Windows and skylights are commonly referred to as the fenestration elements of a home. Each of these fixtures can lose or gain heat through direct conduction, radiation, and air leakage through and around them. Effective designs aim to minimize these unintended heat gains or losses.
Rate at which a window or skylight conducts heat. It is generally expressed in units of Btu/hr-ft2-oF. A lower U-factor for a window indicates less heat gain or loss through the window. With today's technology, a window is considered energy efficient if its U-factor is less than 0.40.
Air leakage through fenestration element
Unintended air exchange through a window or skylight, measured in cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating will be tighter, thus it will lose less energy.
Low-emissivity glass (low-e)
Glass with a very thin layer of material (e.g., metal) on the surface that is engineered to transmit or reject certain frequencies of radiation to reduce energy losses.
Double or triple glazed glass
Glass with two or three layers to reduce conductive and convective energy losses from a home. The air space between glass layers adds additional insulation since air has a low conductivity value. Argon gas can also be used between the glass panels since it has an even lower conductivity value than air.
Solar heat gain coefficient (SHGC)
Fraction of solar radiation admitted through a window or skylight. This includes both the energy transmitted into a home, and the energy absorbed by the fixture that may be released as heat inside a home. A lower SHGC implies that less solar heat is transmitted, thus the fixture has a greater shading effect. A higher SHGC is more effective at collecting solar heat during the winter, and may be desired for your Passive Solar gains. SHGC ranges between 0 and 1, with a higher rating admitting more radiative energy.
Visible Transmittance (VT)
Fraction of the visible light spectrum (380-720 nano-meters), weighted by the sensitivity of the human eye, passed through a window or skylight. VT is expressed as a number between 0 and 1. A product with a higher VT will transmit more visible light.
The ability of a fenestration element to resist condensation formation on its surface.
The National Fenestration Rating Council (NFRC) operates a voluntary program that certifies and labels windows and skylights based on their energy performance. These labels are the most reliable way to compare the energy-efficiency performance of these fixtures. Below is an example of a NFRC label.
Image source: www.glassguides.com
Window Operating Style
A Window's energy perforance is effected by the operating style of the window. The operating style is largely determined by the way a window is opened or closed, and how it is hinged to the window frame. For traditional window types, there are six main operating types: 9
Image source: energy.gov
Windows that are hinged at the top of the window frame, and opens outward. These windows typically have lower air leakage rates when compared to sliding windows
Windows that are hinged on the sides of the window frame. These windows also typically have lower air leakage rates than sliding windows.
Windows don't open at all, thus they have the least amount of air infiltration. These windows should not be installed in places where natural ventilation is desired.
Windows that are hinged at the bottom, and open into the home. They have lower air leakage rates when compared to sliding windows.
Single and double hung
Windows that slide up and down. These are the most common windows we see in older homes. These windows typically have higher air leakage rates when compared to projecting or hinged windows.
Single and double sliding
Windows that slide horizontally. These windows also have higher air leakage rates when compared to projecting or hinged windows.
Window Framing Material
A window's energy efficiency is also impacted by the framing material. Each framing material has its own advantages and disadvantaegs. There are five main framing material types:14
Aluminum or metal frames
The advantages of these frames are that they are strong, lightweight, and durable. The disadvantage to metal frames is that they conduct heat more than other framing materials, which increases a windows overall U-factor. To compensate for this, metal framed windows typically include a thermal break, which is an insulating plastic component placed between the inside and outside of the frame and sash.
These windows are made up of many different wood products. The advantage of these frames are that they have better structural and thermal properties than tranditional wood, and that they are better at resisisting moisture and decay problems.
The advantage of these frames is that have the best thermal performance when compared to wood, vinyl, and metal.
These windows are typically made of polyvincyl chloride (PVC). They have good mositure resistance, and include hollow cavities which are typically filled with insulation to decrease the window's overall U-factor.
These windows have a moderate insulation value, but they do expand and contract with changing temperatures. This makes the windows require more regular maintenance than other framing types.
Window Condensation Resistance
Condensation forms on windows as a result of the surface temperature of the window dropping below the dew point of the inside air. This condensation can lead to mold growth and air quality concerns within a home.
To eliminate condensation problems on the inside surface of your window,
- Choose a window with a lower U value,
- Ensure that your window is properly sealed,
- Control indoor air moisture level, and
- If all of the above cannot solve the problem, increase air movement around the window surface.
For condensation resistance, two different scales are used. The NFRC uses the condensation resistance, or CR value, which ranges from 1-100. The American Architectural Manufacturers Association (AAMA) uses a condensation resistance factor, or CFR value, ranging from 30-80. In each of these scales, a higher value is indicative of greater condensation resistance.
1"Energy Performance Ratings for Windows, Doors, and Skylights" by the Office of Energy Efficiency and Renewable Energy
General overview of fenestration elements (windows, doors, and skylights).
2 "Certified Products [fenestration] Directory" by the National Fenestration Rating Council
Search for windows, doors, and skylights ratings.
3 "Measuring Performance: Energy Star Windows" by Efficient Windows Collaborative
Suggested window specifications depending on your geographical location.
General Overview for Energy Star ratings for windows, doors, and skylights.
5 "Window Selection Tool: New Construction Windows" by Efficient Windows Collaborative
"The Window Selection Tool will take you through a series of design conditions pertaining to your design and location. It is a step-by-step decision-making tool to help determine the most energy efficient window for your house."
6 "Condensation Resistance Factor Tool" by American Architectural Manufacturers Association"
Window condensation resistance factor calculator.
7 "Information Brief – Condensation Resistance, Window" by Minnesota Sustainable Housing Initiative
Good background information to better understand window condensation resistance.
8 "Rating Windows for Condensation Resistance" by Green Building Advisor
In depth article about condensation resistance (good figures and typical values)
9 "Energy Efficient Windows" by the Office of Energy Efficiency and Renewable Energy
Tips for improving energy efficiency for existing windows including a discussion of cold and warm weather strategies, selection criteria for new windows, and installation.
10 "Windows, Glass, Ratings, and Installation" by Green Building Advisor
Learn more about window performance considerations. Windows, Glass, Ratings, and Installation
11 "Casement Windows: Advantages and Disadvantages" by Replacement Windows for Dummies
Discussion on casement window design.
12 “Windows: Understanding Energy Efficient Performance” by the Pual Fisette out of the Building and Construction Technology
13 "Windows and Glass Ratings and Installation" by Green Building Advisor
In-depth discussion on windows and frame selection including material and energy considerations.
14 "Window Types" by the Office of Energy Efficiency and Renewable Energy
An introduction to the differences between types of window frames and types of window glazzing or glass,