There's no such thing as waterproof siding. Over time, moisture
will work its way behind even properly installed siding.
Housewrap provides good protection against the moisture that
does get in, but it must be detailed properly at windows and
other penetrations to work.
In this article, I'll explain the housewrap flashing details I
use when installing flanged window units. These details will
prevent any water that gets past the siding from damaging the
windows or rotting the framing.
Think Like a Roofer
For some reason, builders tend to work less hard at flashing
details on a wall than they do flashing a roof. But when you
think about it, a wall is really a roof with a very, very steep
pitch, so a window is basically a skylight installed in a
vertical roof. I flash windows in much the same way that I
flash skylights: I start at the lowest point and work my way
uphill, with each layer of flashing overlapping the previous
layer, creating positive drainage laps and always avoiding
Cutting the Window Opening
I wrap my houses as soon as I can: The chance of rain is always
present, and I want to protect the shell as quickly as
possible. When it's time to install a window, my first step is
to cut the wrapped window opening. I start by making a level
head cut (1), then two 45-degree cuts upward from the lower
corners (2). I finish with a vertical cut (3).
X-cutting the opening should be avoided: The head flap of an
X-cut will direct any moisture that manages to get behind the
housewrap into the structure.
Next, I trim the flaps of housewrap just a bit shy of the
interior face of the studs and tape them to the framing (4).
This holds the flaps tight to the studs and makes the gap at
the jambs much easier to foam. I also fasten a piece of beveled
siding to the rough sill, with the thick edge toward the
interior (5). This sloped sill will direct any moisture toward
the exterior of the wall.
Finally, I create a head flap by making two 45-degree cuts 6
inches long in the housewrap at the window head corners (6),
and temporarily fold and tape this flap up out of the way
Next Step, Sill Flashing
After I've taped the housewrap to the studs, I install the sill
flashing. I prefer to use Tyvek FlexWrap, a butyl-based
self-healing flashing product I call "peel-and-stick on
steroids." It has all the self-healing characteristics of
generic peel-and-stick, but it also can be stretched to form
seamless sill corners.
I cut the sill flashing one foot longer than the width of the
window opening, so the flashing will extend up the jambs 6
inches. Then I peel off the release paper (8), center the
flashing in the opening (9), and lower it onto the sill. I
press it into place, working from the middle toward the corners
(10), where I carefully stretch the flashing out to create
seamless protection (11). The flashing has a memory, so to
prevent it from curling back, I drive a cap nail (12) at the
outer edge to hold it in place until the adhesive achieves its
full grip (24 to 48 hours). Finally, I smooth the vertical
portion of the flashing against the housewrap (13).
Installing the Window
Before installing the window, I apply a heavy bead of
elastomeric latex caulk at the jambs and the head (14), but I
never caulk the sill flange area. Should any moisture find its
way into the rough opening, this caulk-free sill flange,
coupled with the sloped sill, will provide a weep area for
water to escape.
Next, I install the window, driving roofing nails through the
preformed holes in the flanges, spacing them approximately 6
inches apart (15). Check your window manufacturer's
specifications for the correct spacing.
Flashing the Jambs and Head
Starting 2 to 3 inches above the window head, I apply flashing
membrane over the jamb flanges (16), letting the tape extend at
least to the bottom of the sill flashing.