Windows: construction review
Part of an ongoing series investigating the uses of plastic in construction materials
Since the advent of mass-produced sheet glass in the early 20th century, windows have become bigger, cheaper, and more common. Advances in insulation methods, too, have allowed for their use in greater quantities in cold climates. Windows are usually made of a frame, panes, glazing/coating on the panes, spacers between panes, seals/weatherstripping, adhesives, and hardware. Plastic is a common material choice for all of these components.
Frames
Most window frames are made of either fiberglass (which is plastic, for those deceived by the name) or PVC. This study from 2022 reported that 10% of windows in Japan use PVC frames, while 50% of windows in North America and the EU are PVC. This preference in the Western markets presumably relates to the superior insulation ability of plastics as opposed to aluminum—20% reduction in total building energy cost.1 The common natural options are wood and aluminum, though be warned that aluminum frames are often powder-coated (which is another way to say coated in an epoxy layer, which is plastic) or painted with acrylic-based paints. Wood frames should be fine just like all other wood products, as long as the use of PVA glues, polyurethane finishes, and other plastic treatments are avoided.
Panes
Different glass options provide different thermal properties, single-pane being the cheapest and simplest, up to triple-pane being the best-insulated and most complex. In multi-pane setups, the void between two panes is often filled with an inert gas like argon or krypton. Certain types of glass, such as “Low-E”, are coated with metallic powders (like titanium dioxide, silicon nitrides, or zinc oxide) to reduce heat transfer & UV penetration, and confer other desirable properties. This process is completely plastic-free. Besides cheap windows in which the pane is made from clear plastic like acrylic or plexiglas (which aren’t very common here in the US), the other place to encounter plastic is in laminated glass, where basically two panes of glass are bonded together using an adhesive resin.
Glazing
The layers & coatings applied to windows for color tints, strength & stability, and frosting/texture are frequently made of plastic. Tints are most commonly a plastic film, laminated with dyes, applied to one surface of the window. Tinted/colored glass is definitely possible through non-plastic means, but this is rare in common, commercially-available windows. Frosted glass is sometimes applied as a film as well, but glass can be frosted mechanically (using abrasives), and such frosted glass needn’t contain plastic. If the method for achieving a certain color/finish is not listed, one will need to check with the manufacturer.
Spacers
Spacers between panes in a double-pane or triple-pane window are often made from a combination of cheap metal (like aluminum) with plastic corner pieces and a plastic adhesive/sealant (most often silicone). This video at the given timestamp shows in more detail.
Seals & Weatherstripping
Seals for windows can be internal (to contain the gas in double- and triple-pane windows) and external, to waterproof the joint between the window frame and the panes. Generally window seals are provided by silicone caulk. However, weatherstripping between the frame and the housing it’s installed in can be made from EPDM rubber, thermoplastic elastomer/TPE, and thermoplastic olefin/TPO. In all cases, modern window seals seem to be made from plastics of one sort or another. Also, basic weatherstripping can be made from plastic-derived fabric materials, as seen here.
Adhesives
Adhesives aren’t necessary in plastic-frame windows, as thermoplastics can be heat-welded together. That said, in wood-frame windows, wood glue is used, which uses PVA (polyvinyl acrylate) as a main ingredient. In theory, a wooden window frame could be glued up using hide glue, but I would be shocked to see this already available on the market. Aluminum can also be welded together, but is most commonly glued. Again, I’d be really surprised to see any non-polymer adhesive used for this purpose. As always, this is a good area for engineers and entrepreneurs to explore!
Hardware
Window hardware like locks & other internal catches/mechanisms seem most commonly to be made of cheap metal like brass, zinc, steel, and aluminum. That said, plastic mechanisms & hardware exist too, especially in locks and pulls.
Alternatives
The graphic below shows in great detail one way that windows were constructed before the advent of plastics.
With modern metal construction methods, I’m sure there are alternative ways to construct windows that don’t involve so much PVC, silicone, and polymer adhesive. To achieve modern thermal properties, I almost feel comfortable giving the “okay” to silicone used in window seals—the tradeoff between silicone quantity and energy savings is pretty favorable, especially when we take into account that silicone isn’t a big source of microplastic fallout in indoor air, and that windows are unlikely to work their way into the environment as litter because they’re too big and valuable (and attached to buildings, of course). Would love others to weigh in here, there might be considerations I’m missing.
This is a good example of the tradeoffs involved in zero-plastic construction. As a material, it often strikes an optimal balance between cost, performance, and convenience (for manufacture/install).