Designing Efficient Multifamily Housing

Strategies to improve energy performance, construction efficiency, and operations
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Sponsored by CRL, Mitsubishi Electric Trane HVAC US (METUS), and Tamlyn
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Performance Standards

There are some recognized fenestration standards which are always evolving due to changes in manufacturing technology, building codes and rating system performance requirements. The most widely recognized and relevant standard is the North American Fenestration Standard (NAFS) which governs windows, doors, and skylights. Prepared as a joint collaboration between different organizations, the standard is known as AAMA/WDMA/CSA 101/I.S.2/A440. While this standard is familiar to many specifiers and fenestration manufacturers, the latest version referenced in the current International Building Codes has realigned and changed a bit. Most significantly, a not-so-recent change reduced the total number of Performance Class designations as defined by the Performance Grade (a set of performance requirements corresponding to a Design Pressure range) at the intended location of the building. It is important that specifiers recognize this change and not rely on older versions of this standard which included five performance classes, described as: “R” for Residential, “LC” for Light Commercial, “C” for Commercial, “HC” for Heavy Commercial and “AW” for Architectural. In the most current edition of NAFS, the “C” and “HC” performance classifications have been eliminated and a new “CW” classification has been added for Commercial fenestration. This reduces the total number of performance classifications from five to four and puts the emphasis for testing and performance on meeting the criteria for these Performance Grades.

To be classified into one of these four Performance Grade categories, independent testing is required based on the design pressure of wind forces acting on the fenestration (measured in pounds per square foot—psf) at a stated pressure difference (measured in pascals Pa). The entry-level or “gateway” criteria for tested fenestration units in each of the Performance Grades is as follows:

  • R Class Fenestration: 15 psf (720 Pa) This class is commonly used in one- and two-family dwellings.
  • LC Class Fenestration: 25 psf (1200 Pa) This class is the minimum designation appropriate for low- and mid-rise multifamily dwellings and other buildings where larger sizes and higher loading requirements are expected.
  • CW Class Fenestration: 30 psf (1,440 Pa) This newest class is better suited to low- and mid-rise buildings where larger sizes, higher loading requirements, limits on deflection, and heavier use are expected.
  • AW Class Fenestration: 40 psf (1,920 Pa) This is geared toward mid- and high-rise buildings to meet increased loading requirements and limits on deflection in buildings where frequent and extreme use of the fenestration products is expected.

These descriptions and testing levels are intended to act as a general guide in helping to determine which class is best suited for a particular application. In the case of multifamily buildings, it is clear that LC is the minimum classification needed. However, it is preferable, in most cases, to specify CW class fenestration for low- and mid-rise multifamily buildings to allow optimal performance in a greater range of design conditions.

In the case of large-format glass doors, both sliding and bi-folding versions, CW class units are now available from select manufacturers. That means that they have been independently tested and shown to meet the criteria to qualify for this preferred class of fenestration. This helps design professionals when selecting and specifying such products, but ultimately it means it benefits the owners and occupants of multifamily buildings. The ratings reflect performance features that help achieve the goal of reducing energy use while still meeting design intents. This adds to a building’s overall sustainability and visual appeal.


Creating efficient multifamily housing is a multifaceted task. The Passive House standard is one means to improve the energy efficiency while addressing cost-effective construction techniques. The building envelope can be enhanced with the proper use of WRBs, enhanced with drainable capability, and the cladding finished with aluminum trim. Cooling and heating can be provided with appropriately sized, high efficiency, VRF systems and air-source heat pumps. Fenestration can be addressed by selecting large-scale sliding or folding glass doors that provide light and ventilation along with high performance. These and other similar strategies lead to an overall design that meets the needs of the building owner, the occupants, and the environment.


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Originally published in Architectural Record
Originally published in September 2021


Designing Efficient Multifamily Housing
Buyer's Guide
Palisades S100 Sliding Door & Movable Wall
The Palisades S100 Sliding Door features an ultra-slim 1-5/16” frame that produces expansive views. A 13-foot maximum height and 7-foot maximum panel width creates scale and elegance. Its distinguishing CW40 Performance Grade rating makes it ideal for exterior applications where high loads, limits on deflection, and heavy use are expected.
EZ FIT® Recessed Ceiling Cassette
The ductless EZ FIT® Recessed Ceiling Cassette is ideal for multifamily applications looking to maximize tenant space and meet varied design aesthetics. The cassette fits flush to the ceiling between standard 16-inch joists, appealing to retrofits as well. With adjustable airflow, the unit is effective regardless of ceiling height.
Mitsubishi Electric Trane HVAC US
Tamlyn Moisture Management
Tamlyn's two moisture management products to better the market are TamlynWrap Drainable WRB 1.5 which gives a non-compressible drainage plane, and TamlynWrap Rainscreen. 6.3/10.1 which provides a vented drainage space. Pairing with these is a complete system of flashing, double-sided seam tape, and flashing boots for circular wall penetrations.