The lower the U-factor, the better for you?

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What do the numbers on the window stickers mean?  Who rates them?  How do I know what is best for my home?

These are common questions from homeowners looking to make the right choice when replacing windows.  Like a lot of technical products you buy, windows have their own fancy names, numbers and ways of measuring.  Here’s a typical window sticker listing the most commonly rated factors:

MIlgard WarrantyWhat does it all mean?  Let’s start with this information from the folks that define “Energy Star”:


ENERGY PERFORMANCE TESTING, CERTIFICATION, AND LABELINGThe National Fenestration Rating Council (NFRC) operates a voluntary program that tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings. The NFRC label provides a reliable way to determine a window’s energy properties and to compare products.

The NFRC label can be found on all ENERGY STAR® qualified window, door, and skylight products, but ENERGY STAR bases its qualification only on U-factor and solar heat gain coefficient ratings, which are described below.


Windows, doors, skylights can gain and lose heat through:

  • Direct conduction through the glass or glazing, frame, and/or door
  • The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
  • Air leakage through and around them.

These properties can be measured and rated according to the following energy performance characteristics:

  • U-factor is the rate at which a window, door, or skylight conducts non-solar heat flow. It’s usually expressed in units of Btu/hr-ft2–o For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. NFRC U-factor ratings, however, represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.
  • Solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a window, door, or skylight — either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Your home’s climate, orientation, and external shading will determine the optimal SHGC for a particular window, door, or skylight. For more information about SHGC and windows, see passive solar window design.
  • Air leakage
 is the rate of air movement around a window, door, or skylight in the presence of a specific pressure difference across it. It’s expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.


The ability of glazing in a window, door, or skylight to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:

  • Visible transmittance (VT) is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through the glazing of a window, door, or skylight. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home’s daylighting requirements and/or whether you need to reduce interior glare in a space.

  • Light-to-solar gain (LSG)is the ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn’t always provided.


Okay, so now we know there is a standardized method for testing and rating windows.  We can compare one manufacturer to another and believe that if they are NFRC rated (virtually all are), then we can count on the accuracy of those ratings and what they represent.

So now what?  Who tells us what values constitute the benefits we desire?


The Department of Energy (DOE) and the Environmental Protection Agency (EPA) have developed a designation for products meeting certain energy performance criteria.  Windows that have the ENERGY STAR designation will be labeled showing the zones in which it is qualified.  Since energy efficient performance of windows, doors, and skylights varies by climate, product recommendations are given for four U.S. climate zones.  Country Energy Mapcountry Map

Northern Zone Required Properties (mostly heating)

U-factorSolar Heat Gain Coefficient (SHGC)Visible Transmittance (VT)Air Leakage (AL)

Windows: U≤0.30
Windows: U=0.31
Windows: U=0.32
Skylights: U≤0.55EWC Recommendation:For superior energy performance, use windows with a U-factor of 0.25 or less. If passive solar strategies are to be used, there may be a trade-off with a high SHGC value. If this is the case, make sure proper passive design strategies are implemented (thermal mass, proper shading) to prevent overheating during the summer seasons.Windows: SHGC=No Requirement
EWC Recommendation:If air conditioning is not a concern, look for a high SHGC (0.35-0.60) so that winter solar heat gains can offset a portion of the heating energy need. If cooling is a significant concern and no shading is available, select windows with a SHGC less than 0.35. Select skylights with a SHGC of 0.40 or less.Windows: VT=No Requirement
Skylights: VT=No RequirementEWC Recommendation:Select windows with a higher VT to maximize daylight and view.Windows: AL≤0.30
Skylights: AL≤0.30EWC Recommendation:Select windows with an AL of 0.30 or less.


This is just a start.  Later we will delve in to the question of the value of added efficiency.  How much more does it cost to go to .25 U value versus the savings?  Just like buying that new 4K TV, it costs more but is it worth it?

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