The most thermally-inefficient parts of a high-performance residential building are the windows. The insulating values are usually in the range of R-3/inch (typical) to R-10/inch (very high-quality). When compared to typical wall insulation of R-21/inch, thermal movement is going to take the path of least resistance—the windows.
Even more importantly, if you want to realistically understand how a building component performs, you need to accurately evaluate it. Passive House windows calculations, if done correctly, are noted for their accuracy. Sadly, the California Energy Code, also known as Title 24, is not nearly as accurate, for a variety of reasons. In this article, we discuss one of those reasons.
There are several factors that determine how a window performs. Some of them are well known, others are not. For instance, anyone in the business of modeling, specifying, or ordering windows is probably familiar with Solar Heat Gain Coefficient (SHGC) and U-factor. These are the two most important specifications used by Title 24 to evaluate a window.
SHGC is simply a measure of how much solar radiation can penetrate a window and get to the inside of a building. In the winter, that radiation can reduce the heating load. In the summer it can increase the cooling load. The U-factor, also called conductivity, is a bit more complicated. It’s the overall ability of the window to allow thermal transfer. Just like SHGC can affect heating and cooling loads, window U-factors can do the same. A higher U-factor facilitates more thermal transfer; a lower value reduces it.
The window U-factor is affected by several components. The most important ones are the U-factors of the frame and the glass, the U-factor of the spacer–that assembly where the glass and the frame meet–and the U-factor where the window meets the wall.
When evaluating a window’s performance in the Passive House Planning Package (PHPP), you have to specify all these individual items. That’s one reason why the PHPP is so accurate. By comparison, when evaluating a window in any of California’s approved Title 24 modeling programs, you only specify the SHGC and U-factor of the whole window assembly. These whole-window values are calculated using techniques developed by the National Fenestration Rating Council (NFRC).
All newly-installed California residential windows are supposed to be NFRC rated. If not, the modeling software assigns very unfavorable default values that significantly penalize the modeled energy performance. From California’s perspective, very high-quality Passive House windows that are not NFRC rated perform extremely poorly.
Even NFRC-rated Passive House windows are not treated accurately in California’s modeling software. This is due to the widely-accepted NFCR calculations. Those calculations take into consideration all the items mentioned previously—individual frame and glass U-factors, the spacer, and the installation situation.
The results provide a single U-factor for the entire window assembly. For NFRC certification, the calculations are done for only one window size. The assumption is that this calculated U-factor is accurate for that type of window, regardless of its size. This is a false assumption.
The chart below shows the heat load changes per window square foot for four scenarios using windows of various sizes ranging from four to 100 square feet:
1) A NFRC-rated window with a 0.35 U-factor using Title 24 modeling.
2) A PHPP-calculated window with 0.35 U-factors for the frame and glazing, approximating the Title 24 approach.
3) The PHPP-calculated window with the glass U-factor reduced to 0.15.
4) The PHPP-calculated window with the glass U-factor increased to 0.50.
The chart calculations were done in such as way as to minimize the effects of the SHGC, the window orientation, the spacer, the installation situation, and the reduction of wall area as the window gets larger, effectively isolating the changes due to the glass U-factor.
For the first two cases, the window size has no effect on the results. The overall heat load, which is not shown, changes, but the heat load per square foot of window doesn’t change.
For the third and fourth cases, where the glass U-factor is different than the frame, the glass has more of an effect than the frame as the window size increases. When the glass U-factor is lower than the frame U-factor, the heat load drops as the window gets larger. Conversely, the heat load increases with the higher glass U- factor as the window size increases.
Historically, the NFRC-based calculations used by Title 24 have provided reasonable results. However, as the focus on high-performance buildings increases, it’s clear that more detailed and more accurate modeling is required. The Passive House Planning Package takes all relevant factors into consideration when analyzing windows. That’s one reason why it’s results are more accurate than those calculated by Title 24’s residential software.
As a result, when working with Passive House-savvy California architects, I’m usually asked to do Title 24 calculations first, for permitting. Then I’m asked to provide the more accurate PHPP-calculated loads for mechanical equipment sizing.
By Steve Mann
© 2021 by Home Energy Services.
