Ensuring Energy Efficiency: Specifying windows and curtain walls for cold climates

Performance in the field
Project conditions and test limitations call for caution when using CRF or ‘I’ test results to predict or prevent condensation on installed windows or curtain walls. It is technically preferable to start from basic principles in assessing any design’s robustness to the formation of condensation.

First, a psychometric chart helps determine dewpoint temperature for a given set of interior conditions—both temperature and relative humidity (RH). Condensation or frost forms on any interior surface that falls below the dewpoint.

Next, guarded hot box test results or computer modelling is used to determine the minimum local U-factor at any point of the frame or glass. An appropriate winter outside design temperature is then selected for the project site, usually taken from the ASHRAE Handbook of Fundamentals’ 99 per cent base. By back-calculating from the local U-factor, a quick check can be run to see if any surface will fall below the dewpoint temperature. Finite element computer models can provide more detailed surface temperature profiles.

There is no substitute for the judgment of the mechanical engineer and other design professionals in assessing the importance of condensation in a particular application. Field condensation on interior surfaces is affected by many variables, including:

  • component thermal performance;
  • thermal mass of surrounding materials;
  • interior trim coverage and airflow conditions;
  • weather; and
  • mechanical system design.

CC_Oct13_HR-26Finite element thermal models
In the early 1980s, finite element computer modelling was developed to predict thermal performance of untested frame-glass combinations of custom systems. Thanks to the efforts of NFRC, Lawrence Berkeley National Laboratories (LBNL), and the U.S. Department of Energy (DOE), this software is now publicly available as THERM 5.2 and WINDOW 5.2.2 Many manufacturers, test laboratories, and glass fabricators have modelling capabilities in-house.

Overall U-factor and SHGC, as determined by modelling, are the basis of U.S. code requirements and NFRC labeling programs. Validation tests are run to confirm the model’s accuracy for a particular system type. (Canadian commercial building energy codes are also derived from standards produced by ASHRAE, which notes U-factor and SHGC. The National Energy Code of Buildings [NECB] has its own requirements, but uses the NFRC CPD for its information.)

Design teams should exercise caution when applying modelled surface temperatures of conductive framing materials to real-world conditions.

Energy modelling tools
The value of whole-building tools cannot be overstated. However, for early use in envelope design, the tools of choice are somewhat less comprehensive and focused only on perimeter zones of the building floor plan. Choosing efficient windows for a commercial building can be difficult using published U-factor, SHGC, VT, and CRF because their relative importance depends on site- and building-specific variables.

Fenestration energy modelling tools provide comparative energy performance to optimize product selection. Performance metrics include:

  • annual energy use;
  • peak demand;
  • carbon emissions;
  • daylight;
  • glare; and
  • condensation.

The gold standard of fenestration energy modelling tools is the COMFEN software developed by the Window and Daylighting Group at LBNL, under the auspices of the DOE and other supporting organizations.

COMFEN gives knowledgeable designers and specifiers a tool to systematically evaluate alternatives for fenestration: size, location, and commercially available glass types, as well as shading and light redirection schemes, inside and outside. It is both project-specific and site-sensitive, offering evaluation tools for dozens of locations worldwide.

COMFEN is powered by EnergyPlus energy simulation software, from DOE’s Office of Energy Efficiency and Renewable Energy (EERE), using average hourly weather data and envelope characteristics to model various fenestration design scenarios.

Given its breadth of options, alternatives, and powerful functionality, COMFEN’s user interface was made as simple as possible. Allied industry groups, as well as some North American window and curtain wall manufacturers, use COMFEN to power other applications, further simplifying early product selection and optimization.

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