Wausau Window and Wall Systems
December 2015



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Natural Ventilation
Operable windows allow for views and daylight, provide a seasonal opportunity for natural ventilation, and may help buildings qualify for credits under the U.S. Green Building Council's (USGBC) LEED® green building rating systems. Indoor Environmental Quality points for Increased Ventilation and Controllability of Systems - Thermal Comfort often apply when operable windows are part of an effective natural ventilation strategy, using the recommendations in the Carbon Trust "Good Practice Guide 237"  [1998] and ASHRAE 62.1 -2013.
Wausau's structurally glazed, operable window products -- such as INvent™ Series 4250i-VX , and 4250-Z Zero-Sightline Series  can provide "no-sag" performance -- even for oversized vents. The low air infiltration of Wausau's AAMA AW "Architectural" Class windows helps keep drafts to a minimum.
Openable Area
Codes in certain jurisdictions, including the New York City 2008 Building Code (NYCBC), require a minimum amount of "openable" window area, based on 4% to 5% of occupiable or habitable floor area. In New York, required openable area per window is 5.5 to 6.0 square feet depending on sash mode, and typically, the top of at least one opening in the room must be at least 7 feet above the floor.
As defined, openable area is the entire rectangular frame opening created by the operable sash or vent - width multiplied by height - in square inches or square feet. Per NYCBC-2008 Section 1203.4.4 "Measurement of openings: Dimensions of windows and other openings shall always be taken between stop beads or, if there are no stop beads, between the sides, head and sill of the sash opening."
NFRC Ventilation Rating
Currently in final balloting, NFRC's 401-2013 "Procedure for Determining Fenestration Product Ventilation Rating" uses an alternate methodology. Net ventilation area is determined by calculating the actual opening area created by the operable sash or vent, as limited by the hardware utilized.
For a projected window with less than full-opening capability, this is the familiar "two triangles plus a rectangle" as pictured. Fenestration products are evaluated at their maximum design opening dimension, without consideration for limited opening devices such as those providing child fall protection. A screen obstruction factor is included if applicable.
Air Flow
Air flow through open windows depends on many factors other than openable area or net ventilation area, including, but not limited to wind velocity, building height, stack effect, room configuration, shape factor, and most importantly, provisions for cross ventilation. As estimated on the University of Minnesota Center for Sustainable Building Research's website, www.commercialwindows.org, openings in opposite walls are two- to three-times as effective in promoting air flow as the same total opening area in a single wall, with all other conditions the same.
European Standard prEN 15242 "Ventilation for buildings - Calculation methods for the determination of air flow rates in buildings including infiltration" offers an even more detailed analysis of the factors influencing air flow through open windows. Design teams sometimes employ state-of-the-art computational fluid dynamics analysis tools to optimize natural ventilation strategies in zero-energy building designs.
Accessibility Considerations
Designers can help ensure that fresh air and a connection with the outdoors are made accessible to people with physical disabilities by specifying windows capable of meeting operating force and motion requirements of ICC/ANSI A117.1, "Accessible and Usable Buildings and Facilities." Windows should be tested to AAMA 513-14, "Standard Laboratory Test Method for Determination of Forces and Motions Required to Activate Operable Parts of Operable Windows and Doors in Accessible Spaces."
Gaining the leverage for easy operation can require special location of roto-operators, which may provide only limited opening.
In addition to forces and motions, the designer should ensure operable windows are located and detailed in a way that meets the "reach" limitations of ICC/ANSI A117.1. There are also physical limits on clearances and protrusions, as well as on approach area and threshold height (for terrace doors and sliding glass doors).
Model building codes' and local jurisdictions' requirements address accessibility of operable windows for skilled nursing and personal care facilities, condominiums, apartments, hotels, classrooms and dormitories. Remember, building codes represent only minimum requirements. Even if not required, accessible operating windows may be a very desirable feature of the occupied spaces being designed.
To explore natural ventilation strategies for specific projects in detail, please contact Wausau's market managers for architectural support at  
Featured Project   


1706 Rittenhouse Condominiums 

Location: Philadelphia, PA
Architect: Cope Linder Architects LLC, Philadelpia
Product(s) Used: 4250 Series & Terrace Doors