Window frames can be made from very different materials, which is not a small issue.
Hence the question: which are the best? Wood, composite, aluminum, vinyl or fiberglass windows?
The short answer: in very general terms, foam-filled fiberglass framed windows perform better, with foam-filled vinyl coming second, followed by composite and wood-frame windows.
But there are many details that can make the choice difficult.
There are five main types of window frames: aluminum, wood, clad, vinyl and fiberglass.
Fiberglass-frame windows are the best choice for energy efficiency. But they are pricey.
Aluminum-frame windows are inexpensive and durable, but a bad choice from an energy efficiency standpoint.
Aluminum Window Frames
Aluminum is a light, non-corrosive and strong material, extremely durable and low-maintenance.
Unfortunately, aluminum has a big disadvantage: aluminum conducts heat better than any other building material. And that’s very bad for the performance of the window, even if the frame profile is simple and compact.
In very cold weather, aluminum frames can easily become cold and prone to condensation problems. To minimize it, and to reduce thermal bridging, many manufacturers provide a thermal break - by splitting the frame components, and by joining them with a less conductive material. But that do not make them a really good option.
Even in warmer climates, where they do not pose condensation problems, typical aluminum frame windows are a poor choice for energy-efficiency.
But things are not that simple...
Some manufacturers are now offering what they are calling high efficient aluminum windows, able to meet te PassiveHaus standards (a typical German creation; the highest standard for windows). These windows have their frames filled with insulation foam, and special weather barriers and large decompression chambers, which makes them really efficient. Anyway, they aren't typical aluminum windows. And contrary to typical units they are very expensive.
Wood and metal-Clad wood Window-Frames
Wood windows are popular, but require regular maintenance and have well-known disadvantages. They need either be painted or stained to prevent decay and rot. Wood can also swell and twist over time causing problems on the sash and frame.
Anyway, many "wood-frame windows" are not real solid wood.
They are clad windows, that is, with an exterior layer of vinyl or aluminum. The advantage of these modern clad-wood windows is obvious: they aren't susceptible to rotting or warping, and are mostly maintenance-free.
Vinyl Window Frames
High quality vinyl windows are a great choice for energy-efficiency. They are increasingly common and increasingly cost-competitive.
Be aware, anyway. Low-priced vinyl frame windows can be a problem: they are maintenance free and affordable, but we never know how durable they are.
If you are considering buying vinyl frame windows – because of their good energy-efficiency, good water condensation resistance, low price and low maintenance and no painting requirements – pay attention to the quality of the vinyl. It should have good dimensional stability and resistance to sunlight and extreme temperatures.
Also look for foam-filled vinyl framed windows.
Fiberglass Window Frames
New fiberglass-frame windows are strong and great to control heat transfer. Like vinyl frames, fiberglass frame window cavities can be filled with insulation materials, for higher energy performance.
Fiberglass expands and contracts less than vinyl, which minimizes warpage, leakage and allows a better sealing and integrity. And they can be very strong.
Fiberglass windows look more like natural wood than viny windows, and can be painted (unlike vinyl windows).
Bottom line: fiberglass-frame windows are the best choice, from an energy standpoint. Their only disadvantage: they are more expensive than vinyl windows.
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