Before we get into this topic, please take a few seconds to consider the following questions:
- Do you plan to work, or have you ever worked, on a Passive House building? (If not, the rest of your answers are probably no.)
- Has your Passive House consultant ever told you that the window U-Value you provided “won’t work in their energy model?”
- Has your Passive House consultant ever told you that your window “doesn’t meet the comfort criteria?”
- Have you ever scratched your head when someone asked you to provide the “Psi-spacer” for your window?
If you answered yes to two or more of these queries, please read on. If not, you’ll still learn some useful information, so why not continue?
If you’re still reading, then you are probably somewhat familiar with a “U-Value” and you may know what “SHGC” means. If not, no worries. This article will explain both, and by the end you’ll be able to talk about these terms with most Passive House nerds.
Quick Recap of Window Performance Lingo:
Window type has a major impact on building energy use. For this reason, building energy codes specify minimum efficiencies required for different climate zones and building types. Two values are typically referenced:
- Solar Heat Gain Coefficient or SHGC
What is a U-Value?
In the simplest terms, U-Value is a measurement of how much heat is conducted by a material per unit area per degree of temperature difference. It is measured in Btu/ft2/hr/oF in the United States. This metric is most commonly used for transparent building components such as windows, and is not to be confused with R-Value (the inverse of U-Value), which is used for solid building materials (walls, roofs, floors, etc.). The lower the U-Value, the more resistant to heat flow (opposite of R-Value). This value changes based on the type of window frame material, the frame design, the number of panes of glass, and a number of other factors.
What is Solar Heat Gain Coefficient (SHGC)?
SHGC is the measure of solar radiation that will pass through a window into a building or space. Factors impacting this value include the type of glazing, the films or coatings on the glazing, and the frame thickness. SHGC is a characteristic of the window itself and does not include shading from surrounding objects. It is typically expressed as a unitless fraction but can sometimes be presented as a percentage. The optimal value for a specific project can vary significantly based on the climate and building type.
Not as simple as it seems…
While these terms are simple in concept, it’s not so simple to understand how they are calculated. This becomes critical if you are an energy modeler because it is very easy to use the wrong metric and obtain drastically different results. In North America, we rely mostly on ratings from the National Fenestration Rating Council (NFRC), a non-profit organization that provides independent ratings of window energy performance. For most designers and energy modelers in the U.S., there is no need to become familiar with other rating systems in order to meet code requirements. Passive House certification, however, requires that window performance values be calculated using standards that are not commonly used in the U.S. and can’t be directly compared to NFRC values. By the end of this article, you will understand why different values are used and the main differences between them.
NFRC vs. ISO
The energy model required for Passive House certification uses metrics calculated according to ISO standards, which are set by the International Organization for Standardization. Fun fact for those wondering: ISO is not an acronym. It is derived from the Greek word isos meaning “equal”and was chosen since an acronym would not work across different languages. NFRC values are typically one number for the entire window at a standard size, while ISO values are typically reported for each separate component of a window (frame, glass, etc.). The “whole window” NFRC approach is meant to allow for easy comparison between different products. The component-specific ISO values give the designer the ability to easily calculate the heat loss for windows of varying sizes with accuracy, which is useful in an energy model. So, while you may specify one NFRC U-Value to meet code, for Passive House energy modeling there are a few more values required.
What about the “Psi-spacer” or “U-edge” value?
The connection between the glass and the window frame contributes significantly to the heat loss of a window. This is measured separately from the frame or glass U-Values because the heat loss at this location is often much greater. It can vary widely based on the type of material used to keep the glass in place and the glass/frame combination. For multi-pane windows (such as double or triple pane windows) a material known as a “spacer” keeps the frames separated and adds structure to the glazing unit. The “Psi-spacer” and “U-edge” are two different measures of the heat flow at this juncture, the first used in ISO calculations and the second used by NFRC. At this point you might be wondering about the difference between a Psi-Value and a U-value.
Recap: Psi-value vs. U-Value
First, Psi, the written version of the Greek letter and a Psi-Value (or -Value) is a measurement of heat loss per unit of linear distance. U-Value is the measurement of heat loss per unit area. The units of a Psi-Value are expressed as Btu/ft/hr/oF (notice “ft” instead of “ft2”). A Psi-Value typically accounts for the additional heat loss that occurs at the connection between two materials. You simply multiply the Psi-Value by the length of the connection for the total heat loss, similar to how you multiply a U-Value by the area of a material to calculate total heat loss. In the ISO/Passive House method, a Psi-Value is used to estimate the heat loss at the glass edge/spacer as well as the heat loss at the frame to wall connection. For NFRC calculations, only U-Values are used.
