Allowing for thermal expansion is the biggest concern when
installing vinyl windows
In the past five years, the number of residential vinyl
windows sold in the U.S. has doubled. Once confined to the
replacement window market, today about a third of all windows
used in new homes are vinyl. In fact, most observers predict
that vinyl windows will overtake wood windows in new
residential construction sometime in the next few years.
To explain this steady growth, vinyl window makers point to
advantages vinyl windows have over their aluminum and clad wood
counterparts. None of the three window types require painting,
but most vinyl windows have better thermal performance than
aluminum windows and don’t swell with changes in humidity
like wood windows.
The biggest advantage, however, is lower price, especially
when compared with wood windows. Some vinyl window critics
charge, however, that the strong price competition between
vinyl window manufacturers has led to lower quality. Skeptics
point to several shortcomings, including:
- vinyl’s high rate of thermal expansion and
contraction, which is especially troublesome with wide
- the tendency in the past for some vinyl sash and frames
to become brittle after being exposed to strong
- the fact that vinyl windows are available in a limited
number of colors
In this article, we’ll give you the information you
need to make a sound decision when purchasing vinyl windows.
We’ll also look at installation procedures that will
ensure trouble-free performance.
Quality Issues for Vinyl
Vinyl window production follows three
steps: resin production, vinyl extrusion, and window
fabrication. At all three steps of the process, decisions are
made that can affect the quality of the window.
Vinyl extruders and window
manufacturers have responded to quality concerns by
participating in the certification program
developed by the American Architectural
Manufacturers Association (AAMA). In 1985, the AAMA
standard for steel and aluminum windows was
expanded to include vinyl windows. The most recent
version of the standard, AAMA/NWWDA 101/I.S.2-97,
was approved in 1997, and has separate requirements
for aluminum, wood, and vinyl windows.
The section for vinyl windows,
Section 303, stipulates that vinyl profiles must
meet certain requirements for impact resistance,
dimensional stability, heat resistance, weight
tolerance, color fastness, and weathering. The
weathering tests are performed at three U.S.
locations chosen to represent extremes of climate
(Florida and Arizona) and air pollution levels
(Kentucky). For the weathering tests, vinyl
profiles (not whole windows) are exposed at a
45-degree angle, facing south, for up to two
When purchasing vinyl windows, it is
worth looking for the AAMA certification label,
since certified windows are more likely to be of
consistent quality than uncertified windows. Most,
but not all, vinyl extruders and window
manufacturers participate in the AAMA certification
program. Of the top 15 U.S. manufacturers of vinyl
windows, 14 are AAMA-certified. The exception is
Croft Metals in Mississippi.
The new International Residential
Code, which may eventually be adopted by various
states and local jurisdictions, requires that all
windows comply with the AAMA standard.
Chemistry vs. cost. Resin producers make
polyvinyl chloride (PVC) in the form of pellets or powder. The
vinyl formulations used for window parts usually include heat
stabilizers, as well as pigments like titanium dioxide, which
protects vinyl from ultraviolet light. Manufacturers vary the
percentage of these expensive ingredients in an effort to
strike a balance between durability and price.
When asked whether a builder should worry about the quality
of the vinyl used to manufacture windows, Kevin Jones, vice
president of Dallas Laboratory, which reviews test reports as
part of the AAMA window certification program, answered, "Some
compounds are better than others. Some plants might produce
inconsistent vinyl. In the weathering tests, we have seen some
white vinyl that has gone brown, and some light browns that
have gone chalky." Jones explained that such color changes can
indicate embrittlement, which weakens the structure of the
vinyl. "Embrittlement is more of an issue in hot climates,"
says Jones. "It is caused by UV degradation. The sunlight
actually breaks down the polymers."
According to Jones, manufacturers understand the chemistry
behind making weather-resistant vinyl. "The problem is really
based on cost," says Jones. "If you use the best of the
materials, it will be high-priced, and it will be hard to
compete selling windows."
Since the quality of vinyl cannot be determined by
appearance, the window buyer’s main assurance of quality
is the reputation of the window manufacturer. One way to ensure
that a window meets industry standards is to look for
AAMA-certified windows (see "AAMA Certification").
Structural characteristics. In the second step
of vinyl window production, manufacturers called extruders use
the vinyl resin to produce "profiles," or "lineals" –
extrusions made up of webs and voids for structural rigidity
and stability (see Figure 1, below). To improve the rigidity of
extrusions used in wider windows (or in windows sold in
high-wind regions), some manufacturers insert steel or aluminum
reinforcement into the hollow vinyl profiles. In general, this
results in a window that is stronger but somewhat less
Figure 1. The webs and voids in vinyl
extrusions are designed to give stiffness to the window
using a minimum of material. Some manufacturers fill
the hollows and voids with foam insulation, although
the effect on window U-value is slight.
Some manufacturers will fill the voids in vinyl profiles
with foam insulation. Although this improves thermal
performance, the benefits from the foam may be so slight that
the added cost is not justified. Metal-reinforced profiles,
which are already more expensive, are generally not
Sturdier windows can be built, but in the U.S., price
competition is at the heart of the issue. "European windows are
built stronger than American windows," says Bill Gorman, an
engineer at Milgard Windows. "The problem is cost. Americans
won’t pay the price necessary to build a European-quality
window. In Europe, you see 1/8-inch wall thickness [for vinyl
extrusions], which is .125 inch. Here in the U.S., the wall
thickness varies from .060 inch to .090