Vinyl frames are extruded polyvinyl chloride (PVC) with colors embedded in the frames at the time of manufacturing. The properties of PVC are very successful at arresting heat transfer, making this an ideal choice for projects with energy efficiency goals. PVC is also resistant to moisture, rot, and insects. Yet detracting from vinyl is the fact that the material is not as durable as other types of material as it can warp or degrade over time, especially when exposed to extreme temperatures. While very cost-effective for smaller fenestration needs, vinyl can limit the design options for multifamily buildings as wider frames are required, compared to aluminum windows.

For a traditional design choice, wood frames offer a classic look and strong insulation properties for multifamily projects. Aesthetically wood frames clad in aluminum or vinyl on the exterior can offer exposed wood on the inside to enhance the look and feel of the interior of the home. Wood can be painted or stained and performs well for both energy efficiency and soundproofing. But wood frames do come with several drawbacks. Compared to aluminum and vinyl, upfront costs of wood are usually higher, and wood is susceptible to damage from moisture and insects, meaning regular maintenance is required. In addition, wood frames are thick, reducing the amount of light let into the home and diminishing the view.

Photo courtesy of Aluminum Extruders Council

The pour and debridge process creates a channel filled with polyurethane to halt the flow of heat energy from the glass to the aluminum frame.

As California Goes, So Goes the Nation

California has long been a bellwether state for providing a glimpse of how the nation will move forward from a legislative standpoint, especially related to environmental concerns such as pollution, air and water quality, and energy efficiency. For multifamily building, this may mean a shift in priorities that require a more sustainable design approach.

Achieving green building certifications or satisfying model energy code goals for sustainability is an attractive option for many multifamily projects whether to genuinely protect the environment or as a marketing strategy. However, in California, sustainable design for low-rise multifamily projects is not an option. Title 24 is part of the California Code of Regulations², specifically focused on energy standards that govern the design and construction of buildings to improve energy efficiency and reduce environmental impact. Implemented by the California Energy Commission (CEC), Title 24 is updated every three years with the most recent version released in 2022.

Title 24 sets higher standards for energy efficiency than the International Energy Conservation Code (IECC), which serves as a model energy code for many states. One of the most distinctive features of Title 24 is its Net Zero Energy (NZE) initiative. Under recent updates, all new residential buildings, including low-rise multifamily projects, are required to achieve NZE or near-NZE performance. This means that new homes must produce as much energy as they consume, typically by integrating renewable energy systems, such as solar photovoltaics (PV). While this may not be practical for all projects nationwide, the emphasis on energy use reduction and lowering carbon emissions both embodied and occupied are becoming more relevant.

For the building envelope, particularly in terms of insulation and window performance Title 24 limits the total amount of window or glass area relative to the building’s exterior wall area. For multifamily buildings, the general rule is that the fenestration area should not exceed 40% of the gross exterior wall area. This is to ensure that while windows are used for daylighting, they do not contribute excessively to energy inefficiency.

In terms of design, these requirements allow for some flexibility for the architect. The glass-to-wall ratio is not just about the percentage of glass but also about the performance of the glass itself by designating specific U-Factor and Solar Heat Gain Coefficient (SHGC) ratings based on climate zone. If a project has a higher glass-to-wall ratio, it can still meet Title 24 requirements when high-performance glazing is used to reduce energy loss. Likewise, if the frames and wall systems (basically all components of the building envelope) work in harmony, the building can achieve these aggressive performance goals.

This is where aluminum-framed windows can help achieve project goals by allowing designers to incorporate more glass into projects. This in turn allows more light through doors and windows while also delivering on performance expectations.

Un-Bridging the Window

Aluminum by nature is a naturally conductive material. Heat energy will readily pass through aluminum. To compensate for this, manufacturers of extruded aluminum products have developed different technologies to arrest the heat flow and basically “un-bridge” the thermal bridging.

The first approach to breaking the thermal bridge in aluminum window and door frames is called pour and debridge. The process begins with the extrusion of aluminum profiles for the window frame. A channel is then created along the thermal bridge – the area of the frame where heat could transfer between the inside and outside. In the pour step, a high-performance insulating material, typically polyurethane, is poured into the channel and allowed to cure and harden. Next, the metal along the bottom of the channel, under the polyurethane, is cut away to fully separate the inner and outer aluminum sections, creating a thermal break.

The pour and debridge process follows guidelines published by the American Architectural Manufacturers Association in the AAMA QAG-1-09 specification, or Quality Assurance Processing Guide for Pour and Debridged Polyurethane Thermal Barriers. The guide outlines best practices for processing, handling, and testing polyurethane thermal barriers used to improve the energy efficiency of aluminum frames by reducing thermal conductivity.

Pour and debridge fenestration systems can offer impressive performance, protecting against temperatures as far as -100°C (-148°F). Neither extreme cold nor hot temperatures will transfer through an aluminum frame with a pour and debridge thermal barrier, and the capacity of the frame to structurally support large pieces of glass is not diminished.

The second innovative technology for breaking thermal bridges in aluminum frames is using a polyamide thermal strut. Polyamide is a type of synthetic polymer commonly known as nylon. It is widely used in industrial applications due to its strength, durability, and thermal resistance. For thermal struts, a specific type of polyamide known as PA66 (Nylon 66) is often used, sometimes reinforced with glass fibers to further improve its structural integrity. Polyamide, like polyurethane, is non-conductive, meaning it acts as an insulator, which is key in preventing the transfer of heat or cold between the inside and outside of the aluminum frame.

For this process, two separate aluminum profiles, one for the interior and one for the exterior – are joined together by polyamide thermal struts. The struts are manufactured to exacting specifications separately, and then inserted between the aluminum profiles into grooves created during extrusion. As with the pour and debridged technology, a guide has been produced by AAMA on the specifications named AAMA QAG-2-12 Voluntary Quality Assurance Processing Guide for Polyamide Thermal Barriers.

Besides providing for an effective thermal break in the window, the polyamide strut technology allows for different color profiles or tones on the frame – one choice of color on the inside and another on the outside, for example.

Photo courtesy of All Weather Architectural Aluminum

Polyamide thermal struts are another technology used in fenestration frames to create a thermal break.

IGUs

Choosing the appropriate framing material for fenestration and doors can go a long way in improving the performance of a building, but innovations in glazing technology and design are also contributing to performance goals. As design trends in multifamily projects move toward increasing window sizes to improve views and invite more daylight into the living space, the performance of the window unit becomes more critical. Of note, the specification of Insulating Glass Units (IGUs) has changed the performance landscape – and expectations – for low-rise multifamily projects, both improving thermal performance and reducing noise within the home.

IGUs consist of two or more panes of glass separated by a gas-filled space. The gas, usually argon, is a better insulator than air and reduces heat transfer between panes. Spacers between the panes help center and support the glass. Historically spacers were made from aluminum, however, the glass to aluminum connection point served as a thermal bridge. Today, a new spacer technology called a “warm-edge spacer” has been developed to help arrest this heat flow. Made from less conductive materials like stainless steel, silicone, and neoprene or foam, warm-edge spacers improve the energy efficiency of the window while also helping secure the panes in place. The glass panes are carefully sealed around the edges to prevent moisture intrusion and gas leakage.

The insulation and air sealing allow the interior surface of the glass to remain warmer, relatively the same as the indoor ambient temperature. By keeping the interior glass surface warmer, IGUs reduce the likelihood of condensation forming on windows, which can lead to mold growth or damage to interior finishes over time.