(C) J Wiley, S Bliss Guide to Window Materials & Construction Methods

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What are the different materials used to construct windows & what are their different performance properties?

Here we explain the properties of current window types and what they are made-of, including wood windows, clad wood windows, vinyl cladding on wood windows, fiberglass cladding on wood windows, solid vinyl windows, fiberglass windows, and aluminum windows.

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Guide to Window Materials and Construction

Antique wood framed window, Rugat, Spain (C) Daniel FriedmanIn this article series we discuss the selection and installation of windows and doors, following best construction and design practices for building lighting and ventilation, with attention to the impact on building heating and cooling costs, indoor air quality, and comfort of occupants.

We review the proper installation details for windows and doors, and we compare the durability of different window and door materials and types. This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Article Contents

As described in Best Practices Guide to Residential Construction Chapter 3, BEST PRACTICES GUIDE: WINDOWS & DOORS:

For centuries, even before glass was used for glazing, windows were framed with wood set into building walls. Our photo (left) shows an antique wood-framed window in Rugat, Spain (DF).

Earliest wood framed windows were left open (such as this example from Xotolar, Mexico). Later in areas of more hostile climate windows were glazed with animal skins, then parchment, and finally, glass.

For many years, the material choices for "modern" or new residential windows were limited to wood, clad wood, and aluminum. Wood and clad wood remain the leading materials, accounting for almost 50% of the new and replacement window market. Wood use has been declining, however, with the rapid growth of solid vinyl windows.

Solid vinyl windows made inroads into the replacement window market in the mid-1980s; but they were not widely accepted in new homes until the 1990s, when their use skyrocketed. Solid vinyl windows now account for an estimated 30% of the new-home market and 60% of the replacement market.

Aluminum windows account for about 15% of window sales, with the remaining share of the market spread among fiberglass windows and a variety of hybrids and composites that have entered the fray, making window selection today anything but simple.

Wood and Clad-Wood Windows

Antique wood casement window, Barcelona, Spain (C) Daniel FriedmanWood is the traditional material of choice for residential windows. It can be milled into highly detailed designs, such as true divided lites, and easily fashioned into just about any custom configuration.

Photo at left, masonry window details, Barcelona, Spain demonstrate stone masonry window trim details.

In addition to its aesthetic appeal, wood has excellent insulation value, and if well maintained can last indefinitely. Our photo (left) of a wood-built casement window set into this stone wall in Barcelona, Spain, shows a window that is more than 100 years old.

On the downside, wood must be stained or painted and well-maintained or, over time, it will be vulnerable to peeling paint and decay. Windows with wood exposed directly to the weather, such as open casements, are particularly vulnerable.

Although wood is dimensionally stable with temperature changes, it does swell and shrink with changes in relative humidity, causing tight-fitting windows to stick in humid summer weather.

When selecting a wood window, look for materials that are factory-treated with a water-repellant preservative, which will help prevent decay and also improve paint retention and dimensional stability.

The vast majority of wood windows made today have a vinyl- or aluminumclad exterior. A clad exterior is the most practical choice, providing a maintenance-free exterior with the look of a wood window on the interior. The only downside is a limited color choice.

Construction Details for Wood Windows

Rotted window sash (C) Daniel FriedmanSome very high-end wood windows are manufactured from rot-resistant species, such as mahogany, but most use clear pine for the sash and visible portions of the frame.

The hidden portions of most wood windows use some combination of solid wood, finger jointed lumber, and various types of engineered lumber, including laminated-strand lumber (similar to oriented strand board, or OSB).

Watch out: Leaks into wood window or door components can cause severe rot even on newer products, as our photo (left) illustrates.

And where finger-jointed wood is used for window or other wood parts exposed to the weather, if the wood is not kept painted and sealed, we find that finger joints may swell, separate, and invite rot or insect damage - DF.

Composite windows are beginning to be used as well, such as Fibrex, a proprietary wood-vinyl composite used in the subsills of some of Andersen’s replacement windows and patio doors.

In general, wood windows are sufficiently strong and rigid for most residential applications. However, it is always best to use windows approved by one of the three agencies that certify windows and doors (see Window Certification).

In coastal areas or other areas prone to high winds or hurricanes, look for products with a suitable pressure rating from the American Architectural Manufacturers Association (AAMA) or the Window and Door Manufacturer’s Association (WDMA).

Vinyl Cladding on Wood Windows

While vinyl cladding is only offered by a few manufacturers, one of them is Andersen Windows, by far the largest supplier of residential windows in the United States. Andersen’s clad windows’ strong record of durability, reliability, and moderate cost has helped make vinyl-clad windows one the most popular options today.

Good quality vinyl-clad windows have a heavy-gauge covering, and heat-welded corners to provide a durable seal against water entry. Vinyl cladding is also more energy efficient than aluminum and is preferred by some in coastal environments subject to salt spray.

Watch out: on some older versions of vinyl clad windows that leave wood exposed (typically at the ends of sills projecting outdoors) or where vinyl cladding over wood has been damaged, broken, or cracked, we often find rot as well.

(C) J Wiley, S Bliss

On the downside, vinyl comes in only a few colors, typically white and beige, and cannot be painted.

Some manufacturers, such as Andersen, now offer a limited number of dark tones as well, using newer technologies that resist the fading and heat problems characteristic of dark colored vinyl.

Vinyl is also vulnerable to cracking in cold weather if struck by an errant baseball or hammer.

While all manufacturers cover the exterior frame and exterior face of the sash, some also protect the top of the sash, which is important in casement windows.

Andersen wraps the entire sash inside and out, providing excellent protection (see Figure 3-3 at left).

But for those seeking the look of wood on the interior of the sash, consider windows with exterior-only cladding, such as those from Weathershield and MW Manufacturing.

See vinyl window details at VINYL WINDOWS.

Aluminum Cladding on Wood Windows

A number of manufacturers offer aluminum cladding, using either relatively thin roll formed aluminum or heavier-gauge extruded aluminum. The advantage of extruded aluminum is that it adds strength and rigidity to the window and resists denting better than thinner stock.

Also, aluminum can be formed to crisper profiles than vinyl, creating a less bulky appearance. Other advantages of aluminum include a wider choice of colors and the ability to be painted if desired. One downside to aluminum cladding is a slight reduction in energy-efficiency compared to vinyl-clad windows. On average, a vinyl-clad unit has an R-value about 10% higher than for a comparable aluminum-clad window.

See aluminum window details at ALUMINUM WINDOWS.

Properties of Fiberglass Clad Windows

A few manufacturers, including Marvin and Milgard, offer wood windows clad on the exterior with a tough fiberglass composite manufactured in a process called pultrusion (see FIBERGLASS WINDOWS).

Pultruded fiberglass is an ideal cladding material due to its durability, energy efficiency, and very low rate of thermal expansion. Unaffected by heat, cold, and moisture, manufacturers claim that pultruded fiberglass will never crack, peel, or warp. Fiberglass-clad windows come prepainted with a factory finish and can be repainted on site if desired.

Where to buy window and door products

As noted in Best Practices Guide to Residential Construction Chapter 3, BEST PRACTICES GUIDE: WINDOWS & DOORS:


-- Adapted and paraphrased, edited, and supplemented, with permission from Best Practices Guide to Residential Construction.


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