Of course, windows are an expressive design element, an architectural statement really. A view to your outdoor world; of ocean, city, mountains, or a cornfield (my view). They act as a means to extend the indoor space visually. But in addition to their beauty, and just as important, windows are a vital part of the building envelope, providing thermal barriers, light transmission, sound control, and natural ventilation. Something else windows can do? Contribute to sustainable design and improve occupant health! Because of the importance these fenestrations (or window and door openings) are to a building’s envelope, manufacturers apply NFRC labels to their products. So, what are these labels supposed to tell us exactly?
Ok, if we zero in on just the window or door’s thermal and energy performance, ta-da, that information is all on the:
NFRC LabelThe National Fenestration Rating Council (NFRC) is a nonprofit organization providing an independent, uniform rating system for the energy performance of fenestration products (including windows, curtain walls, doors, and skylights). A window’s NFRC label includes the two most utilized ratings: U-factor and Solar Heat Gain Coefficient (SHGC). It also includes ratings for Visible Light Transmittance (VT), Air Leakage (AL), and sometimes Condensation Resistance (CR). These ratings allow designers, architects, builders, code officials, contractors, homeowners, and specifiers, an easy way to compare energy efficiency and determine whether a product meets code requirements. But what do they really measure? We’re going to get into that now.
Super important: The NFRC window rating is the performance rating of the entire window assembly.
Let’s go…lots to unpack:
U-Factor measures how well a window keeps heat from escaping from the indoor environment. The U-Factor is an important balancing act, depending mostly on the climate. Generally speaking, our Northern Climate is plagued with cold winters and requires great reliance on heating. The energy efficiency in such places focuses on keeping heat inside with the lowest U-Factor and the maximum solar heat gain. The Central and South Climate Zones have a ying-yang climate, relying upon heating or cooling the indoor space depending on the season. The sun's heat is beneficial in the winter, but in the summer, it’s unwelcome. Typically, we see these "mixed" regions go with a U-Factor of 0.30 or less. If you want a warmer space, go with a lower U-Factor.
If you prefer a cooler indoor space, combine a higher U-Factor with a lower Solar Heat Gain Coefficient. Use minimum standards from Energy STAR to receive tax credits and achieve maximum energy efficiency. Check here for the details.
Understanding the numbers: U-Factor ratings measure the amount of heat flow through a material and fall between: 0.20-1.20. The lower the number, the better the window assembly is at keeping heat in.
Well, let me fill ya in. Windows react to outside air temperatures and wind, as well as indoor air temperatures and occupant use. Today and every day, our sun gives us immense quantities of energy. And windows are strongly affected by light, heat – the sun.
R-value measures resistance to heat flow, usually referring to insulation or the wall assembly. It does not accurately reflect window energy efficiency. Therefore, the window industry measures in terms of thermal transmission.
U-Factor measures the rate of heat flow. The U-Factor is the mathematical inverse (so, 1 divided by R-value gives you U-Factor), and measures the rate of heat transfer through the window assembly.
If U-Factor measures from the inside out, the Solar Heat Gain Coefficient measures how much solar heat reaches though the window from the outside to the inside environment. The sun (again, plays a big role) warms our indoor space and reduces the need for heating. In colder climates and winter months, this energy can be quite beneficial. However, for buildings in the warm summer months or hot climates (like y’all in Texas or Arizona), unmanaged solar energy creates a thermal heating load that must be removed by air-conditioning. A simple way to explain SHGC is in terms of a ratio; 1 is the maximum amount of solar heat gain that can come through a window and 0 is the least amount. An SHGC of 0.40 then means that 40% of the available solar heat is coming through the window. The most effective way to manage the amount of solar gain that enters, is to block it before it gets in. How do you do that? With tinted, reflective, or spectrally selective glass that can block or re-radiate much of the energy from the sun. So just like with U-Factor, the lower the number, the better the window is blocking; in this case the ability of the window to block heat from the sun.
Visible Transmittance is the amount of light in the visible portion of the spectrum that passes through the glazing. Allowing daylight into the indoor space will potentially reduce reliance on artificial lighting. Plus, we all know daylight is super important for health and well-being! (Hopefully, you have a window in your COVID home office). Studies have shown that increased daylighting helps crank up our productivity, stimulates faster patient recovery, improves students’ grades, keeps our biological clocks in check, relieves stress, and helps with depression and anxiety. VT ratings are between 0 and 1. The higher the number, the more natural light is being let in through the glazing, providing occupants with more benefits.