Passive House Taking it One Step Further: Evaluating the Window Installation Detail
An important note is that Passive House certification requires one additional step when modeling windows that is not used in most standards/energy modeling tools. This is the process of accounting for the additional heat loss at the juncture between the frame and the wall by calculating what is called the “Psi-Install.” This is one of the aspects of Passive House certification that makes it so robust – it forces you to thoughtfully design and evaluate how your windows will be installed from early on in the project. Typically, the Passive House consultant will work with the design team to create good details that over-insulate to reduce heat loss around the frame. Then the Passive House consultant will model this detail in a heat flow modeling tool, such as THERM, to quantify the linear heat loss (or Psi-Value). This additional heat loss is incorporated into the final window U-Value (the “installed” U-Value) used for energy modeling, but it is not typically referenced outside of the energy modeling process.
Figure 1 compares the individual components calculated by each standard and how they generate a whole window U-Value as well as a whole window installed U-Value. As described above, if you are referencing an NFRC value, it is likely the whole window U-Value, however with ISO you are likely referencing the components (noted by the boxes highlighted in green). The addition of the Psi-Install is unique to Passive House certification.
Solar Heat Gain – Just the Glass or the Whole Window?
Figure 2 summarizes the main critical difference between NFRC values for SHGC and the g-value used in Passive House modeling. (Note that g-value is the total solar transmittance, proportion of the solar energy available for the room). Similar to the whole window U-Value concept, the SHGC is typically a “whole window” value, while the g-value is specific to the glass itself and does not include the frame. This may seem like a small difference, but it is critical for energy modeling. Since frames are typically opaque and only allow a small fraction of solar heat to transfer through them, including frames in this calculation will result in a much lower fraction (up to 50% lower) than a “center of glass” g-value for the same window.
Just to make everything more complicated, the underlying assumptions used in these standards vary significantly, so even for an identical window that is the exact same size, NFRC and ISO methods will result in different values. The main source referenced for this article is a research paper published by RDH Building Science, which investigates the differences in the values calculated by these standards for the exact same window (see references below). In general, what was found is that center of glass U-Value, frame U-Values, and overall window U-Values tend to be higher when calculated by NFRC methods. Alternatively, SHGC values tend to be lower, even when comparing only “center of glass” values. One example in the RDH paper is that a specified g-value of above 0.5 for a Passive House project might be achievable with a window that has a NFRC center of glass SHGC value of 0.45 and even a whole window SHGC of 0.35 depending on the type of glass and frame. So, if you wrongly referenced the NFRC value you might rule out a window that might work for your project. All this to say that the standards should not be directly compared.
Two Final Notes: Passive House Certified Windows and the “Comfort Criteria”
Finally, for those who are deeply involved in Passive House design, there are two additional topics to touch on. First, both the Passive House Institute in Germany (PHI) and the Passive House Institute U.S. (PHIUS) have processes for certifying windows for use in Passive House certified buildings, which can be looked up in online databases (see references below). While it is not required that Passive House certified windows be used for building certification, it does make the process easier and it ensures that the correct values are used in the energy modeling tools. The second item of note are the “comfort criteria.” Both programs have methods for evaluating all windows on a project to ensure a minimum interior surface temperature is maintained to provide optimal thermal comfort year-round. This check is climate specific and typically includes the heat loss from the Psi-install calculation. It requires the design team to specify a good window and design a well-insulated installation detail, typically checked within the energy modeling tool itself for each window on the project. The project is often further along in the design process, however using a certified window for your climate zone is a good bet that you will pass (assuming your details are good).
Main Takeaways for Designers
If you would like to learn more, check out the paper by RDH titled “International Window Standards,” linked in the references. Many topics were not covered in this post, such as air tightness, visual transmittance, and affect on other fenestration types. Look forward to these being covered in a later post! For now, here are a few takeaways for designers looking to work on a Passive House project.
- When asked for a U-value, be sure to clarify:
- What standard (ISO, NFRC, or other)?
- Is it specific to the glass (also called “center of glass”), frame, or whole window?
- When asked for an SHGC, be sure to clarify:
- What standard (ISO, NFRC, or other)?
- Is it center of glass or whole window?
- Do not compare between NFRC and ISO values:
- In general, however, NFRC U-values are likely higher and SHGC values are likely lower than ISO values.
- When evaluating a window for a Passive House project, look to see if it is a “Passive House certified component:”
- Check the online database for the program you are using (see references below);
- Use the values reported for energy modeling.
RDH, International Window Standards, 2014
National Fenestration Rating Council
International Organization for Standardization
PHI certified components database
PHI window certification criteria
PHIUS certified windows database
PHIUS window certification criteria
Written by Chris Hamm, Senior Building Systems Engineer