These kinds of inconsistencies
carry over into the installation instructions provided by
various manufacturers. Here, for example, are some of the most
striking contradictions we encountered when asking about basic
Should the sill be shimmed? Several
manufacturers’ instructions recommend shimming between
the rough sill and the window frame in two or three locations,
just as with wood windows. Others recommend that their windows
be installed on a continuous, level rough sill. "There is no
guarantee that [a vinyl window] won’t sag in the center
if you shim it in two spots," says Doug McDougall, a window
certification manager at CertainTeed. "You’re better off
providing full support under the sill."
One manufacturer, Simonton, introduces a third option. It
instructs the installer to use shims as with a wood window,
then to remove the shims after nailing fins are fastened
– leaving the window hanging by the nailing fins.
Should the fasteners be driven home or left
proud? The instructions from Willmar Windows recommend
leaving the fasteners proud: "Do not over-tighten screws or set
nails too tight. This will restrict the window from expanding
and contracting." Installers of Jeld-Wen Windows, on the other
hand, are instructed to use roofing nails driven all the way
in. When we posed this question to other manufacturers’
technical experts, four recommended driving the fasteners home,
while four others recommended leaving the fasteners a little
Should the nailing fin at the head of the window be
left unfastened? Some manufacturers recommend fastening
the nailing fin at the head of the window, while others warn
against the practice (see, Figure 4). Some installers avoid the
problem by driving a nail above the fin, then bending it down
to pinch the fin. This procedure is also part of some
companies’ installation instructions. Superior’s
instructions, for example, say, "Do not nail through the nail
fin at the head – put the nails 1/2 inch above the fin
and bend them over the fin." But the nail head in this case can
tear the building paper and interfere with other flashing and
Figure 4. Comparing installation instructions from
different vinyl window manufacturers reveals a jumble
of contradictions. For example, some recommend
attaching the fin at the top of the window, while
others warn against it. The conflicting advice causes
many installers – including the one who installed
this window – to risk voiding the warranty by
ignoring the manufacturer’s instructions.
Among the variety of reasons given for not fastening the
head fin is one that might be called "the myth of the settling
header." For example, CertainTeed’s McDougall says, "You
don’t want the rough header to settle down and distort
the window." A variation of this myth comes from Scott Clauss,
product development engineer at Insulate Windows, who says, "If
the house settles, it will pull the sill down, but the head
Given a properly sized and installed header, this concern
seems misplaced. While headers may shrink a little, this
movement is far less than that of the vinyl window itself due
to thermal expansion.
Should a vinyl window be fastened at the
corners? This question, too, produces no consensus. In
some instructions, including those from Vinylcrest, the
installer is told to fasten the nailing fins at the corners.
Other manufacturers warn against nailing near the corners.
"When the corners are fastened tight," explains
Jeld-Wen’s Jeff Ward, "that’s where the window can
crack, because it won’t have room to expand."
Installing a Vinyl Window
All of this conflicting advice from window experts is confusing
for installers. It can also be costly, since the warranty will
be void if you fail to follow the manufacturer’s written
instructions. This is a particular problem for installers who
work with several brands of windows, since it is easier for
them to confuse instructions or fail to notice subtle
differences in recommended procedures.
Finally, some of the recommended procedures may run counter
to reliable methods that you have developed over the years. If
the manufacturer’s written instructions seem
ill-conceived, your best recourse may be to buy windows from a
manufacturer whose instructions you can live with.
The installation procedures listed below are consistent with
those of many, but not all, vinyl window manufacturers. These
procedures also incorporate best practices for avoiding leaks
when installing flanged windows of any kind (
Avoid cold-weather installation, when a hammer
can shatter the brittle vinyl nailing fins. If you must install
a window when the temperature is below 20ûF, use
flat-head screws instead of nails.
Size the rough opening according to the
manufacturer’s instructions. Most manufacturers
specify an opening 1/2 inch larger than the window frame. The
window should then be centered, left to right, leaving a
1/4-inch gap on either side. Be careful: If the rough opening
is even a little bit larger, the nailing fins might not catch
Don’t shim the sill – instead, install the
window on a continuous, level base. Some installers
prefer to double up the rough sill, using stacked 2x4s or 2x6s.
This permits easy shimming between the two sills, and also
gives wide blocking for stucco installers to fasten their wire
lath without puncturing the window’s nailing fins.
Install flexible flashing around the perimeter of the
window opening. Flexible flashing, which generally
comes in 9-inch wide rolls, is made from a variety of
materials, including polyethylene and rubberized asphalt (see,
Figure 5). Flashing suppliers are listed at the end of the
Figure 5. Flexible flashing at the side of the
rough opening should extend over the flashing at the
bottom. Here, the side flashing was cut too short, and
the installer incorrectly failed to lap the filler
piece over the bottom flashing. When housewrap or felt
paper is later installed, it should be tucked under the
horizontal flashing at the bottom of the window.
Lap the flashing in the proper sequence to
ensure that the flashing conveys moisture away from the
Fasten the nailing fins according to the
manufacturer’s instructions. Fasteners should be
installed tight, unless the manufacturer directs you to leave
them proud (see, Figure 6).
Don’t puncture the nailing fin with extra holes
when installing housewrap, exterior trim, wire lath, or siding.
In all cases, fasteners should be held back from the nailing
Leave a 1/4-inch gap between the siding and the window
to allow for expansion. Otherwise, wood or fiber-cement
siding may restrict window expansion, causing the window to
deform; in the case of stucco, window expansion may cause
You may need to adjust the gap depending on the temperature
at the time the siding is installed. In cold weather, for
example, the window will be at its smallest dimension, so the
gap between the window and the siding may need to be larger.
The reverse is true during very hot weather.
Don’t use spray foam to fill the gap
between a vinyl window and the rough opening. Vinyl windows
frames, being less rigid than wood or aluminum frames, are
particularly susceptible to being deformed by expanding
Repair and Maintenance
Vinyl is a relatively soft material that scratches easily.
Fortunately, shallow scratches can be rubbed out with light
steel wool, fine emery cloth, or Soft Scrub cleaner (an
abrasive cleaner available in grocery stores). Scratches can
also be removed with acetone, a solvent – but be careful,
because too much acetone can dissolve the vinyl. For deeper
gouges, some manufacturers recommend a liquid vinyl product
called Stelmax Gap Filler (available for about $22 from G-U
Hardware; 800/927-1097). Before repairing a window, contact the
manufacturer so as not to void the warranty.
How long will vinyl windows last? Eventually,
any window will need to be replaced, and vinyl windows are no
exception. "Some vinyl window manufacturers are now replacing
the first vinyl windows from 20 years ago," says Jon Hills,
sales manager for Dayton Technologies, a vinyl extruder. "But
the formulation of vinyl has evolved since then. Will
today’s vinyl windows last longer than 20 years? To know
that, we’ll have to wait and see."
is an assistant
editor at the Journal of Light Construction