This post originally appeared on the Andersen Windows website.
Where do you live? Is it cold or hot, wet or dry, or something in between? I live in an area that is considered cold and moist, Northern Minnesota. My climate is much different than Tucson, Arizona or Kansas City, Missouri. Because of the climate differences, some of my choices in building materials will be different than if I were located in one of those other areas.
Figure N1101.7 (R301.1) Climate Zones-2021 International Residential Code (IRC)
This climate zone map is published by the American Society of Heating and Air-Conditioning Engineers (ASHRAE) and is included in the International Residential Code, Chapter 11, Energy Efficiency. ASHRAE’s purpose is to create standards of how an engineer or HVAC professional calculates and designs heating, cooling, and ventilations systems to match the insulation, air sealing, and moisture profile of a building. When designing these often-complicated systems, where the structure is located becomes key, this is the reason the map was created.
The map is broken down by individual counties in each state by their historic climate data. Based on that data, each county is placed in a numbered climate zone, the numbers start at 0, which is extremely hot, (there are no locations within the United States that have a climate zone 0 designation, yet), and move to climate zone 8, subarctic/arctic (extremely cold). There are also subcategories to each climate zone indicating whether the location is moist, dry or marine. Texas, for example, has serval different climate zones both in humid and dry categories ranging from climate zone 1 to climate zone 4.
Each climate zone gets its designation based on historic weather patterns gathered from nearly 5,000 weather sites across the US. These sites monitor several different weather metrics, such as temperature, wind speed and direction, precipitation, humidity, solar radiation along with other weather-related data. The most pertinent information used in developing the climate zone map is temperature and precipitation.
So, how does this information influence window selection? Codes base window performance on location, the climate of the project. There are two specific metrics the codes address, U-factor and Solar Heat Gain Coefficient.
U-factor is the insulation value of a window but is listed differently than what most in the building industry are familiar with. R-value is better understood, the higher the number, the better insulation performance. U-factor is the inverse of R-value, lower numbers are better with the values being between 0 and 1. They will almost always be a decimal point. The formula for calculating U-factor is 1 divided by the R-value. As an example, let’s say we have a wall R-value of R-20, to calculate the U-factor, use the formula 1/R-value. In the case of the R-20 wall, the U-factor is U-0.05. The inverse is also true, to calculate the R-value from a U-factor, you use the formula 1/U-factor. A window listed at U-.25 will have an overall R-value of R-4. Codes will require better insulating properties from windows in cold climate, less insulation will be required in warmer climate.
Solar heat gain coefficient (SHGC) is a measurement of how much solar heat gain is generated when sunlight shines through a window. In hot climates, we want to limit sunlight through the glass, but in cold climates, we may want as much heat from the sun entering the home as possible. Changes in SHGC are typically achieved by the addition of different coating applied to the glass during construction of the window. Codes for the SHGC metric have listed values between 0 and 1 with lower numbers allowing less solar heat gain through a window. Much like U-factor, energy codes specify what is allowed based on climate location. 0.25 or less is required in hot climates, climate zones 0-3. 0.40 or less in more moderate climates, zones 4 and 5, and there are no requirements for SHGC in cold climates (zones 6, 7, and 8).
Heat loss and heat gain are important performance considerations when choosing a window, but there are other considerations. As I said earlier, I live in a very cold climate where it is common to see both liquid and frozen condensation on our windows during the heating season. This is the result of water vapor in the air coming in contact with the very cold surface of the window glass. The temperature of the glass is below the dew point temperature and the water vapor in the air condenses on the glass, the same principles of how a dehumidifier works. (Window condensation can also occur in warmer climates but is usually on the exterior of the window because of very high exterior humidity levels and cooler temperatures inside an air-conditioned home.) There are a few ways to control window condensation, warm the surface of the glass and/or lower the humidity level. Another strategy might be to keep air moving across the surface of the window. Most homes in my very cold climate will choose to lower humidity levels, often wintertime indoor relative humidity will be held in the 20-30% range, sometimes even lower during very cold weather. A better option might be to warm the surface of the glass. By choosing a triple pane window, the surfaces of the glass can be warmed, condensation control is then improved. This is just one example of how a window choice can affect the overall performance of a home.
As a builder or homeowner, how do you choose the right window for your project? Are you making window performance decisions based on what is best for the project or simply choosing to use the minimum requirements of the energy code? Of course, style is the biggest influence, but how the window performs based on where you live should always be a part of the decision.