When it comes to performing Manual J calculations, window heating loads represent one of the most challenging elements to estimate given the variations in window placement and sizing. As a result, software programs such as CoolCalc are becoming increasingly popular because they provide an easier way to approximate heating loads. Despite this, window heating load calculations are often grossly overestimated in Manual J reports as observed by Abode’s Training & Development Manager, Mike Simons. This article explores the considerations of using software to estimate heating load from windows. 

The most important variable to consider when estimating heating load is the NFRC U-factor rating of the window. This is defined as the rate at which the window transmits non-solar heat flow (i.e., how well it insulates) including the frame and spacer material. The default and most commonly used setting for window U-factor in CoolCalc is 0.87, which assumes that the windows are not well insulated. This is populated based on the home age selected by users at the beginning. However, a cursory glance at the U-factors of commonly used windows shows that this value is overestimated. Double-paned windows typically have a U-factor of about 0.3 while even single-paned windows with storms use a factor of 0.5. The impact of this difference on heating load calculations is shown below through an exercise:

Outdoor Temperature = 32°F

Indoor (Design) Temperature = 70°F

Surface Area of windows = 500 ft2

Window Type = Single-pane storm windows (U = 0.5)


Rate of heat loss calculation = 0.5 × 500 ft2 × (70°-32°) = 9,500 BTU/h

CoolCalc rate of heat loss = 0.87 × 500 ft2 × (70°-32°) = 16,500 BTU/h

These simple calculations serve to illustrate the impact that incorrect U-factor values can have on Manual J estimates. The CoolCalc heat loss calculated is almost double the actual heat loss and is equivalent to having 870 ft2 of windows with the correct U-factor applied. It is critical to verify the accuracy of the values used in Manual J calculation software to ensure the accuracy of the heating load estimate. Generally, if the window heat loss accounts for more than 40% of the entire home’s heating load, these values should be re-checked before moving forward. The importance of using accurately-calculated Manual J estimates cannot be overstated as it prevents system over-sizing and improves overall customer satisfaction, which are key to accelerating electrification.