Finding a Winning Combination
August 2, 2009
FEATURE ARTICLE | Codes & Standards, Energy Efficiency, Materials & Components
With the impending changes to Energy Star and the .30/.30 tax credit requirements of the American Rescue & Recovery Act, now, more than ever, fenestration manufacturers are looking at every option available for performance improvement and making product adjustments to take advantage of the sudden flurry of activity in the market.
There are literally endless possibilities for designing windows with a variety of framing materials, glass coatings, spacer systems, gas fills and glazing gap dimensions. But how much will you gain by changing your configuration and which ones will have the greatest impact on performance?
CALCULATING IG PERFORMANCE
Before getting started, it’s important to understand how both the window (or glazed door) and insulating glass unit thermal performance are measured and calculated. First, a window system is divided into three primary and distinct areas as illustrated in Fig. 1 and defined in the National Fenestration Ratings Council 100 as:
- Frame area (F): the projected area of frame and sash in the plane(s) parallel to the glazing surface
- Center-of-glazing area (COG): all glazing areas except those within 63.5 mm (2.5 inches) of any part of a primary sash and/or frame and/or divider
- Edge-of-glazing area (EOG): all glazed vision areas within 63.5 mm (2.5 inches) of any part of the frame and sash or of the door lite frame sight line, excluding any divider or edge of divider
Fig. 1–Window schematic illustrating the three
distinct window areas defined by NFRC.
Without getting into too much detail, there is a basic relationship between these areas. Thermal transmittance (U-factor) and solar heat gain coefficient (SHGC) are “area-weighted” average calculations of the performance of each of the three section areas. This means that the performance of each section area (frame, COG, EOG) is averaged into the performance of the entire window based on the section’s area relative to the entire window. For instance, a window might have 15 percent frame area, 25 percent EOG area and 60 percent COG area for a total of 100 percent of the window system.
The performance impact of each area is a function of what percentage it is of the entire window. In this case, as in most, the biggest improvement to the window system’s thermal performance is by improving the COG performance. In this example, halving the U-factor of the COG will yield a 60 percent x ½ or 30 percent reduction in the window system U-factor. The same halving of U-factor for the frame equals 15 percent x ½ or a 7.5 percent improvement in thermal transmittance.
Understanding this correlation will help a window fabricator understand how to obtain the most cost-effective improvement in thermal performance—in other words, the best “bang-for-the-buck.”
From a window fabricator’s perspective, it isn’t easy or practical to start switching framing materials and fabrication equipment to meet these new industry requirements. This discussion will, therefore, focus on the IG unit and means of improving its thermal performance. The IG unit is typically the largest single component of a window system by exposed area, and therefore has the greatest ability to improve the performance of an entire window or door system.
The key components of the IGU construction to be addressed are:
- Glass coatings
- Insulating gases and glazing gap dimensions
- Spacer and sealant types
Glass and Coatings–The first and best option for improving the thermal performance of an IG unit is to incorporate low-emissivity or low-E coatings. These coatings are designed to improve U-factor and SHGC by reducing the absorption and transmission of infrared energy through the glass. There are a multitude of different types and this is only meant to serve as an example of the broad range effects of choosing different types.
As most fenestrators are aware, there are two principal types of low-E, which are based on how the coating is applied: pyrolytic (or hard-coat) and vacuum sputter deposition (or soft-coat). The key to any effort to upgrade performance is to understand that the performance characteristics of both types can be fine-tuned to specific application needs. Specifically, windows installed in hot, Southern geographies of North America should reduce the amount of radiant infrared solar energy into a building as this otherwise heats the house and increases cooling costs. Windows in these cooling dominated climate zones, however, don’t necessarily need a very low U-factor because insulating demands are relatively low. Accordingly, the key focus in these geographies is low solar gain low-E coatings that provide low SHGC values.
Conversely, windows installed in Northern, heating dominated climate zones need to have excellent insulating properties (i.e., low U-factor) to keep heat inside the building and reduce heating costs. Infrared solar radiation through these windows might not, however, be such a bad thing since, in a properly designed building envelope, this may help to offset heating costs in the colder months. Low-E coatings for these markets may have moderate or high solar heat gain characteristics that allow the sun’s heat to get into the building while reducing the flow of heat energy associated with HVAC systems out through the windows.
