is usually quantified as R-value. The higher the R, the better the insulation. Just to confuse us, the insulation value of windows is expressed as U-value which is the inverse of R. To find out the R-value of a window, divide 1 by its U-value. For example, U= .4; 1÷.4 = R2.5
Other than keeping rain and snow out of your bed, perhaps the most pivotal function of your house is its ability to create a different temperature inside than the temperature outside. This is accomplished by wrapping the interior space with insulation, a generic term for a material designed to resist the flow of heat. To oversimplify for our purposes, the better this insulation cocoon functions, the less heating or cooling the building will need. Since heating and cooling both cost money and usually involve global warming creating carbon emissions (our buildings are responsible for about 50% of our collective carbon footprint), improving insulation has been a focus of the green building movement. In recent years, we’ve made incredible strides and now have access to insulation systems that can produce walls systems with R-values (see sidebar) in the 20’s, 30’s, and even 40’s. Typical new windows, however, have R-values of only 2 or 3, 10 or more times worse than the wall itself. This is almost equivalent to a thermal hole in the wall. Therefore, the main performance flop for windows is their inadequate resistance to the flow of heat.
Mean Radiant Temperature
Mean radiant temperature is basically the average temperature of the surfaces of everything in the vicinity of your body. In a house, that means the surface of windows, walls, furniture, dusty knickknacks, and everything else. All of these surfaces radiate heat outward toward your skin, and your skin in turn radiates toward them. Since windows are so bad at slowing heat movement, their surface temperature will tend to be very different than that of other surfaces in your house. If the surface temperature of an object near you is considerably less or more than that of your body, you feel it as cold or warmth. This is why on a cold winter day, the thermostat can read 70◦F and you’ll still feel cold standing by a window. Low surface temperature, then, is another way windows drag down the overall thermal performance of our wall system.
Doors and operable windows are basically huge holes that can be opened and closed. By definition, though, that closure is never perfect. The hole always leaks. Gaps and cracks in our wall will allow air to bypass insulation resulting in the movement of heat in or out of our building. Therefore, another strike against windows and doors is their contribution to this air leakage.
Solar Heat Gain
Responsible energy efficient designs incorporate a basically infinite, free source of energy: the sun. In our climate this means letting the sun in during the winter. We need glass-filled openings to accomplish this. Different glass types and configurations let in more or less of the sunlight that hits them. This is quantified as a number called the solar heat gain coefficient (SHGC) which is basically the percentage of potential solar heat that glass lets into the building. For example, a SHCG of .5 means that 50% of the potential solar heat is making it through the glass. There are situations where we want solar heat gain and others where we don’t, so the wrong glass type in the wrong place can be a major detriment to building performance.
The point I’m making here is that windows and doors are typically VERY weak spots in the performance of a modern building. Next month, I’ll give you the skinny on how to choose the right windows and doors for new construction and remodeling or how to spiff up the performance of your existing underachieving glass units.