High R Value Windows
FINDINGS #7, December 2013
Hi-R Panel Retrofits Provide Significant Energy Savings for Low Investment
In a recent assessment, GSA’s Green Proving Ground (GPG) program achieved a 41% reduction in winter energy use at a Provo, Utah federal office building by retrofitting 21 single-pane windows with triple-pane highly insulating window panels (Hi-R panels). Hi-R panels are pre-manufactured units designed to improve the insulating power of low-performing windows, without the need for major renovations or costly modifications to existing buildings. Hi-R panels use low emissivity coating and single-, double- and triple-pane configurations to build upon the concept behind residential storm windows, providing greater thermal performance with a long-lasting, simple application. These framed units can easily be installed on the interior side of existing windows, improving their performance at minimal cost with little or no disruption to building occupants. Findings from this study support the consideration of Hi-R panel retrofits as a low-cost option for achieving significant savings in heating and cooling commercial buildings, particularly those located in cold climates. The potential energy savings to be gained from installing high-performance windows in commercial buildings nationwide equals more than 1% of the total U.S. energy consumption—enough to power 5.5 million U.S. households(1).
What We Did
COMPARED PERFORMANCE DATA BEFORE AND AFTER RETROFIT
In February 2012, GSA retrofitted 21 single-pane windows with triple-pane Hi-R windows in a 6,400 square foot, single-story office building in Provo, Utah, at a cost of $7,500 for labor and materials, or $32.40 per square foot of glass. Data logging sensors were deployed to measure window an wall surface temperatures for all four orientations over an eight-month period—three pre-retrofit and five post-retrofit, to discern the heating load reduction resulting from the Hi-R panel installation. Before and after the retrofit, 12 employees who regularly occupy the space participated in a survey that measured their thermal and visual comfort.
What We Measured
HEAT TRANSFER, SOLAR RADIATION, AND VISIBLE LIGHT
The test case measured U-factor, solar heat gain coefficient (SHGC), and visible light transmission (VT) of various wall and window surfaces. The U-factor of glazing is related to the resistance of heat transfer between the interior and exterior of windows; the smaller the U-factor, the better the insulator. The SHGC reports the amount of solar radiation that flows through the window from direct sunlight. VT refers to the amount of light that passes through a window. In conjunction, these three factors determine the balance of heating load reductions, cooling load reductions, and visual comfort generated by specific window types. Local climate, building orientation, internal and external window shading systems, and building type play large roles in the measurement and prioritization of these factors.
[Modelled Heating Energy Savings in Three Retrofit Configurations, COMFEN results compared to base configuration of single pane + bronze film]
What We Concluded
FAVORABLE FINDINGS FOR LOW INVESTMENT
The Provo test case provided a 34-41% reduction in winter heating load for a project cost of $7,500, resulting in a conservative simple payback estimate of 9 years. Additionally, building occupants reported improved thermal and visual comfort as a result of the retrofit. Significant energy savings, increased occupant satisfaction, and relatively non-intrusive installation methods make Hi-R panels a quick and manageable alternative to the full replacement of low performance windows.
SITE-SPECIFIC EVALUATION NECESSARY FOR OPTIMUM RESULTS
Condensation sensitivity is a potential challenge in Hi-R panels. The dry climate in Provo, Utah and the absence of a building humidification system made this test case a poor gauge of potential weaknesses caused by condensation. Further evaluation should be conducted for site-specific applications.
In general, Hi-R panel retrofits have proven most effective in buildings with lowper forming windows in colder climates; however, the impact of windows on energy demand is extremely context-specific. The payback and performance of Hi-R panels are highly dependent on building conditions including climate, existing window types, interior space configuration, and building form. While initial testing generally recommends Hi-R panels as a strong contender for improving thermal performance, site-specific evaluation is essential to gauging the potential success of Hi-R panel retrofits.
For more information, contact Green Proving Ground email@example.com.
1 Apte, J., Arasteh, D. (2006), Window-Related Energy Consumption in the US Residential and Commercial Building Stock. Berkeley, CA: Lawrence Berkeley National Laboratory report, LBNL-60146 http://gaia.lbl.gov/btech/papers/60146.pdf
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