The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) weighed in on windows and glazing recently, through a new guideline aimed at increasing energy efficiency among historic homes and buildings. ASHRAE Guideline 34-2019, Energy Guideline for Historic Buildings, cues in on minimizing the disturbance of a building’s historic character, including Section 6.4, which covers glazing requirements for retrofitting. Among the requirements are detailed descriptions for the processes and procedures that should be used to achieve improved energy efficiency, with a further suggestion that historic doors and windows should be retained when possible.

“The committee members writing this guideline are exceptionally knowledgeable about the special issues related to historic buildings and the care needed to preserve them,” says 2018-19 ASHRAE president Sheila J. Hayter, who also served as chair for the international guideline committee. “The committee’s intent was to provide guidance for worldwide communities and specifically for entire project teams—not just engineers.”

The guideline, ASHRAE officials suggest, is particularly aimed at providing guidance for ‘listed’ historic buildings, or those formally designated or eligible to be designated as historically significant by a governing body.

“Historically, windows, in particular, have provided the critical functions of a visual connection with the outdoors, a light source … and, when operable, a source for natural ventilation,” reads the guideline. “Accordingly, these features should not be compromised with energy upgrades,” it suggests.

For example, if divided-lite windows are replaced with monolithic units, “the provision of a distinct character of framing the view is lost,” the guideline declares. Similarly, replacing original frames with those made of different materials or wood species, or with those of a different shape or size, also risks destroying components of a building’s architectural character, it suggests. For this reason, the guideline recommends any proposed upgrades consider historic significance and appearance when analyzing cost- and energy-saving benefits, such as storm windows, which assist in regulating both light and heat flow, it says.

“In some cases, the reinstating of passive elements, e.g., operable shutters and operable windows, will reduce the need for some of the mechanical heating, cooling, and ventilating being considered or already in use,” the guideline says.

Storm Windows

Section 6.4.1 of Guideline 34 explains that it is typically desirable to prevent condensation on original sashes, which may affect the outer glazing of window units with the onset of cold weather. Therefore, the use of exterior storm windows, it suggests, is preferable. The guideline further recommends that exterior storm windows be designed with double glazing or low-E coatings, which reduce the likelihood of condensation and provide energy savings.

Regarding the use of interior-mounted secondary (storm-style) windows, “The success of efforts to reduce condensation using interior storms depends not only on airtightness of the storm to the frame, but also on the closure of other airflow paths at the sill, jamb, and head,” the guideline states. “Double-hung windows are particularly difficult to air-seal because of the weight pockets and sash cord openings.”

Weather-Stripping and Caulking

Section 6.4.2 of the guideline explains that caulking and weather-stripping assist in curbing air infiltration and improving energy efficiency, but that contractors undertaking those measures should check the air tightness before and after to ensure that expected improvements are realized. At the same time, “It should be noted that window units with cracks that permit air exchange are rarely major contributors to overall air exchange in a building,” the guideline suggests. “This is because the cracks are quite small and because the windows are located close to a neutral pressure plane, where buoyancy drive is not active.”

For additional protection from solar heat gain, sections 6.4.3 and 6.4.4 outline the benefits of shutters and shades, and window films, respectively. Those products, it suggests, will not only provide improved security, but reduce the required loads on air conditioning.

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