It has been estimated that a quarter of the annual energy that is used to heat and cool buildings in the United States is lost through existing inefficient windows. This is roughly equivalent to the energy delivered by the Alaska Pipeline every year. Through the use of modern energy-efficient windows, all this energy could be saved in addition to providing more comfortable spaces for us to work and live.
Modern energy-efficient windows incorporate new materials and technologies that have greatly improved the performance, application and durability of fenestration systems. Although the use of new technologies like Insulating Glazing Units (IG Units), and Low-e coatings increase the overall energy efficiency of windows, it also adds to the complexity of the decision making process that an architect or home owner must go through in selecting the proper window. The key is knowing what to look for.
The annual thermal performance of any fenestration system can be determined from three energy performance characteristics; Solar Heat Gain Coefficient (SHGC, which was formally known as Shading Coefficient), U-factor (sometimes referred to as U-value), and the Air Leakage rating. The SHGC and U-factor have been determined for many of the windows sold in North America by a nationwide certification and labeling program developed by the National Fenestration Rating Council (NFRC). NFRC uses a combination of testing and computer simulation to determine the actual values used to characterize the thermal performance of a particular fenestration system. Therefore it is important that the most accurate test and simulation methods are used to evaluate these products, and these testing and computer simulation methods produce results that are comparable.
The Large-Scale Climate Simulator (LSCS) is unique in that it has the metering chamber located below the climate chamber so it is best suited to testing test specimens mounted horizontally such as roof systems. For this reason, it is also well suited for testing large commercial skylight systems that are too big to fit in conventional ASTM C 1363 thermal chambers.
The Rotatable Guarded Hot Box (RGHB) is typically used to measure the thermal transmittance (U-factor) of test specimens mounted in a vertical orientation, but the entire chamber can rotate so that measurements can be performed with the test specimen at any tilt between vertical and horizontal. Not only can this chamber be used to test to the conditions specified in ASTM C 1363, but it can also perform thermal transmittance tests of fenestration systems that are smaller than the metering area as required by ASTM C 1199-97.
The test facilities at ORNL are also unique in that they have been calibrated to operate at unusual environmental conditions. Unlike commercial laboratories which typically perform ASTM C 1363 tests at ASHRAE or NFRC design environmental conditions, the test facilities at ORNL can produce a wide variety of environmental conditions at steady state, as well as perform transient tests to determine the dynamic performance of massive building systems. The facilities can also be pressurized to study the impacts of air leakage and energy transfer simultaneously.
Andre O. Desjarlais