Building Science 4 Dummies: NFRC Labels (Windows, Doors, Skylights)

The NFRC or National Fenestration Rating Council basically tests & labels glass products used on buildings like doors, windows, and skylights. The label will include who the manufacturer is, a brief description of the product, and the official ratings for one or more of performance characteristics for that product. The NFRC’s also has an online Certified Products Directory which contains all the information contained on the label to allow one to pre-shop or to verify a label in case it is smudged, missing, or something appears off.

The word fenestration comes from the Latin word “fenestra” which roughly translates to opening or window. For most of the population, fenestration refers to openings in a structure, while in the medical field it is commonly defined as; the act of perforating or condition of being perforated or, the surgical creation of a new opening in the labyrinth of the ear for restoration of hearing in otosclerosis.

The U-Factor or value is a measurement of how well a product prevents heat from escaping or entering the building. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value. Most products rate between 0.20 & 1.20 (U is the inverse of an R-value so a .2 equates to an R5 while a 1.2 has an R value of less than 1.)

While a lower SHGC number is better in the desert, when you live in a heating dominant climate or are looking into passive solar / Passive Haus designs, you would generally want a higher number to help heat your place during the day. The catch is you need to remember to shut the drapes & blinds at night to prevent the loss of said heat  (it does go both ways) & make sure you have your shading dialed in for the summer months so your house doesn’t turn into a sauna.

The Solar Heat Gain Coefficient (SHGC)measures how well a product blocks radiant heat gain. Technically it is “the fraction of incidental solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward” which is also expressed as a number between 0 and 1. The lower the number is, the less heat is transmitted into the building which is great in hot climates.

Visible Transmittance (VT) measures just how much natural light comes through. It is also expressed as a number between 0 and 1 with the higher the VT rating, the more visible light being allowed in.

The values on the label represent the rating of the “whole system” which includes the glass (aka glazing) & the frame. It does not account for the flange area or how it was installed which is generally the biggest problem area nullifying the “windows” performance.

Air Leakage (AL)is a measurement of the equivalent cubic feet of air that can pass through a square foot of window area. The lower the AL value, the less air will pass through & correspondingly the heat loss and gain caused by infiltration through cracks in the assembly. This rating is optional & manufacturers are not required to test it.

The FTC has lately been cracking down lately on Window Manufacturers regarding claims being made. For example all claims & marketing material should:

  • Back energy-saving claims with scientific evidence;
  • Be specific about the type of savings consumers can expect;
  • Avoid deception when making “up-to” claims;
  • Avoid deception when selecting home characteristics for modeling;
  • Clearly and prominently disclose any assumptions;
  • Exercise care in using testimonials or “case studies;” and
  • Realize they may be liable for misleading or unsubstantiated claims made to dealers or retailers, in addition to claims made directly to consumers.

While we largely applaud the FTC for doing their job, please remember that they have basically only hit the very tip of the iceberg. So be careful out there as bogus claims are made every day by not only window companies but many other product manufacturers & even by some government sponsored programs.

Condensation Resistance (CR) is expressed as a number between 1 and 100, and rates the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. The biggest factors influencing this rating are the U Factor & the spacer bar used between the two panes. If you live in Antarctica &/or interior moistureis an issue in your house (and for some reason you can’t control it) then this rating might be worth something to you. This rating is also optional & manufacturers are not required to test it.

Historic Windows vs. Performance: It is real easy to believe that an older historic window will never perform as well as a newer one but the fact of the matter is unless you are comparing your windows to a high performance triple pane, your windows can outperform many newer ones with a little work. (yeah if you sense an article or two coming up, you would be right…)

Sample NFRC label for an ENERGY STAR labeled product

  • Notice the yellow shading – that is where the label applies
  • The label also does not need to be attached together as some manufacturers like to keep them separate

The current ENERGY STAR specs for fenestration products based on location

  • As an FYI – the current ES specs exceed the 2009 IECC but in some areas are less than the 2012 IECC which will hopefully be changing soon (For more on this – NorthernStar, DOE, & Windows)
  • Just remember the lower the U-Factor & the Air Leakage numbers the better for all types of fenestration’s
  • EnergyVanguard

    Nice post, Sean. I wasn’t aware of the condensation rating and am sure I’ve never seen one. Have you?

    • SLS Construction

      Thanks Allison & neither was I, till I was on NTRC’s site – my guess is this is something only a few northern manufacturers use in Anschel’s area or maybe up in Canada. I guess we could always pop into their database & see just how many do it…

  • jb @BuildingMoxie

    Indispensable info Sean … great post.