Fenestration refers to the arrangement, proportioning, and design
of windows and doors in a building. The energy efficiency of the building
envelope is greatly impacted by the fenestration systems. Windows strongly
influence the use of the building and the productivity and comfort of its occupants.
It has been estimated that in 1991, a quarter of the energy that was used to
heat and cool buildings in the United States was lost through existing inefficient
windows. Modern energy efficient windows could save much of this energy
and provide more comfortable spaces in which to live and work.
issues
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,
they also add 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 characteristics to look for in order to choose the proper window
for a particular application and location. The annual thermal performance
of any fenestration system can be determined from three energy performance characteristics:
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Solar heat gain coefficient (SHGC) formerly
known as the shading coefficient |
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U-factor sometimes referred to as U-value or
thermal transmittance |
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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). Because NFRC combines testing and computer
simulation to determine the actual values used to characterize the thermal performance of
a particular fenestration system, it is important that the most accurate test and
simulation methods be used to evaluate the products.
objectives
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Modify the Rotatable Guarded Hot Box (RGHB)
to be able to test fenestration systems to the criteria specified in ASTM C 1199-97:
Standard Test Method for Measuring the Steady-State Thermal Transmittance of Fenestration
Systems Using Hot Box Methods, and in ISO/DIS 12567: Thermal Insulation - Determination of
Steady-State Thermal Resistance of Components - Hot Box Method for Windows and Doors
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Use the RGHB to measure the surface heat transfer
coefficients on both sides of projecting fenestration products, such as
curb-mounted skylights, at different tilt angles. Use these results
to refine FEA (Finite Element Analysis) models of the same test specimens
so that the results from the FEA can be used to modify the NFRC rating system
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Compare the ASTM (North American) and ISO
(European) test methods for window systems to identify the differences in the methods and
results
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research
Because of the unique testing facilities that
ORNL maintains, we have been able to take part in the development of the test methods
and computer algorithms used to evaluate building envelope components, including
windows. Unlike commercial laboratories, the test facilities at ORNL have been
calibrated to operate at unusual environmental conditions and can produce a wide variety
of environmental conditions at a steady state. They can also perform transient tests
to determine the dynamic performance of massive building systems and can be pressurized to
study the impacts of air leakage and energy transfer simultaneously.
The 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 in accordance with ASTM C 1363, but also it can perform thermal
transmittance tests of fenestration systems that are smaller than the area required
by ASTM C 1199-97.
The Large-Scale Climate Simulator (
LSCS) and
the Rotatable Guarded Hot Box
RGHB are both
capable of testing the U-factors of building envelope systems in accordance
with ASTM C 1363-97. The 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 skylight systems. The chamber is well suited
for testing large commercial skylight systems that are too big to fit in conventional
ASTM C 1363 thermal chambers; it can also perform thermal transmittance tests
of fenestration systems that are smaller than the metering area required by
ASTM 1199-97.
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