Condensation, condensation, condensation – frustrating to experience and often hard to control. Condensation is a light coating of water or frost, which forms when the temperature of the surface is less than its dew point temperature. For example, if the temperature of the glass in a window is 50°F and the dew point temperature for the glass is 55°F, condensation will form. The Condensation Resistance rating is a measure of how well a window resists the formation of condensation on the inside surface. CR is expressed as a number between 1 and 100, the higher the number the better it is at resisting condensation. Helping to achieve these higher resistance numbers are insulated glass units with appropriate low-e coatings, thermally broken frames, and occupant behavior all help reduce the chance of condensation. Did we mention how important airflow and circulation are to help reduce condensation?
Also worth mentioning, the lower the U-factor of a window, the better the window is at keeping heat inside the room and the lower the potential that condensation will form.
Air infiltration, or air leakage, is commonly overlooked – but it shouldn’t be. AL - calculated by cubic feet of leakage per minute per square foot - measures how much air will enter the indoor environment through the window; the lower the number means less air can pass through with fewer drafts. So, with a possible range between .1 to .3, look for the lowest numbers. If the window assembly is poorly designed and manufactured, this can result in significant heat loss or gain, and drafts. The primary cause of air leaks in windows is failed seals. Look for a 10 year seal failure warranty. More importantly, preventing window leaks begins with proper handling and installation. If your budget allows, specify a window mock-up and have field testing done on the mock-up before all the windows are installed.
Working hand in hand with the NFRC Label is Energy Star. The EPA (Environmental Protection Agency) released Energy Star in 1992 and sets energy performance by climate zones. And, wow, Energy Star has helped Americans save 5 trillion kilowatt-hours of electricity, avoid more than $450 billion in energy costs, and achieve 4 billion metric tons of greenhouse gas reductions.
The NFRC window labels give ratings for U-factor, Solar Heat Gain Coefficient (SHGC), Visible Light Transmittance (VT), Air Leakage (AL), and some include Condensation Resistance (CR). Energy Star maps can help you determine what those numbers should be for your climate. Got it. Well, sorry, still need to unpack some more:
A lot. Pretty much every little part and piece of the window can change up all the NFRC numbers.
Regarding the Insulated Glass Unit (IGU), features that can affect IGU efficiencies and therefore NFRC numbers, are the use of thermally broken frames, use of gas infills over air, airspace, low-e coatings and other glazing types, spacer bar material, frame material and more.
Glazing, where to begin –might have to be its own blog. However, my favorite thing to yak about. Double, triple glazing. Low-E. Tinted. So many options. Low-E, means "low emissivity” the ability of a surface to emit radiant energy. Many layers make up a single low E coating. By changing the types of materials used in the "stack" of Low-E, or by increasing or decreasing the number of layers, or even which glass surface it is applied to, it all changes the performance.
I’ll just focus on one, the LoĒ-i89 Cardinal glazing coating. LoĒ-i89 keeps more heat inside during cold winter months. The coating is applied to the #4, the interior surface of an insulated glass unit, reflecting escaping heat back into the room and lowering U-factors. This conductive oxide sputtering creates longwave energy. From what I hear, no haze, same scratch resistance as regular glass, chemical resistance, low reflectivity (not a mirror). The whole window U-factor can be as low as 0.22 with argon. An insulating glass unit with LoĒ³-366 and LoĒ-i89 can meet the most stringent energy standard – without going to a triple-pane unit. For sure, check it out.
Reach out to your Sierra Pacific Windows Architectural Consultant to chat more on cool specialty glazing options like: Electrochromic Glass (AKA Smart Glass or Dynamic Glass), Ultra Clear / Low Iron Glass, Fritted Glass, Low Reflectivity Glass, Bird Protection, Turtle Glass, Spandrel, and Neat / Easier to Clean.
Yep, there’s more, and just as important.
Goodness, did not even address sustainability, structural performance and mulling, water and acoustical ratings, security performance, oh my…. You can usually find these ratings on the manufacturer’s website. Will have to be another blog.
Fenestration standards are always evolving due to advances in technology, building codes, and rating system performance requirements. Always feel free to reach out to our Architectural Consultants(here) for the latest and greatest – we love to talk all this stuff.
Wrapping this up.
Windows = unique funestration (Oh, where did the word fenestration come from?? From Latin fenestrare, from fenestra "window, opening for light”). There are SO many benefits of a high-performance window, advocate for the best quality window the budget will allow!!!
Hoping you understand the complex window world a tad more. Cheers to 2022!
Finding the Extraordinary within the Ordinary
Photo credits: Photo 1 Whitney Kamman Photography – Adams Construction (Brooke Adams) Photo 3 Chad Holder Photography for This Old House Photo 4 Skylab Architecture & Stephan A. Miller Photography