by Arlene Zavocki Stewart

Taitem Engineering released a field methodology for obtaining U-factors using an infrared thermometer on February 28 at the Residential Energy Services Network (RESNET) conference. Based on a single point temperature reading at the bottom corner of the sash where the glass meets the frame, principle investigator Ian Shapiro and his team have developed a heat-transfer calculation derived from National Fenestration Rating Council (NFRC) procedures. Shapiro said the calculation mimics published NFRC U-factors within a 10 percent margin of error and that he hopes the method can be used by energy raters and weatherization auditors in their calculations and cost analysis to increase justification for window replacements where there is no label on the installed window.

The RESNET conference marked the initial release of Taitem Engineering’s research. During the year-long project, 12 vinyl and four wood windows were installed in a climate-controlled chamber where temperature measurements were taken at various positions on the window. The researchers then correlated their results to the published U-factors of the ten labeled windows in the study (six were not available). They ascertained that the measurement point most consistently close to the published NFRC U-factor was the lower corner of the window.

The researchers then adapted the test chamber method for the field, developing self-adhesive stickers to place in the corner to ensure that the infrared thermometer would focus to take an accurate temperature measurement. They also used the stickers as a focal point to take the air measurement six inches in front of the window, required for the calculation. They field-measured more than 200 installed windows in a number of multi-family projects. Researcher Javier E. Rosa indicated that once they became experienced in the method, it took as little as six minutes in the field to obtain the necessary data for a window, half of which was spent waiting for the stickers to equalize.

The research project is one of four the firm currently has underway, funded by the New York State Energy Research and Development Authority (NYSERDA). The firm’s direct experience in conducting comprehensive energy studies for programs like NYSERDA’s FlexTech and Energy Audit likely was the inspiration for the project.

“We have a hard time justifying replacement windows. Sometimes it works, sometimes it doesn’t,” Shapiro said. “Even single-pane windows, we struggle in various government programs we operate in to justify window replacement. We would like to.”

More than half the windows in this country were installed before NFRC even existed, he noted.

Some expressed concerns about the methodology.

“We use the Home Energy Rating System (HERS) to rate a good number of our homes,” said a production home builder who wished to remain anonymous. “What I want to be careful of is that we don’t get ourselves caught in a place where we purchased a window that meets the requirements, we’ve installed it in the house and rater comes out there a year later, tests the glass and says ‘that’s not a 0.28, it’s a 0.35.’ And all of a sudden, you’re into this dispute…”

Jim Larsen of Cardinal Glass Industries, who also was in attendance, questioned its applicability.

“The single-point temperature measurement can likely correlate for a given window size and design (glass, spacer and frame type) but all of these are variables in the field, so the usefulness of the method is questionable,” said Larsen. “A review of Table 4 in Chapter 15 of ASHRAE Handbook of Fundamentals tells us if we know the number of glass layers, frame material and window type, the window U-factor can be estimated with all the precision needed for energy analysis, particularly for discussions of retrofit and upgrade decisions.”

Shapiro contends that his team would like to go one step farther than the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). “If this is a tool that gets us one step more accurate than opening up an ASHRAE book, that’s good,” he said.

Still, they are not looking to replace NFRC and would be willing to share information with the organization. “NFRC has a tremendously important role,” said Shapiro. “My interest is to try and accurately characterize existing windows to make sure we are not missing opportunities to replace [them]. My fear is that we are underestimating the energy loss–especially single pane. Hopefully, in many instances, it’s going to show the losses and justify replacing the window or adding a storm window.”

“Our interest is to promote energy conservation, and to do it honestly,” he added.

When a session attendee asked where to obtain a copy of the method procedure, Shapiro said Taitem needed to further refine the procedure and had submitted a proposal to NYSERDA for phase 2 development.

Taitem Engineering’s PowerPoint presentation may be found on the RESNET website.

The RESNET conference was held February 28 to March 2 in Orlando.

Arlene Zavocki Stewart is a columnist for DWM magazine and owner of AZS Consulting. The company works to facilitate field application of energy codes for both residential and commercial construction. Follow her on twitter at ArleneOnEnergy.


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