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Technical Resources Tips for Selecting High Performance Windows and Curtainwall What you need to know in selecting these products By Ken Brenden To select the best window and curtainwall systems for a building, one must first understand a project's unique characteristics and requirements to ensure an apples-to-apples comparison of performance and value. The next critical step is to involve the manufacturer early in the design process and to work together closely through the project's completion. The first questions a manufacturer will ask are: Where is the project located? What is the building's purpose? What are its dimensions? What environmental conditions will it encounter? What specific thermal, acoustical, and other performance characteristics must the building address? From these answers, the manufacturer can help determine if the solution will require a mixture of windows and curtainwall, unitized curtainwall, ribbon windows, or storefront applications (single spans). Thermal Performance High performance windows and curtainwalls are available with a variety of thermal barriers. To match the best product with a project, consider the differences between conduction, convection, and radiation. Conduction is heat transfer through a solid medium, which can be controlled by the addition of low-conductance thermal barrier materials such as polyurethane, Insulbar, or PVC into the framing. Convection is heat transfer through a fluid medium such as the air space of insulating glass. Radiation is heat transfer that does not require an intervening material. This can be reduced by the addition of low-emissivity (low-E) coatings on glass surfaces. Fenestration thermal performance is quantified by two tested parameters: thermal transmittance, or U-factor, and condensation resistance factor (CRF). Both are determined in a guarded hot box test. In this test, the window unit divides a cold chamber (with fan-generated, 15 mph wind velocity) from a highly insulated/guarded, warm chamber. By measuring the energy input required to keep the warm side at a constant temperature, the U-factor of the window unit is indirectly determined-the lower the U-factor, the better the thermal performance. U-factors are the reciprocal of the more familiar R value, often used in describing wall insulation. In the same test, glass and framing surfaces are fitted with thermocouples to measure surface temperatures at 20 pre-specified locations, as well as at the four coldest points on the frame. These are weighted and combined to generate "average" frame and glass temperatures which are used in the CRF calculation (the higher the CRF, the better the thermal performance). Because CRF is a "unit-less" temperature ratio, frame and glass CRFs are usually specified separately, but the "minimum" governs condensation performance. CRFs typically are recommended as a comparison tool only. Field condensation performance is affected by many variables outside the manufacturer's control, such as interior air movement and humidity, wind speed and direction, interior finish materials, etc. There can be no guarantee of thermal performance under transient field conditions, only under controlled and steady-state, laboratory conditions on defined configurations. Expansion and Contraction Expansion and contraction of material is the result of temperature change. Because all projects will be subject to temperature changes, all materials will change length. The extent of change is dependent on the type of material, temperature change, and the material's thermal expansion coefficient. As many commercial windows and curtainwalls include aluminum framing, a workable rule-of-thumb is that aluminum's thermal expansion will equal 1/8 inch in 10 feet for a 100° temperature change. When selecting a system, be sure to allow for vertical and horizontal movements to ensure future water and air performance while remaining aesthetically pleasing. If movement is not considered in the design of the product or its application to a structure, problems may occur including frame distortion, window seal failure, undesirable noise, and air and water infiltration. Perimeter sealant joints should be large enough to accommodate thermal expansion, as well as the anticipated live load floor deflections, deflections due to windload, and other building movements. The anticipated thermal expansion and contraction movements and other anticipated building movement should be considered when selecting a sealant material. Based on the height of the opening, if the perimeter joint becomes too large for practical application, a head starter or receptor should be incorporated into the design and budget. The anchorage system and method of anchorage also should be capable of providing for movement. Ken Brenden, chief designer with 25 years of experience in windows and curtainwalls, leads the Wausau Window and Wall Systems prebid engineering department where he helps customers with architectural project specifications, details, and thermal and structural analyses. An active member of AAMA, he currently serves as second vice president of its Curtain Wall/Storefront/ Sloped Glazing Council. He also is a certified document technologist (CDT) with a degree in architectural and residential design. Source: Appeared in Glass Guide for Architects, Glass Magazine June 2000. |
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