Highly Insulating Commercial Framing

Highly Insulating Commercial Framing

Introduction

Thermal Performance
Thermal Performance

Commercial fenestration systems mostly employ aluminum framing because of the aluminum alloy's relatively low cost, high strength, easy manufacturability and long service life. However, aluminum has one serious inherent disadvantage in its high thermal conductivity. This limits the ability of most framed commercial windows and facades to meet current thermal requirements. Traditionally, poor thermal performance and low condensation resistance have plagued aluminum framing. Because of this, over the past 20+ years, the design of aluminum framing has been modified to include thermal breaks of various designs, but the thermal performance, while improved, still remains a major limitation to attaining DOE's performance goals. Commercial builders seek window framing solutions with the cost and strength of aluminum products plus improved thermal efficiency to meet more stringent building codes. Window framing approaches, such as those incorporating pultruded fiberglass and other fiber reinforced polymers, have proven to date to be prohibitively expensive for high market panetration, and they cannot be integrated into existing manufacturing processes.

In this project, we develop a new thermal break technology that allows aluminum framing to have thermal performance comparable or better than wood and PVC, while preserving the inherent structural benefits of the aluminum alloy material. In order to ensure our frame design is practical and easily brought from prototype to market, we have worked closely with major commercial window frame manufacturers.

Background

Because of the inherent high conductivity of aluminum framing systems, the designs of Aluminum framing have been modified to include thermal breaks of various designs over the past couple of decades. Technologies used for thermal break are generally divided into two categories, as shown in the figure.

  1. Pour-and-debridge method, where the framing is extruded as a single piece with the pocket for thermal break. Liquid polyurethane is poured into the pocket and after solidifying, the backing Aluminum section is ground away. This is the older of the two methods, but still in widespread use. The disadvantage of this method is that thermal break width is limited (typically it is about 0.25 in.) by the structural requirements, and the thickness of the thermal break is fairly large, thus limiting the effectiveness of the thermal break. Windows incorporating this type of thermal break have general performance of about U = 0.5 Btu/(hr·ft2·°F), or R2.
  2. Crimped strips (sometimes called I-bars), where the frame is extruded into two dies and Polyamide strips (usually two) are crimped on each side to create single framing cross-section. Even though Polyamide has higher conductivity than Polyurethane, these strips have smaller cross-section (i.e., thinner) and can have larger widths than pour-and-debrigde systems (normally around 0.50 in.), which allows for better frame performance (typically U = 0.35 to 0.4 Btu/(hr·ft2·°F) or up to R3). Their disadvantage is that this thermal performance cannot be easily be improved further.

Additional, and much less used, methods in use today consist of partial de-bridging of the framing web or steel bolts at regular intervals to fasten indoor and outdoor frame sections. While these methods have improved thermal performance of Aluminum framing, their relatively poor thermal performance still remains an issue and has resulted in relaxed code compliance requirements for commercial framing, as compared to residential framing. Namely, stricter structural requirements for commercial framing have prevented the use wood and PVC framing materials in commercial buildings, which are common materials in residential framing.

Design and Analysis

The goals of our completed frame system are two-fold, first to meet strict structural requirements, and second to meet thermal requirements. Structural requirements are standardized in North America and outlined in the 2011 North American Fenestration Standard/Specification for windows, doors, and skylights (AAMA 101). The outlined fixed window Architectural Window structural requirements include a minimum design pressure of 40 lb/ft2, maximum of L/175 in deflection at design pressure, and life cycle testing, all at a minimum window size of 60 in x 99 in.

Truss Frame
Truss Frame

The structural performance of the frame design itself is benchmarked against the performance of current industry frames and to structural criteria using the AAMA technical information report Structural Performance of Composite Thermal Barrier Framing Systems (AAMA 04). The structural calculation procedures outlined in this work are followed for our initial approximations of structural performance. We plan to also follow several industry validation test procedures upon completion of the prototype frame.

While there are no standard commercial frame profile dimensions, the width, 3.25 in., and height (sightline), 2.25 in., overall dimensions of our frame are based on typical sizes found in industry. The frame is designed to accommodate up to a 1.50 in. glazing package with the appropriate glazing bead, but is initially built with a glazing bead to accommodate a more common 1 in. glazing.

Thermal requirements are not as well defined. For commercial and residential applications, windows are typically designed to meet the requirements of the International Energy Conservation Code (IECC), the most commonly adopted model energy code in the U.S. for buildings. Commercial applications often use the ASHRAE 90.1 standard to meet performance or prescriptive based whole building energy requirements. The prescriptive path limits window to wall ratio to 40%, in part to account for the typically low thermal performance of commercial glazing and framing systems.

Glazing systems modeled in a NFRC size fixed window (47 in x 59 in) with three frame types (standard aluminum, traditional thermal break, and the highly insulating frame) show that the traditional thermal break frame ranges from a 40% to 90% improvement over traditional aluminum, and the highly insulating frame achieves a 20% to 90% further improvement over the traditional thermal break frame (80% to 170% over the aluminum frame).

Future Work

Continued development of the frame design is currently being supported with funding from California Energy Commision. Under this funding we plan to complete development of a prototye and perform validation testing.