For reference here is the Minneapolis temperature statistics for the FLIR thermal image of the patio door and window. Outdoor temp -9 degree Fahrenheit with a north wind at 6 mph. Indoor air temp 69 degrees. The patio door and window face west. The window is a single hung with double pane insulated glass rated at U-0.28 and is low e coated. Patio door is a slider, double pane insulated glass, low e coated U-0.28. Exterior wall is 2×6 construction.

We have already discussed foundation and attic insulation, which are significant parts of the thermal envelope. Windows take up the least square footage of the thermal envelope but account for around 30% of heat loss or gain for a home. We need windows for safe egress from bedrooms and to let light into the house. Very few exterior walls don’t have windows, and some cities have percentage requirements of square feet of windows to square feet of exterior walls.

Here is some background information when we talk about windows and how well they insulate. A Window is rated in U-value vs R-value, which gets confusing. The reason for the U-value rating is that it’s a combination of all the window materials plus the insulated glass. Make sure when buying windows that the U-value rating for the window is the overall rating for the entire window, not a center of glass measurement, which will always be the highest U-value rating of the window. For this article, we will use both U-value and R-value ratings to make the comparisons easier. While it looks confusing, It’s pretty straightforward to figure out the R-value of a window. The formula is (1/u-value = R-value), and the opposite works to get the U-value (1/R-value=U-value). Here is an example: 1/ U-0.40 = R-2.5. As we look at the U/R values, you will notice that as the U-value goes down, the R-value goes up. You will also notice that R-values of windows are really low compared to all other insulation values for a home. With that in mind, let’s look at the requirements for the 2021/2024 Energy Code, Energy Star, and Energy Star ZERH programs for climates zones 6 & 7.

The U-factor requirement in the 2021/2024 Energy Code for climate zones 6 & 7 is not overly strict, allowing for a U-factor of 0.30 (R-3.3). This provides a wide range of window options, including double-pane insulated glass windows from many manufacturers. However, the U-factor requirement in the Energy Star program is slightly more demanding at U-0.27 (R-3.7), which means the choices become more limited. Despite this, many manufacturers still offer affordable double-pane insulated glass windows, mostly in casement and awning styles, that meet the Energy Star requirements.

The Energy Star ZERH requirements are even more stringent, with a U-factor requirement of U-0.25 (R-4). This means that triple, quad-pane, or vacuum glass windows are mostly needed to meet and exceed the requirement. However, some double pane glass can also meet the requirement due to excellent edge of glass insulation by manufacturers. While these tighter requirements used to limit the choice of manufacturers, the good news is that more manufacturers have been offering triple pane windows, especially in their base windows lines, in the last couple of years. This trend has significantly lowered the costs of triple pane windows, making them more accessible.

For this Energy Star ZERH build, the builder selected a combination of triple pane casement and awning windows rated at U-0.20 (R-5). The triple pane option adds about another $1285 to the window cost from this manufacturer. The gain in window insulation is well worth the extra cost because the change from the U-0.30 (R-3.3) window for the 2012/2024 Energy Code to the U-0.20 (R-5) window for the ZERH requirements improves the window insulation by 51%. That will equate to lower energy bills in both the winter and summer and a more comfortable quiet home.

Up next what everyone has been waiting for! The EcoSmart™ Stud exterior wall system and the Energy Star ZERH build.