However, recent testing shows that U-factor improvement does not automatically lead to condensation resistance improvement. Condensation resistance performance can actually degrade under certain combinations of low-E coatings and spacer construction.
Recent attention on achieving R-5 performance in glazed window systems (i.e., those having a U-factor of 0.20 or less for fixed units and 0.22 for operable) has worked to push insulating glass (IG) technology to new heights through the use of:
- Sputtered low-E coatings (“soft-coat”) on internal IG surfaces
- IG infills (argon, krypton, etc.)
- Low conductivity/warm-edge spacer systems
- Triple glazed construction (a suspended low-E film inside dual-glazed insulating glass units [IGUs] provides similar performance results)
A Closer Look at 4th Surface Low-E Coatings
Concerns regarding the weight, thickness and cost of triple-glazed IGUs have led to advances in coatings for use on the exposed interior fourth surface of traditional double-glazed IGUs. In this configuration, a standard soft-coat low-E coating is placed on surface #2 inside the IGU with a second pyrolytic or “hard-coat” low-E coating applied to the 4th surface. This construction is designed to reflect long wave infrared radiation back to the interior, thereby decreasing thermal transmittance and achieving a U-factor approaching 0.20.
However, this infrared reflection of heat to the interior environment also reduces the surface temperature of the interior (#4) surface of the interior glass lite. The reduction can be significant, with center-of-glass temperatures as much as 12 degrees F lower for air-filled 4th surface low-E units than would be the case for units having 90 percent argon fill and uncoated 4th surface. Edge-of-glass temperatures for the same comparison are more than 20 degrees F different. During extreme cold weather, the interior edge-of-glass temperature can fall below 32 degrees F. Regardless of interior relative humidity level, the potential exists for moisture to condense on the perimeter of the glazing system as water or, potentially, as ice.
Note that the indicated U-factor of a window expresses the heat flux averaged over its entire area. Individual components of the system, however, are principally responsible for the ability of the system to resist condensation formation. The area of the window system having the lowest surface temperature is most likely to create an opportunity for condensation to form.
While perhaps having minimal influence on the system U-factor, a thermal short circuit can be introduced that may create significant problems concerning condensation. This is true given the localized effects due to thermal conductivity differences of discrete components.
Evaluated by Simulation and Testing
To quantify this effect, 25 IGUs of varying configurations were prepared for simulation analysis and laboratory testing to compare thermal performance (both U-factor and condensation resistance). Each configuration was simulated in accordance with the procedures of NFRC 100 to determine interior EOG surface temperatures and Condensation Resistance (CR) rating. Representative specimens were also physically tested in a guarded hot box in accordance with the setup of AAMA 1503 for comparison. With the interior side temperature held at 70 degrees F, the outside temperature was varied between 0 degrees and 40 degrees F, while the interior relative humidity was varied between 15 and 50 percent.
Even at the higher exterior temperature, there were no less than six IGU configurations having fourth surface low-E coatings that presented with interior edge-of-glass temperatures at or below the dew point temperature.
While a modest improvement in U-factor was realized due to fourth surface low-E coatings versus clear fourth surface units this comes with a significant reduction in the average CR rating from 67.7 for the clear fourth surface units including the clear/clear construction unit to 52.7 for the fourth surface low-E units.
Therefore, it can be concluded that when fourth surface low-E coatings are used in dual-glazed IGUs for improvement in thermal transmittance, it is imperative that all additional available performance features are integrated into an IGU to minimize the negative impact on condensation resistance. Primary of these is improvement to edge-of-glass components to reduce heat flow through the edge-seal/spacer system.
There is a significant improvement on the edge-of-glass temperature profiles of each of the IGU configurations incorporating non-metal spacer systems. Warm-edge spacer technology reduces heat flow through the edge-of-glass where the temperature profile reduction is most prevalent. The use of a non-metal foam spacer versus a metallic spacer system raised the edge-of-glass temperature by nearly 14 degrees F. Identical IGU systems represent an edge-of-glass temperature elevation of 11 degrees F when comparing a tinplate steel spacer system to a foam warm-edge system. These systems also represent a 17 point (121 percent) and 13 point (78 percent) improvement in CR rating, respectively.
Bottom line: While utilization of fourth surface, pyrolytic coatings in dual-glazed insulating glass units can provide some degree of improvement on thermal transmittance performance (U-factor), it must be understood that a compromise must be considered on the ability of the IGU to resist condensation formation on interior surfaces, particularly at the edge-of-glass.