Gary Katz and Bill Robinson
Most of us have worked on old homes where the exterior trim and
siding were still in great shape: no cupping, bowing, twisting,
or rot. As often as not, the wood is old-growth Doug fir, and
none of it — the back of the siding, the back of the
brick mold, the back and bottoms of the exterior jambs —
is back-primed. In many cases, not even the bottoms of the
doors were painted! Try that today.
Lumber these days is hardly even "new growth" — a lot of
it is more like "instant growth." Just look at the growth rings
in a piece of siding or a 1x6 trim board. All this new-growth
wood requires new building practices. We can no longer install
doors without priming and finishing all six sides. In fact,
most door manufacturers will no longer guarantee doors finished
in dark colors or installed without adequate overhang
New Products, Old Solution
The need for new building practices is most critical with
siding and exterior trim — and not just because of
new-growth lumber. There are also new manufactured lumber
products on the market that perform differently than old-growth
wood, and energy practices and housewrap technology have
changed since those older homes were built.
Walls are no longer breathable cavities supported by let-in
braces and wrapped with layers of felt paper. Instead, they've
become sealed envelopes wrapped with plastic. Housewrap
manufacturers are constantly improving their products by
dimpling, creasing, and texturing them to encourage drainage.
But the fractional stand-off space provided by bumps and
wrinkles isn't always enough to offset the quality of marginal
building materials, poor design and detailing, or the pressure
differentials that drive moisture vapor through siding and
housewrap and into wall cavities.
Oddly enough, one of the most effective moisture-control
techniques has been used — at least in part — for
decades. What carpenters once called furring strips are used
today to build "rain screen" walls, which are the best way to
ensure long-lasting trim and siding installations.
But there's more to a rain screen than simply nailing spacers
on top of your housewrap. In this article, we'll review the
advantages of rain-screen walls and describe the details that
make them work.
Reasons for Failure
Let's begin by looking at the reasons modern siding and trim
fail. Growth rings are only part of this puzzle. Read any
recent article by building-science experts like Joe Lstiburek
or Paul Fisette and you'll learn that, except for vinyl siding,
all exterior claddings are "reservoir" products. They absorb
moisture even if they're primed on all sides.
For a variety of reasons, moisture will find its way through
minute cracks and crevices no matter how tightly the siding and
trim are sealed. Poor water-shedding designs, failure to prime
end cuts, installation directly against pressure-treated or
high-moisture-content substrates, and installation too close to
grade are all obvious reasons why moisture penetrates siding
There are also a couple of less visible causes, namely the
air-pressure difference between the outside of the wall and the
back of the siding, and the capillary action of water moving
between materials tightly sandwiched together. Wind and
wind-driven rain cause positive pressure against the exterior
of a building — but there's no pressure on the back of
exterior cladding. This pressure difference creates a
vacuumlike effect, so that moisture in the form of water vapor
is both driven and sucked through capillary action into the
exterior siding and trim. Even if proper water-shedding details
are in place, capillary suction alone can draw water vertically
up behind siding boards, and especially through small cracks at
butt joints and even nail holes.
Once the exterior cladding absorbs enough moisture, the
pressure differential can, in extreme cases, drive water vapor
through the housewrap into the exterior sheathing and wall
cavity. If this vapor ever reaches its dew-point temperature,
it can condense inside the building envelope. Since liquid
water cannot pass through plastic housewrap, it's trapped in
the wall, where it can cause rot and mold.
Felt paper is the only housewrap that will absorb water within
the wall and allow it to dry toward the outside, but the force
of positive pressure will often prevent that. At the very
least, saturated housewrap and moisture-laden walls will
accelerate the deterioration of exterior siding, trim, and
A Ventilated Air Space
Rain-screen walls, sometimes referred to as pressure-equalized
assemblies, solve these problems because they provide a
ventilated air space that defeats the air-pressure difference.
Properly detailed, they also provide a drainage plane — a
way for any water that does get behind the siding to escape.
And compared with the cost of replacing trim and siding that
have failed prematurely, building a rain-screen wall is by no
means prohibitively expensive.
