In this third and final part on our window series we will look at the different glass options and glazing efficiency used to make window sashes. We have all heard of single and double-pane glass. But what does it mean? If double-pane is better than single-pane, is triple-pane better than double-pane? However, along with the number of panes of glass there are other factors such as the edge seals, glass coatings and gas filled spaces to consider.
Let’s begin by defining some terms used when talking about window efficiency:
- U-Factor: A measure of heat transfer through a material. Lower numbers indicate less heat transfer (more insulation). U-Factor can be measured for just the glass or for the window assembly. Depending on the materials used to manufacture the sash and frame the overall unit U-Factor can be lower than the glass U-Factor. When comparing U-Factors between window manufacturers, make sure to compare using overall unit-performance and not just glass performance.
- Solar Heat Gain Coefficient (SHGC): Measures how much heat from the sun is blocked. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the more a product is blocking solar heat gain. Blocking solar heat gain is particularly important during the summer cooling season in hot Southern climates. By contrast, people in Northern climates may want solar heat gain during the cold winter months to lessen the cost of heating the home.
- Argon Gas: An inert gas that is often injected within the dead airspace of an insulating glass unit to enhance its efficiency. Since it is a naturally occurring component of the atmosphere, it is 100% safe. Argon gas-filled windows are approximately 6% more energy efficient than windows without argon gas.
- Krypton Gas: An inert gas that is often mixed with argon and injected into the dead airspace. It is more prevalent in triple-pane windows since it is more effective when the glass panes are close together.
- Low E Glass: Low-E glass contains a highly engineered surface designed to reflect heat before it passes through the window. While early versions were often mirror-like or tinted, today’s Low-E glass is designed to allow most visible light to pass through it and doesn’t appear any different than ordinary glass. In addition, low-E coatings are often designed to reflect UV light to protect draperies and interior furnishings from sun bleaching. By incorporating low-E glass, manufacturers can decrease energy loss through double-pane glass by up to 50%.
- Edge spacer: A spacer between the panes of glass on double and triple pane windows to seal the panes together keeping any argon or other airspace gases contained. A warm edge spacer has the advantage that heat transfer by conduction is minimized through the spacer bar. Ordinary spacer bars are often made of aluminum or other alloys that readily conduct heat or cold from the glass surfaces, while warm-edge spacer bars provide an insulating barrier. As a result, windows with warm-edge spacers are less prone to condensation at the perimeter of the glass during cold winter months.
Let’s look at how all this applies in the case of a particular window brand—Andersen Windows. Andersen uses warm-edge spacers between their glass and an enhanced Low-E coating on the exterior. They also apply a proprietary interior coating they call Heatlock for additional performance. So applying the best technologies Andersen has at its disposal here is the data for a triple-pane window and a double-pane window that they manufacture. An Andersen A-Series double hung window with triple-pane Low-E glass with Heatlock has a U-Factor of 0.20 and an SHGC of 0.26. An Andersen 200 Series double hung with double-pane Low-E glass with Heatlock has a U-Factor of 0.28 and an SHGC of 0.21. Andersen doesn’t offer any single-pane window options to compare; as is true of most window manufacturers. But as a reference point the U-Factor of a single-pane window is around 1.1 which is substantially worse than any double-pane option.
As you can see there is an enormous advantage in moving from single-pane to double-pane with the U-Factor dropping from 1.1 to 0.28. However the performance gain is not as significant moving from a double-pane to a triple-pane window with the U-Factor only dropping from 0.28 to 0.20. Yes, the triple-pane is more efficient and will save energy but is it worth the additional cost?
As technology continues to improve we will likely see continued performance gains for both the double-pane and triple-pane windows. These gains are from a combination of glass technologies and coatings as well as the materials and processes used to manufacture the sashes and frames. Based on current technology Brennan Builders is still recommending good quality double-pane windows for most applications.