Different types of low-E coatings that control radiant heat flow differently are what are known as “spectrally selective.” That is, the coating can be tuned to control the transmission of different parts of the electromagnetic spectrum – light, high infrared (solar), low infrared (heat), UV, etc.–depending upon its construction.
According to the Efficient Windows Collaborative, typical ranges of SHGC by low-E type are:
- High solar heat gain: SHGC = 0.71
- Moderate solar heat gain: SHGC = 0.58
- Low solar heat gain: SHGC = 0.39
Obviously only high-performance, low solar heat gain products can approach the maximum 0.30 SHGC requirement of the ARRA tax incentive for high-performance replacement windows. This must also take into consideration the impact of the frame and EOG on total window SHGC, but provides a first target for product evaluation.
Finally, consideration must be given to the surface orientation of low-E coating on the lites of glass. Glass lite surfaces are numbered beginning with the exterior surface of the outboard lite being surface No. 1 and each subsequent surface consecutively higher until the final surface is reached at the interior surface of the inboard lite (Fig. 2). Low-E coatings reflect radiant energy back in the direction of the source and coatings on surface No. 2 frequently improve the resistance of an IG to heating due to solar radiation by preventing solar energy from getting into the IG–thereby improving SHGC.
Insulating Gases & Glazing Gap–The next step to improving COG performance is to incorporate insulating gases. These range from relatively inexpensive argon, to the more expensive with krypton to exorbitant xenon. These are “noble” gases, which simply means that they don’t react with anything and are, accordingly, very stable molecules.
The primary benefit of insulating gases is that they have a thermal conductivity lower than air. Consequently, in a properly designed IG, insulating gases allow less heat transfer from one side of the IG to the other side. This addresses only conductive (through a material) and convective (due to movement of air/gas) because insulating gases have no impact on radiant heat transfer.
A key to proper IG design for insulating gases is to optimize the space dimension between the glazing layers for the gas being used. While the typical understanding is bigger is better, this isn’t the case with an IG glazing space. Glazing gap dimensions that are too small will allow for higher thermal conductivity across the IG unit. Gaps that are too large, however, will allow for convection inside the IG, which will degrade the thermal performance of the unit.
Convection is the process by which a fluid (gas or air in an IG unit) transfers heat from hotter areas to colder areas through the movement of the fluid. An IG unit should be designed to prevent movement of gas within the glazing space that creates a condition known as “stratification”. The stratified glazing space is small enough that the gas within it cannot move within the unit and create ‘convection loops’ (Fig. 3A). The gas within the glazing space becomes stratified or layered within the IG with the warmer air at the top and the cooler air at the bottom and little/no movement between them (Fig. 3B).
Fig. 3–A shows thermal convection loops. B illustrates gas that has stratified or layered within the IG unit.
Different gases have different optimal glazing space gaps based on their respective densities to prevent convection inside the IGU, but have the greatest possible distance to reduce conduction. The optimal glazing space dimension for air and argon is approximately 0.5 inches (12.7 mm) while the optimal dimension for krypton is closer to 0.25 inches (6.4 mm). Anything greater will allow convection inside the IGU, decreasing thermal performance.
Using data from the NFRC Certified Product Directory, Table 1 illustrates the performance differences of a generic, fixed vinyl window with different types of low-E configurations, insulating gas fills and glazing gap dimensions. This example is a small subset of hundreds of glazing options that are available. While all meet the minimum performance requirements of the ARRA stimulus plan, it is evident that there are multiple glazing configurations and component options available to yield these results.
Table 1–Performance numbers achieved by a generic vinyl window using different combinations
of low-E configurations, gas fills, and glazing gap dimensions.
Spacer Systems–As noted above, the EOG is the 2.5-inch (63.5 mm) perimeter area between the frame and the glazed COG area. While the key focus here is spacer and edge seal effects, it is important to recognize that the factors noted above also affect the EOG because a portion of the glazing area is included in the EOG.
