High-Efficiency Commercial Window Inserts

Instead of new construction, many firms are retrofitting older buildings in an attempt to reduce a project’s embodied carbon. The problem is that the operational energy required to run these often poorly insulated buildings can offset many of those gains. And windows are one of the biggest challenges, since they account for “about 10% of the energy use in buildings and influence end uses that comprise 40% of building energy use,” according to the U.S. Department of Energy.

Alpen High Performance Products is offering a solution to this problem in the form of its WinSert interior window inserts (otherwise known as a secondary glazing systems, or SGS). WinSerts have been in development for a number of years as prototypes, but Alpen is now marketing them for wider distribution. Available for commercial, multifamily, or residential applications, these inserts use the company’s thin-glass window technology (as found in the company’s triple and quad pane windows, a 2020 BuildingGreen Top 10 Product winner) and fiberglass frames to transform aging windows into high-performance units, reducing solar heat gain and air infiltration while improving whole-window thermal value.

Why use thin-glass inserts?

Single and dual-pane products with poorly insulated wood and aluminum frames are still common in old buildings, leading to excessive air and sound infiltration, condensation, glare, and poor thermal performance. With all these issues, you’d think improving the performance of these openings would be a top priority, but this is no easy task.

The cost of replacing old commercial windows is significant, and, in the case of historic buildings, matching the aesthetics of the exterior with contemporary materials can be prohibitively expensive. And though window inserts are available for larger commercial projects, they can require complex installation, and they use standard glazing that can make them too heavy and thick to be used in some structures.

Alpen addresses these problems with two versions of WinSert: the single-pane WinSert Lite and the dual-pane WinSert Plus. Both use the company’s thin-glass technology and are available with different coating options, depending on performance needs. The Plus version comes with krypton gas fill. Both use the same narrow-profile insulated fiberglass frames, which include exterior-quality gasketing to create a thermal break, a significant weak point of the aluminum frames common in most commercial buildings. A novel neodymium magnetic attachment system is used to speed installation and avoid any penetrations into the structure.

According to the company, adding WinSerts to an aluminum-framed, single-glazed system with a whole-window U-factor of 1.12 (R-1.12) and a solar-heat-gain coefficient (SHGC) of 0.73 improves the U-factor to 0.53 (R-1.9) using WinSert Lite and to 0.19 (R-5.3) using WinSert Plus while improving the SHGC to 0.53 and 0.38, respectively (for more on window performance basics, see our Glazing Performance Metrics primer). Though these U-factors are not as impressive as the 0.10 (R-10) version of the company’s Zenith series and other Passive House products, they turn a poorly performing, single-pane window into a high-performance option.

Less weight, less carbon

The thin glass used in WinSerts is similar to the glass used in smartphones. At only 1 mm thick and weighing only 0.53 pounds per square foot compared with standard commercial glass that is 5.7 mm (1/4 inch) thick and 3 lb/ft², a full WinSert averages less than 1 lb/ft² and is only 1-½ inch thick.

This is part of why using WinSert Lite—despite its lower performance—can be the better option for some applications. According to Brad Begin, Alpen’s president, “In almost every case so far, virtually everyone is choosing the WinSert Plus,” but “in some situations simply adding a single pane can offer a great benefit.” He points to historic buildings and those in mild climates where, he says, “Solar-control management and air infiltration can be solved at a lower price point.” But there are other factors to consider, such as noise, condensation problems, and structural loads. The light weight of the Lite, for example, can be a viable choice where other solutions are prohibitively heavy, like in old high rises.

The 27 ft² WinSert Lite units installed as prototypes in a historic New York City high-rise improve thermal performance and create a quieter interior, according to the company, but weigh only 32 pounds each, compared to standard glass inserts that would weigh more than 90 pounds.

With less glass per square foot, there is also the potential for lower embodied carbon when compared with standard-glazed products. Glass production is an energy-intensive process, and Alpen points out that one ton of float glass (5/32") covers just under 1,000 ft², whereas one ton of thin glass covers over 5,500 ft². All things being equal, this would add up to significantly less embodied carbon for thin glass. But it’s important to note that standard glazing is manufactured using a “float” process, and thin glass uses a “fusion” process, so until we have more life-cycle data on thin glass, it’s difficult to make an apples-to-apples carbon comparison.

Installation and cost

Getting WinSerts installed on a project is a multi-step process. “You need a certain amount of space around the perimeter of the window, 1.5 inches or so,” according to Craig Maierhofer, vice president of business development at Alpen. If there is room, the company looks at the current window materials and condition to assess thermal stress risks and air infiltration (to reduce condensation risk). They then create a prototype so clients can see how it will look and perform.

Installing WinSerts is done using a clip system that includes neodymium magnets and a heat-resistant tape. This means no penetrations into the building, which speeds installation and lowers overall costs, but it also makes it easier to remove and clean the inserts. Meierhofer said that for one installation, the building has large, historically significant metal frame windows with a center pivot for cleaning. “It was critical to them to preserve that,” he said, making WinSerts a good fit for the project.

The cost of WinSerts differs based on job and scale, but on the majority of commercial projects, they will be significantly less expensive than full replacements. And there are often savings due to easier installation, easier maintenance, and other factors, making them a viable option for those looking to upgrade aging building stock on a budget.

The 200,000-square-foot Veterans Administration office in Houston used WinSert to improve the thermal performance of its aging 9' x 12' single-pane windows. Bill Abbott, part of the building’s facilities team, said they were told the windows would have to be replaced since gaskets for the aluminum frames were no longer made. After the bidding process for replacements started, Alpen solved the problem using its WinSert Plus combined with sealing the current window frames, which was necessary to address potential condensation concerns. The team also installed an interior shading system.

“We wound up spending $2 million total, and $½ million of that was for the window shade,” Abbott said, claiming it would have been even less expensive if Alpen had been brought in from the start. Other bids came in above $2 million for double-pane replacements, he continued, noting that installation of full replacements would have taken much longer and been logistically difficult since it would have meant creating openings to the elements, a challenge in any climate but especially Houston where temperatures and humidity can be extreme.

When finished, the VA facility essentially had triple-pane performance at less cost and fewer delays than that of a double-pane replacement. Abbot said utility bills dropped $3,000 per month immediately after installation, but since the WinSerts were installed during COVID when the office was closed and utility use was less, he expects monthly savings to be significantly more once the building is occupied again.