Daylight is one of the most desirable qualities for any building, especially a green building.  From my Base Zero lighting design perspective (follow the link for my previous blog), daylight is fundamental. An aperture in a roof or wall introduces daytime illuminance into an otherwise dark enclosure. But there is an art and a science to optimizing that raw light into a usable resource that is visually comfortable. The successful placement, size, and design of skylights and windows depends on an understanding of location, orientation, climate, and solar positioning as well as glazing qualities such as visual transmittance, shading coefficient, emissivity, tint, spectral selectivity, and U-value or heat transfer coefficient of the glazing. It also requires considerations of visual task requirements, occupant preferences, interior finishes, and space planning. In other words, a window is highly desirable but deciding on the right window can be complicated.

Windows for historic buildings present a particular challenge. From the perspective of lighting quality, the design, size, and placement of these windows was most likely based on very well-understood principles for bringing light effectively into pre-20th century buildings that otherwise depended on feeble candlelight, torchlight, oil lanterns, gaslight, or early incandescent illumination. If operable, these same windows were essential for also ventilating the building on pleasant days. But these apertures were also likely drafty and offered little insulation against winter chill or summer heat.

From a dMass perspective, the preservation of an existing building is highly desirable, as long as the building can perform up to modern energy efficiency standards. A common "green building" assumption has been that historic windows should be replaced with new high-performance windows to improve thermal performance and energy efficiency. However, that assumption is too hasty and, in many cases, wrong. The National Trust for Historic Preservation has recently published some compelling information and case studies indicating that the careful restoration and weatherization of existing windows and/or the addition of well-designed storm windows can result in better energy performance than replacement windows.

Wood casement windows, prevalent in 19th to mid-20th century US residential construction can be refurbished and augmented with new storm windows to provide insulating values comparable to or better than new aluminum or vinyl replacement windows. This saves the historic integrity of the building exterior and avoids the use of environmentally questionable new materials and the energy and resources used to manufacture new windows.

For more complex restoration projects, there are technologies such as Insulated Glazing Units (IGUs) that can replace old single-pane clear glass with new double-paned IGUs to greatly improve the thermal performance of the building envelope while preserving the historic façade. However, IGUs are substantially thicker than original window glass, often precluding their use in delicate frames. In those cases there are a variety of film products that can be applied to windows to filter light and heat. There are also many window manufacturers that specialize in replicating historic windows to recreate mullion patterns and frame designs that meet preservation approval. Fortunately, there is some good information available from reliable sources to help owners and architects make good decisions regarding window restorations.  (For more reading, see "What Replacement Windows Can’t Replace: The Real Cost of Removing Historic Windows" (PDF), "The Effects of Energy Efficiency Treatments on Historic Windows" (PDF), and "Getting the Most from Old Windows: A Tale of Attachments."

For Yale University's Stoeckel Hall (PDF), a late 19th century Venetian-style building, the intricate stone-framed decorative windows were carefully rebuilt using IGUs. The deteriorating rectangular wooden windows were exchanged with operable replica windows with double-pane, low-emissivity glazing. This met preservation goals, reduced energy consumption, and helped the building achieve a LEED Gold rating.

In other words, historic building performance and preservation are compatible principles, especially when it comes to windows. Old windows are valuable. The bigger challenge will be the rehabilitation of buildings constructed after 1950. The vast majority of buildings now in existence were built after World War II. Many suffer from failing envelopes including substandard windows. Next time, I'll explore what's happening with new window technology to improve daylighting performance in those existing, but not quite beloved, historic buildings. Stay tuned.

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