For this discussion, the spacer system includes the spacer, desiccant and any required sealants because they all affect the thermal conductivity at the EOG. Aluminum-based spacer systems have been used for more than 50 years, but have become less popular since the advent of warm edge technology. While aluminum spacers are strong and work well for structural strength, they are thermally conductive and, therefore, facilitate heat transfer at the EOG.
To meet Energy Star and the ARRA Stimulus Plan requirements with aluminum spacers, IG fabricators must rely heavily on high-performance coatings and gas filling to improve U-factors to offset spacer conductivity, and may even then be challenged to obtain the required performance. Stainless steel spacers provide thermal performance improvement over aluminum and, while this is a step in the right direction, it is often not sufficient to make significant improvement in overall window U-value performance. Lower conductivity metal spacers offer improvement over aluminum spacers, but they are still metal and therefore may conduct sufficient heat to negatively impact window U-factor–particularly at the EOG, possibly causing issues with condensation.
Reduced metal and non-metal spacer products offer the best option for improving window U-factor over highly conductive spacers such as aluminum based products. To keep things in perspective, however, go back to the area weighting discussion above. The spacer system is a proportionately small area of the entire window unit and can, therefore, have only a moderate effect on window U-factor. A very significant improvement in spacer system thermal performance is necessary to have a measurable effect on total window system performance.
As evidenced in the referenced NFRC CPD tables, the best performance benefit that can be expected by changing from an aluminum based spacer system to the most thermally efficient, non-metal spacer system is typically 0.03 – 0.04 Btu/hr-ft2-°F (0.17 – 0.23 W/m2-K). While this may seem a marginal difference, it can be critical for a window manufacturer providing a window system that has a U-factor rating of 0.33 Btu/hr-ft2-°F with an aluminum or similar metal spacer, and needs to meet the ARRA or new Energy Star performance requirements of 0.30 Btu/hr-ft2-°F.
Table 2 illustrates the performance difference of separate fixed vinyl window products having multiple different spacer options as listed in the NFRC CPD. Unfortunately, it is not possible to directly compare a broad cross-section of the vast number of different spacer types in a given window system as most manufacturers only run one or two spacer products.
Table 2–Performance numbers achieved by a generic vinyl window using multiple IG spacer options.
Triple vs. Double-Glazed IG
Some manufacturers are looking to triple and even quadruple-glazed IG to meet the new energy performance requirements. Triples do offer a significant improvement in U-factor. For example, a double-hung triple using a dual sealed silicone foam spacer system, argon gas filling and two surfaces of low-E glass can achieve a U-Value of 0.20 Btu/hr-ft2-°F /SHGC = 0.17, whereas a double glazed IG with the same specifications (low-E on one surface) would achieve a U-factor of 0.30 Btu/hr-ft2-°F / SHGC= 0.20
The thermal performance with triples can be tempting, but manufacturers are urged to plan well when considering adding this new glazing option. Triple glazed units have several issues that an IG fabricator must consider, including heavier IG weight, handling and coordination of a third internal lite that typically has high performance low-E coatings and the potential for increased IG failure rate.
Because most IG failures for durability performance are due to moisture transmission between the spacer sealants and the glass, double glazed IG have two paths by which this can occur. Adding a third lite adds two additional moisture vapor paths that effectively double the potential for IG failure over time. Manufacturers need to ensure the effective adhesion and sealing of all sealants between all paths of moisture vapor transmission. An alternative is to incorporate spacer systems that support the center lite in a “U” type channel without creating additional moisture vapor paths. There is much to consider if you want to do it right.
Whether or not you approve of the .30/.30 tax credit requirements, they are here. Whether or not you approve of the revised Energy Star requirements, they are coming. One thing to keep in mind for both is the upcoming requirements for IG certification. The ARRA stimulus incentive requires products to meet the compliance criteria of the International Energy Conservation Code (IECC) for windows which includes NFRC certification and labeling. The requirements for NFRC certification and labeling effective July 1, 2010 include mandatory IG certification.
For those manufacturers who have not yet certified IG, book time in the test lab as soon as possible as this process can take up to 6 months to complete.