Volume 24, Issue 6 - November/December 2010

Metal Matters



The Next Level Assuring Peak Thermal Performance of Fenestration
by Ken Brenden

he discussion about the thermal performance of fenestration has moved beyond yesteryear’s debate over the thermal properties of a product’s predominant framing material. Today’s highly engineered profiles take maximum advantage of an assemblage of components of various materials to optimize all aspects of thermal performance. The key elements of this are minimizing heat conduction between the interior and exterior (as measured by the U-factor) and optimizing solar heat gain (as expressed by the Solar Heat Gain Coefficient [SHGC]).

Note that the proper term is optimizing, not minimizing SHGC. While energy use in commercial buildings is typically dominated by cooling loads that dictate a lower SHGC, buildings in northern climates can benefit from higher SHGCs to offset thermal transmittance losses.

Thermal Performance Contributors
Aside from green scoring systems, energy-specific rating systems and energy codes, specifiers (and others) should be aware of the specific physical attributes that contribute to maximum thermal performance.

1. Insulating glass unit (IGU): Double glazing is well-recognized as a key means to minimize U-factor;

2. Gas fill: Replacing the “dead air” within IG units with an inert gas, such as Argon or Krypton, further reduces heat flow;

3. Warm-edge spacers: This technology upgrades the more traditional metal spacers to material that reduces heat flow around the edges of IG units and also reduces the tendency for condensation to form around the perimeter of the glass;

4. Low-E glazing: These glass coatings have been shown to reduce solar heat gain and the loss of internal heat via re-radiation. This solution can be fine-tuned for heating-dominant or cooling-dominant climates by varying the IGU surface to which the coating is applied;

5. Tinted or reflective glazing: Especially viable in hot, southern or southwestern climates, this is a well-known option for reducing unwanted solar heat gain;

6. Thermal barriers: For commercial applications, aluminum is commonly used due to its inherent structural strength and light weight. Aluminum’s naturally high thermal conductivity has been largely overcome by advanced thermal barrier technology; and

7. Effective sealing and weatherstripping: Air leakage around frame and sash members can work against all other energy-conserving solutions. Reasonably tight construction and effective weatherstripping will do the job, but its application must not unduly increase operating force.

Added Benefits
Minimizing thermal transmission also carries the added benefit of minimizing the tendency to form condensation. In commercial facilities, condensation resistance can be an important factor in the building’s functionality. For example, in hospitals moisture provides fertile ground for growth of potentially dangerous molds or bacteria and can lead to hospital-acquired illnesses in patients as well as compromise sensitive diagnostic equipment.

The tendency of a window design to form condensation can be predicted by its Condensation Resistance Factor (CRF). AAMA offers an online calculator to provide general guidance on defining a target minimum CRF based on a project-specific set of environmental conditions. (Visit http://www.aamanet.org/CRFtool for details.)

Thermal performance of a completed commercial fenestration unit is quantified by testing according to industry consensus standards, such as AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, initially published in 1980.


Also, AAMA 507-07, Standard Practice for Determining the Thermal Performance Characteristics of Fenestration Systems Installed in Commercial Buildings, provides a simple means to verify the energy performance of commercial fenestration using graphs and linear interpolation based on one-time test results.

Ken Brenden is the technical services manager for the American Architectural Manufacturers Association (AAMA) based in Schaumburg, Ill. Mr. Brenden’s opinions are solely his own and not necessarily those of this magazine.

 

Architects' Guide to Glass & Metal
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