Volume 13, Issue 6 - July/August 2012

Eye on Energy

Designing For the Big Picture
Surface #4 Low-E Coatings Could Impact Condensation
by Tracy Rogers

Condensation is unsightly, potentially unhealthy and can be damaging to the window frame and window treatments. When a homeowner invests in thermally efficient fenestration systems, they don’t expect to see moisture. Programs, such as the R5 Windows Volume Purchase Program, are requiring U-factors as low as 0.20 for fixed units and 0.22 for operable units. As this focus on improving building envelope thermal performance continues to grow, so have alternative solutions for improving the U-factor of glazed fenestration systems. But, are we considering the big picture, including condensation resistance, in our designs?

The Basics
U-factor represents the amount of heat energy transmitted through a product per hour, per degree temperature difference from the hot side to the cold side, averaged over the projected area of the system. While certain sections of the fenestration product may transmit more heat energy than others, the U-factor represents the heat flux averaged over the entire system.

In most fenestration products the glazing area, particularly the center of glass, has the most significant influence as it has the greatest percentage of projected area. The next area of greatest influence is dependent on a number of variables, including window type (casement, fixed, hung, etc.), frame material, reinforcement, cross section, the spacer and sealant system and the location of the spacer system in the frame. Each of these may have multiple components of varying size and thermal conductivity, and therefore varying impact on the overall U-factor.

On the other hand, condensation formation on interior surfaces is directly related to interior surface temperatures and interior relative humidity. Once the interior surfaces of a fenestration system fall below the dew point temperature of the interior ambient air, moisture has the ability to condense on the surface. The area of the window system having the lowest surface temperature is most likely to create an opportunity for condensation to form.

As compared to U-factor, condensation resistance has far less to do with the system performance versus component performance. The fenestration system’s ability to resist the formation of condensation is determined by an evaluation of localized effects due to thermal conductivity differences of discrete components. For example, a highly conductive material spanning warm side and cold side surfaces, such as the spacer, will create a localized heat flux that is significantly greater than low-conductivity materials.

Improved U-factors
Various IG unit constructions and component improvements can be incorporated to achieve improved thermal performance. Primarily, methods include:
• Sputtered low-E coatings on internal IG unit surfaces;
• Insulating gas in-fills, including argon and krypton;
• Low-conductivity warm-edge spacer systems; and
• Triple-glazed construction

Each of these has a relative impact on U-factor and most are used in combination with the others to achieve the target performance numbers. Continued focus on high-performance U-factors has emphasized triple-glazed designs incorporating multiple low-E coatings and gas in-fills. The addition of a third lite of glass, however, adds to the cost, weight and overall dimension of the IG unit–causing many manufacturers to seek alternative solutions.

One such alternative is to add a specially formulated low-E coating designed for use on the fourth surface of a traditional double-glazed IG unit. In this configuration, a standard hard-coat low-E coating is applied to surface #4, which is the exposed interior glass surface. The purpose of the fourth surface coating is to reflect heat back to the interior of the building. Specifically, it reflects medium- and long-wave infrared radiation, which is radiated by materials and components within the structure, thereby reducing heat flow through the glazing.

Impacting Condensation Resistance
Adding low-E to the fourth surface of a double-glazed unit is appealing because it will cost less than converting to a triple-pane system and still achieve U-factors approaching 0.20.

According to recent research by Quanex in which 25 IG units of varying configurations were compared for U-factor and condensation resistance performance through physical and simulated testing, a principal concern of fourth-surface low-E is the overall reduction in surface temperature of the interior lite of glass. Since the low-E coating is designed to reflect heat back to the interior, it inherently lowers the surface temperature of the glazing.

The surface temperature reduction can be significant with surface center-of-glass temperatures as much as 14 degrees Fahrenheit lower than clear glass and edge-of-glass temperatures more than 25 degrees Fahrenheit lower than clear glass. This represents a significant potential for condensation formation on the interior glazing of IG units specifically designed for improved thermal performance.

Looking at the big picture is essential for creating units that will provide optimal overall performance now and in the future.

Tracy Rogers is director of industry relations for Quanex Building Products.


DWM

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