Looking into spacer systems
In the past, spacer systems weren’t regarded as key determinants of a window unit’s overall thermal and structural performance. That perspective has changed. As energy costs continue to rise, framing systems offer better insulation values, and pricing pressures continue unabated. Manufacturers throughout North America are therefore compelled to offer products with bottom-line benefits that relate directly to new market expectations. Consequently, they now look to window component manufacturers that can provide real performance advantages. As a result, the impact of warm-edge spacer systems on a window unit’s sightline temperature, U-factor, and condensation resistance has become increasingly relevant.
Spacer system manufacturers are working harder than ever to convince window manufacturers, developers, building owners, architects and specifiers that their particular systems outperform others. Many make performance claims based on either thermal simulation testing data or actual measurement of thermal performance.
Purchasers of spacer systems deserve factual and reliable data in order to make informed decisions. However, variables exist in window system and IG spacer system simulation and testing methodologies that can affect the accuracy of results generated. So long as these variations remain, truth and clarity will remain beyond a purchaser’s reach.
Both simulation and physical testing are valid tools for assessing thermal performance. But each of the critical elements inherent in these approaches must be carefully understood and evaluated in order to achieve accurate and reliable results.
Let’s first consider the physical construction of window units. When windows are constructed for thermal measurement, several factors can degrade the integrity of the resulting data. Units should be built under strict manufacturing controls so that the effects of these variables on the data are minimized. For one, proper geometry of framing and glazing profiles must be compared to those called for in print specifications in order to ensure proper glazing bead contact, weatherseal gaps, tolerances and gasket compressions. For example, increased compression of glazing beads over tolerance can result in over-compressed IG units and inaccurate thermal measurement data.
Exact placement of the IG unit in the finished window unit also is critical. This includes proper setting block placement to prevent shifting during the test procedure. Additionally, it’s essential to measure and record all critical variables upon completion of unit assembly. Should inconsistencies occur, it’s then possible to conduct a root cause analysis to trace their origin.
Another critical variable to consider is the environment in which system performance is measured. Spacer systems should be simulated and tested under normal conditions of their construction. To illustrate this, consider the use of secondary sealant and its effect on spacer placement relative to the edge of the glass. Spacer systems often exhibit dramatic variation in the thickness of secondary sealant that they use. This is due to materials used in spacer construction, specifications by sealant suppliers for their materials, structural performance specifications, application equipment requirements, and other variables.
Some spacer systems require significant increases in the thickness of secondary sealants due to the materials used in their construction. Others, especially those in select residential markets, require no secondary sealant across the back of some stainless steel spacers because the stainless steel’s corrosion-resistant properties, and the properties of the system’s elements, allow the unit to meet the required performance specifications. This factor alone allows some stainless steel-backed spacers to be placed at or very close to the edge of the glass, while others reside an additional 3⁄16 inch to 3⁄8 inch from the glass edge. As the distance between a spacer and the edge of glass increases, thermal performance declines while the cost of additional secondary sealant rises significantly.
In addition to sealant thickness, the type of secondary sealants used in building the IG units influence simulations and physical test results. To create a true comparison between spacer systems, the edge construction—comprising the spacer, its setback from edge of glass, and the type and thickness of the secondary sealant should be evaluated in the exact manner as it would be used by the IG fabricator.
The parameter of “backset from the edge of glass” is a factor that should also be used to standardize results and ensure accuracy. Generally, it’s acceptable to leave the backset as equal to the thickness of secondary sealant. If manufacturers provide a small gap between the edge of glass to the back side of the secondary sealant, that gap should be standard for all unit constructions.
In each case, simulation and testing should be performed with identical window systems, so that performance differences between IG spacer systems can be compared on an apples-to-apples basis. Additionally, testing should be done in complete products, rather than as isolated IG units. More than a decade ago, the fenestration industry moved to adopt thermal simulation and testing of IG units exclusively in the framing systems in which they would be used. Consumers and building owners don’t use insulating glass alone; rather, they employ it as part of a complete fenestration unit. That is why the market moved away from simulation or testing of isolated IG units.
The relative performance difference between IG spacer systems can be positively or negatively impacted, depending on the thermal insulation properties of the framing systems used in simulations or actual physical testing. For example, the performance difference between spacer systems of a thermally broken aluminum system using a wider polyamide insulating strut would be significantly better than a system that utilizes a polyurethane separation.
Correlation between thermal simulation and actual window testing has significant value. In addition to extensive use of outside simulation and testing labs, Technoform, headquartered in Germany, has generated thousands of simulations and window tests, and evolved simulation and testing routines that accurately correlate to each other. The lessons learned from correlating both aspects help us to provide better accuracy in both the simulation and testing processes and a greater understanding of the thermal movement of window systems.
Over time, I hope the considerations I’ve outlined become reality. Until then, there are several steps that manufacturers can take to ensure they are purchasing the spacer system that best meets their needs:
• Ask the spacer manufacturer if independent testing was conducted on the conductivity values of the materials used in its spacer, and then compare those to the material property values used in the National Fenestration Rating Council library. Go to www.nfrc.org for more information. Additionally, simulation results are reliable only if the materials being simulated, and the exact sizes of these materials, are identical to those being considered for use by a fabricator. Many product, size and material variations exist in today’s market, so look closely to ensure accuracy.
• Make sure that all simulations are performed by an independent NFRC- certified simulator, and that all values are calculated using the latest generation of approved simulation software.
• Always ask for a copy of the simulation or testing results upon which published documentations are based. The test results can help you identify any inconsistencies to ensure that the data has integrity and the product decisions that you make on the basis of this data are solid and reliable.
• Remember, thermal performance is only one determinant of an IG spacer system’s overall performance. Additional considerations include manufacturing technologies, component manufacturer requirements, sealant compatibility considerations, inert gas retention properties, total manufactured cost, and most critically, long-term unit durability.
By addressing the need to make testing more transparent to end users, the fenestration industry can continue to lead energy conservation initiatives while providing optimal solutions to manufacturers. In the end, that’s a win-win proposition for everyone.