10.10 Understanding Window Performance Labels

10.10 Understanding Window Performance Labels

Introduction to NFRC Ratings

The National Fenestration Rating Council (NFRC) label provides objective, third-party ratings that allow reliable comparisons between products. Fenestration refers to the arrangement and proportion and design of windows and doors in a building.  to the arrangement Each label includes:

  • Manufacturer name
  • Product description
  • Source for additional information
  • Energy performance ratings for key characteristics

Standardized Testing: NFRC rates all products using two standard sizes to ensure fair comparisons. The label displays ratings for up to five performance factors: U-Factor, Solar Heat Gain Coefficient (SHGC), Visible Transmittance (VT), Air Leakage (AL), and Condensation Resistance (CR).

NFRC window label
National Fenestration Rating Council's window label
Text description of the National Fenestration Rating Council's window label.

The image is a certification label for energy performance ratings of a window product named "World's Best Window Co. Series '2000' Casement." The top section features an ENERGY STAR map of the United States, with certified regions highlighted in blue. The ENERGY STAR logo is present, along with the caption "ENERGY STAR Certified in Highlighted Regions," written in both English and Spanish. Below the map in a blue bar, the text "energystar.gov/windows" is listed alongside the word "Certified" in English and Spanish.

The middle section includes a logo for the National Fenestration Rating Council (NFRC), indicating certification. Below the NFRC logo, details about the window product are presented, including its frame material, glazing, and model number.

In the bottom section, the energy performance ratings are displayed. It lists values for U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage. Condensation Resistance is also mentioned.

[Transcribed Text]

ENERGY STAR Certified in Highlighted Regions
Certificado por ENERGY STAR en las regiones resaltadas
energystar.gov/windows Certified / Certificado
National Fenestration Rating Council NFRC CERTIFIED
World's Best Window Co.
Series "2000" Casement
Vinyl Clad Wood Frame
Double Glazing-Argon Fill - Low E
XYZ-X-1-00001-00001
ENERGY PERFORMANCE RATINGS
U-Factor (U.S. / I-P) 0.22
Solar Heat Gain Coefficient 0.23
ADDITIONAL PERFORMANCE RATINGS
Visible Transmittance 0.51
Air Leakage (U.S. / I-P) ≤0.3
Condensation Resistance 51
Manufacturer stipulates that these ratings conform to applicable NFRC procedures for determining whole product performance. NFRC ratings are determined for a fixed set of environmental conditions and specific product size. NFRC does not recommend any product, nor does it warrant the suitability of any product for any use. Consult manufacturer's literature for other product performance information.

www.nfrc.org

Credit: National Fenestration Rating Council

U-Factor: Measuring Heat Loss

What It Measures

U-Factor (or U-Value) measures how well a window prevents heat from escaping. It indicates the rate of heat transfer through the window assembly.

Understanding the Scale

  • Range: 0.20 to 1.20 (typical residential windows)
  • Lower is better: A lower U-Factor means less heat loss and better insulation
  • Testing conditions: Based on 0°F (-18°C) outdoor temperature and 70°F (21°C) indoor temperature

U-Factor vs. R-Value

U-Factor and R-Value are inversely related:

  • U-Factor = thermal conductance (heat flow through a material)
  • R-Value = thermal resistance (heat flow resistance of a material)

U-values are the reciprocals of R-values (h °F ft 2/Btu).

Thus, the U-value is the inverse of the R-value or:

R = 1/U  U = 1/R 

Some manufacturers rate thermal performance using R-Value. For example, an R-factor of 4.0
is the same as a U-Factor of 0.25.

U = 1/ 4.0  U = 0.25 
U- and R- Factor Ranges
Window AssemblyU-FactorR-Value
Single Glazed0.91–1.111.1–0.9
Double Glazed0.43–0.572.3–1.7
Triple Glazed0.15–0.336.7–3.3

Solar Heat Gain Coefficient (SHGC): Blocking Solar Heat

What It Measures

Solar Heat Gain Coefficient (SHGC) measures how well a window blocks heat from sunlight. Specifically, it represents the fraction of incident solar radiation admitted through a window—both directly transmitted and absorbed then released inward as heat.

Understanding the Scale

  • Range: 0 to 1
  • Lower is better for cooling: A lower SHGC means less solar heat transmission
  • Higher may be better for heating: In cold climates, higher SHGC can provide passive solar heating

Visible Transmittance (VT): Maximizing Daylight

What It Measures

Visible Transmittance (VT) measures how much visible light passes through a window. It's an optical property that indicates the amount of daylight transmitted into a space.

Understanding the Scale

  • Range: 0 to 1
  • Higher is better for daylight: A higher VT means more natural light enters the room
  • Typical range: 0.20 to 0.80 for most windows

Balancing VT and SHGC

While you want high VT for daylighting, you also want to control heat gain (SHGC). 

Light-to-Solar Gain Ratio (LSG) indicates how efficiently a window transmits daylight while blocking heat gain. It's calculated as:

LSG = VT/SHGC

Understanding the Ratio

  • Higher LSG = Better performance: More light with less heat
  • LSG > 1.0: Window transmits more light than heat (ideal for hot climates)
  • LSG < 1.0: Window transmits more heat than light

Test Yourself

Calculate the R-value and LSG

For each of the following examples, calculate the R-value and LSG. (Round your answers up to two decimal places.) After you enter your answers in the boxes below, check your work by clicking on the “check” buttons below.


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Calculating R-Value and LSG

Based on the following energy performance ratings, calculate the R-Value and LSGR for each.

  1. Window #1
    • U-Factor: 0.30
    • Solar Heat Gain Coefficient: 0.36
    • Visible Transmittance: 0.59
    • Air Leakage: 0.2
  2. Window #2
    • U-Factor: 0.35
    • Solar Heat Gain Coefficient: 0.30
    • Visible Transmittance: 0.46
    • Air Leakage: 0.2
  3. Window #3
    • U-Factor: 0.32
    • Solar Heat Gain Coefficient: 0.45
    • Visible Transmittance: 0.58
    • Air Leakage: 0.3

Answers:

  1. Window #1
    • R-Value = 3.33
    • LSG = 1.64
  2. Window #2
    • R-Value = 2.86
    • LSG = 1.53
  3. Window #3
    • R-Value = 3.13
    • LSG = 1.29

Air Leakage (AL): Preventing Infiltration

What It Measures

Air Leakage (AL) rating indicates the rate of air infiltration through cracks in the window assembly. It's expressed as:

AL = cubic feet of air per minute per square foot of window area (cfm/ft²)

Understanding the Scale

  • Lower is better: Less air infiltration means better energy efficiency
  • Typical range: 0.1 to 0.3 cfm/ft² for high-performance windows
  • Industry standard: ≤0.3 cfm/ft² meets most energy code requirements

Why It Matters

Air leakage causes:

  • Heat loss/gain through convection
  • Increased energy bills from conditioned air escaping
  • Comfort issues from drafts
  • Moisture problems from humid air infiltration

Condensation Resistance (CR): Preventing Moisture Buildup

What It Measures

Condensation Resistance (CR) measures a window's ability to resist condensation formation on the interior surface.

Understanding the Scale

  • Range: 0 to 100
  • Higher is better: A higher CR rating indicates better resistance to condensation
  • Typical range: 20 to 80 for residential windows
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