Is a Formula an Acceptable Substitute in Place of Use Phase Results?

At the recent Insulating Glass Manufacturers Alliance Winter Meeting, members were shown an example of the formula in use for the energy calculation for a sample window in a mixed climate zone.

At the recent Insulating Glass Manufacturers Alliance Winter Meeting, members were shown an example of the formula in use for the energy calculation for a sample window in a mixed climate zone.

At the recent Insulating Glass Manufacturers Alliance Winter Meeting held in San Francisco, the idea of using a formula in place of use phase results was addressed. Jeff Haberer, director of technical services for Trulite Industries, and a developer of the formula, discussed how regression equations can be used in place of use phase results for life cycle assessments.

The equation is: U-factor*M1*5.6738) + (SGHC*M2) = Total energy (GJ/yr) with M1 representing U-factor scalar and M2 as SGHC scalar. The span is 30 years.

When initially questioned at the conference about the many considerations, including decay, the equations may not accurately represent, Haberer told attendees, “There’s no known way to establish what the decay would be. It’s a quite difficult task to do this and we’re trying to make it as simple as possible … These are comparative numbers so one product compared to the next, you can at least get an idea of numbers.”

In response to people who may doubt the use of the formula, Haberer said, “I’d say it’s not a perfect solution, it’s only a comparison from one window to the next.

“Energy modeling has a larger number of variables that affect the energy flow through windows,” he adds. “Two of the largest variables are climate and building type. The Product Category Rules establishes rules to measure windows consistently so products can be compared. The committee chose five simplified models to represent a spectrum of building types. The computer models of these buildings were based on existing IECC and ASHRAE models created by the Pacific Northwest National Laboratory. In addition, three specific climates were also chosen to represent the wide spectrum of climate zones across North America.”

Haberer said the formula is based on regressions from existing models and represents the most current, cost-effective option.

“For ease of use and to reduce simulation costs, computer simulations of the various window products were regressed into linear equations. With the system in place, use phase energy of windows can be simply and economically determined by entering already know NFRC window performance values (U-factor, SHGC, VT) into a simple equation,” he notes. “The results are not to be taken as ‘THE’ window energy result for all buildings and climates. Rather, the results provide a consistent measuring technique for comparison of one window product to another, using consistent assumptions of the building and climate in a convenient and economical manner.”

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