Windows and doors must perform two conflicting tasks: Be a barrier, and be a portal. While it is frequently noted that the easiest way to design a net-zero building is to eliminate openings, few of us can actually bring ourselves to design, build, or inhabit a windowless building. We are thus faced with the task of designing an envelope that is at once porous and sealed.
Window choice is important here, and while the window/wall ratio must be a part of the equation, I will discuss the window itself.
We have a small residential design project hoping to achieve Passive House certification in an ASHRAE 6B (cold and wet) climate zone: so it’s cold at the site. The project also has a tight budget. We looked at a variety of window manufacturers to determine whose windows might be appropriate. There are a number of factors to look at when choosing a window manufacturer and window type, but for this discussion on energy performance, we looked at U value and Performance Grade.
U value is of course the inverse of R value, and is a metric for determining resistance to thermal energy flows. U values have a maximum of 1 (perfectly unable to stop the flow of energy) and a theoretical minimum of 0 (perfectly able to stop energy flow). A very energy efficient window has a U value of .2 or .16, which corresponds to an R value of 5 or 6.
U value in windows is a function of the U value of the glazing and the U value of the frame. Window manufacturers may report center of glass U value, which is the U value of the glazing. They may also report whole unit U value, which is the better metric. In general, it appears that a window’s glass is almost always more thermally resistive than the window’s frame. Therefore whole unit U value will generally be lower than center of glass U value. Because most window frames have less area in elevation than the glazing they support, glass U-value will dominate. There is only limited benefit in designing a window frame that is significantly more energy efficient than the glass.
Performance Grade (PG) is the industry standard measurement for leakage. A good performance grade depends on the frame. It is based on several tests: air leakage, structural stability, water leakage, and, in the case of doors, opening force. U values are somewhat influenced by window type (casement, fixed, slider, double hung), while Performance Grade is significantly influenced by window type. Because thermal performance is the result of energy and air flows through the window, it is important to look at U value and Performance Grade together.
Because thermal performance is the result of energy and air flows through the window, it is important to look at U value and Performance Grade together.
In surveying commonly specified windows in Maine, we see U values in the range of .17 to .24 for triple glazed casements, and Performance Grades in the PG45 to PG70 range. Manufacturer web sites are not consistent in their reporting. On the high end, Kawneer has good data supporting their PG70 claims, while Loewen may have PG70 windows, but there is some ambiguity in their reporting. Interestingly, windows from Matthews Brothers, Marvin, Pella and Andersen all appear to have casements in the PG40 to PG 50 range, which means these windows should all perform similarly with regard to air and water leakage. The same is true for U value: U.17 is about as good as one can get from a triple-glazed window with two low-E coatings and argon/krypton gas. Intus, which is popular amongst Passive House folk, also maxes out at .17. Intus does not have much literature supporting their Performance Grade. They assert ratings will be available this year (2015).
With regard to U value, Alpen does offer windows with reported values in the .12-.15 range. In other words, their windows seem to break through the R-5 or R-6 barrier and head toward the R-7 or R-8 realm. Alpen’s web site does not report Performance Grade.
It appears, then, that windows available in Maine have similar performance, both with U value and Performance Grade. Again, other factors come into play when choosing a window, but our recent research suggests no compelling reason to spend the farm an a high-performance window from a brand-name manufacturer.