ASHRAE has published a new guideline for increasing energy efficiency in historic buildings while minimizing the disturbance of the building’s historic character. Section 6.4 of ASHRAE Guideline 34-2019, Energy Guideline for Historic Buildings covers glazing requirements for retrofitting historic buildings, including detailed descriptions of the processes and procedures to achieve greater measured efficiency.
The guideline is particularly aimed at providing guidance for ‘listed’ historic buildings; i.e., those formally designated or eligible to be designated as historically significant by a governing body.
“The committee members writing this guideline are exceptionally knowledgeable about the special issues related to historic buildings and the care needed to preserve them,” said 2018-2019 ASHRAE president Sheila J. Hayter, P.E., who also served as chair of 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 recommends that historic windows and doors with glazing be retained when possible.
“Historically, windows, in particular, have provided the critical functions of a visual connection with the outdoors, a light source (see Section 9: Lighting) and, when operable, a source for natural ventilation,” reads the guideline. “Accordingly, these features should not be compromised with energy upgrades, e.g., if multi-light windows are replaced with large-pane single units, the provision of a distinct character of framing the view is lost or replacing original frames, especially with those of a different material, wood type, shape or size from the original, also likely destroys their architectural character.”
The guideline also recommends any proposed upgrades consider historic significance and appearance when analyzing the cost- and energy-saving benefits of historical elements such as storm windows, which assist in regulating both light and heat flow.
“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,” reads the guideline.
6.4.1 Storm Windows
Guideline 34 explains that it is usually desirable to prevent condensation on the original sash, which may affect the outer glazing of window units with the onset of cold weather. Therefore, the use of exterior storms is preferable. The guideline recommends that exterior storms be designed with double glazing or low-E coatings, which reduce the likelihood of condensation and provide energy savings.
“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. Double-hung windows are particularly difficult to air-seal because of the weight pockets and sash cord openings,” reads the guideline.
6.4.2 Weather-stripping and Caulking Windows and Doors
The guideline explains that caulking and weather-stripping assist in curving air infiltration and improving energy efficiency. It recommends that when undertaking weather-stripping, contractors measure the tightness before and after to ensure that expected energy savings are realized.
“It should be noted that window units with cracks that permit air exchange are rarely major contributors to overall air exchange in a building. 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,” reads the guideline.
The guideline outlines the benefits of shutters and shades, such as providing security and contributing to good solar protection and overall energy efficiency.
Films may be added to glazing to reduce infrared solar gain, according to the guideline, to reduce air conditioning load.