Sizing up cavities

Cavity design, size and location play important roles in the designing process of a thermally improved aluminum profile. Improper cavity sizes or styles can lead to structural and thermal inefficiencies. However, the properly fitted cavity will show increased thermal energy efficiency and correct structural support. To achieve such optimal results, the extrusion or product designer should answer certain questions before selecting the proper cavity size and design.

These questions include:
•  What is the end use of the profile?
•  What is the nominal wall thickness of the profile?
•  What is the overall size of the profile?
•  What are the structural requirements of the window, door or skylight assembly?

The answers to these questions must be thoroughly explored, as an improper cavity threatens structural strength and security. Often, the cavity size or extrusion-gap separation selected will be too small or too large for the profile wall thickness and overall profile size, as shown in the diagram below.

Strength at the core
Properly formulated thermal barriers, including those made with polyurethane, can provide composite strength for heavy architectural aluminum framing members. The effectiveness of polyurethane and other thermal barriers include resistance to atmospheric corrosion, and high shear and impact strength for safety from wind load, blast and deflection.

The designer also should be aware that the location and size of the cavity is important. The location of the cavity within the framing member, in addition to the debridge width or size of the cavity, affects the thermal performance of the frame.

Guidelines for the cavity
The basic cavity design for pour-and-debridge extrusions has not changed significantly for years. Instead, the cavity sizes and designs have just been refined to give designers guidelines that can be used to select the most effective cavity size and design for his or her application.

The AAMA TIR-08-04, “Structural Performance of Composite Thermal Barrier Framing Systems” of the American Architectural Manufacturers Association of Schaumburg, Ill., provides the designer with guidelines for selection of cavity size and design. As stated in the AAMA publication, “They [the guidelines] are not intended to be prescriptive or all inclusive. Commonly accepted engineering practices should be followed in the design of any component of a framing system.”

In addition, Azon of Kalamazoo, Mich., has created a set of guidelines for cavity size based on wall thickness and profile size, depicted in the tables above, to use as a reference when designing or evaluating pour-and-debridge profiles for use in fenestration products.

By taking precautions and implementing AAMA guidelines and common engineering practices, the thermal-break cavity will successfully isolate the exterior of the aluminum from the interior. Once the extrusion is filled and the metal bridge removed, the composite member reduces thermal conductivity of the frame by 1,000 times, eliminating the formation of moisture and frost on the interior profile, while securing the structural soundness of the fenestration product.