By Randy Williams
How to account for climate when picking out windows
Understanding the local climate is key to selecting the right windows for your project. Guest contributor Randy Williams explains what you need to know.
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, including my choice of windows. Windows should also be chosen with local climate in mind.
In this piece, I’ll help you understand how climate affects code, what measures you should look at when assessing windows for your climate, and how window selections can improve performance.
How climate affects code
The American Society of Heating and Air-Conditioning Engineers (ASHRAE) has developed a climate zone map that’s included in the International Residential Code (IRC), Chapter 11, Energy Efficiency. It’s meant to aid in designing heating, cooling, and ventilating systems that match the insulation, air sealing, and moisture profile of a building.
The map was created using historic climate data to divide the U.S. into distinct climate zones by county. Climate zones range from 0 to 8, where 0 is extremely hot and 8 is extremely cold. Within each climate zone, there are subcategories indicating whether the location is moist, dry, or marine. For example, Texas spans climate zones 1 through 4 and within those climate zones has both humid and dry subcategories. Temperature and precipitation are key data points used in developing the climate zone map, but other weather metrics are considered as well, such as temperature, wind speed and direction, precipitation, humidity, and solar radiation.
Building codes base window performance on local climate. There are two window-specific metrics that the codes address, U-Factor and Solar Heat Gain Coefficient (SHGC) values. U-Factor values are key in cold climates, and SHGC values are key in warm climates.
Understanding heat loss in cold climates
Building codes will require better insulating properties from windows in cold climates, than in warm climates. To understand how well a window keeps heat from escaping, you need to look at its U-Factor value.
U-Factor values measure the rate of energy (heat) loss. These values are listed differently than what you might be familiar with if you're aware of R-Value, which measures resistance to heat flow. With R-Value, the higher the number, the better the insulation performance. With U-Factor, lower numbers are better because it’s a measure of how fast heat moves through the window assembly, which means it’s the inverse of R-Value. U-Factor values are between 0 and 1 and will almost always be a decimal point.
Here’s what the IRC lists for U-Factor values in each climate zone:*
- No U-Factor requirements in climates zones 0 and 1
- 0.40 or lower in climate zone 2
- 0.30 or lower in climate zones 3 through 8
* Based on the 2021 IRC table R1102.1.2 (R402.1.2))
In addition to the product’s performance label, you can find U-Factor values for Andersen products by working with your local dealer or by looking up Andersen products in the NFRC Certified Product Directory.
Factors affecting window performance in a cold climate
Glass, frame construction, and installation can all improve the insulation value of a window.
A key selection is the right low-emissivity (Low-E) glass coating or combination of coatings. In most climates, a Low-E4® glass coating is a good choice. It reflects heat away in summer and keeps heat in during winter. However, there are other options that can help make the most of the sun in heating a home.
The number of panes of glass used in a window’s construction also makes a difference. While dual-pane glass is standard on Andersen windows, and an improvement over the single-pane glass sometimes found in older homes, triple-pane glass is the most insulating option. Triple-pane glass is available with windows from the Andersen® A-Series, E-Series, and 400 Series product lines — read more about this option.
Thermally broken frames are window frames where none of the materials on the exterior of the window are continuously connected to the interior of the window. This break helps limit the transfer of energy between outside and inside. Most Andersen® windows are thermally broken, including the best-performing A-Series product line.
Finally, a proper installation, i.e., one that connects all the control layers around the window, will work in conjunction with thermally broken frames.