The requirements for an effective rain screen are:
•a minimum 3/8-inch air gap between the back of the
cladding and the drainage plane;
•ventilation to assist in drying and to partially equalize
the pressure on the cladding;
•drainage at the bottom;
•a rigid drainage plane and air barrier.
In essence, a rain-screen wall — a ventilated gap or
cavity providing drainage and a capillary break — works
by promoting air circulation.
Prepping for Trim
As with most construction projects, the first step in building
a rain screen is establishing the correct sequence. On the
recent remodeling project shown here, we began by wrapping the
wall, making sure that all window and door penetrations were
properly flashed and sealed (see "Flashing a Flanged Window,"
6/05). The housewrap or drainage plane must be air- and
watertight to prevent liquid water and air from passing through
from outside to inside.
Rather than furring out the windows, which would create an
offset in the drainage plane, we installed them against the
sheathing and housewrap, then applied furring for trim and
siding on top of the flashing (Figure 1). By leaving the trim
— simple flat stock — slightly proud of the
windows, we avoided creating a potential dam in the drainage
blocks of pressure-treated plywood will provide an air space
behind the wood window trim. To separate wood from wood, the
authors covered the blocks with building paper before nailing
up the trim.
To match the window trim, we chose to install a door with no
brick mold; we added plinth blocks and flat casing afterward.
This allowed us to run the drainage plane right onto the face
of the door frame (Figure 2). Since we were using
1/2-inch-thick furring strips for the siding and furring blocks
for the trim, we had to add 1/2-inch jamb extensions to the
doors before installing the casing. We opted for the individual
furring blocks instead of a continuous furring strip because we
thought they would provide better air circulation behind the
flashing tape seals the gap between the door jamb and the
We covered the pressure-treated furring blocks with flashing or
housewrap to isolate the trim from the blocks (Figure 3). We
also sealed every cut and notch in the trim with two coats of
oil-based acrylic primer.
Figure 3. Covering the
1/2-inch plywood spacer with building paper (top) helps prevent
water from moving between the wood trim and the plywood block.
All cut edges should be primed before installation (bottom
left). Note the 1/2-inch jamb extension (bottom right), which
brings the back of the trim out flush with the top of the
The height of backing blocks above the doors and windows was
determined by the upper trim detail, where head flashings were
later installed (Figure 4).
flashings tuck in beneath the housewrap above doors and windows
(top) and at the water table (middle and bottom), bringing any
water that reaches the drainage plane back out to the
Once the door and window trim was installed, we turned to the
water table, where scant clearance to grade required special
attention. Before wrapping the house, we applied a
self-adhesive membrane at the bottom of the wall to protect the
wood sheathing, then used PVC trim — a rotproof,
nonreservoir material — for the water table.
After attaching the water table, we carefully sliced through
the housewrap and inserted a custom-bent PVC flashing.
We decided to use Cor-A-Vent (800/837-8368,
www.cor-a-vent.com) at the top and bottom
of the wall. First we attached a layer of ordinary window
screening, and then we installed Cor-A-Vent SV-3, wrapping the
window screen over the SV-3 to prevent bugs from infiltrating
the rain-screen cavity (Figure 5).
Figure 5. Two
3-inch-wide polypropylene-mesh vent strips — one at the
top, one at the bottom — help provide continuous
ventilation behind the siding (top and center). For good
measure against bugs, the strips were wrapped in insect
screening, installed first (bottom).
We repeated the same steps at the top of the wall, but because
this overhang had no soffit, we installed backing for the eaves
trim beforehand. This allowed us to hide the ventilation strip
behind the trim at the top of the wall (see
Finally, we installed corner boards and intervening furring
strips (see photo on the top of this article), and the wall was
ready for siding.
Material and labor costs for the rain-screen wall were not
excessive. On an average home, a rain screen might add $2,000
to the cost, but the savings in maintenance and the increased
longevity of the exterior paint more than compensate for the
additional investment.a finish carpenter in Reseda, Calif., and
moderator of the jlconline.com finish-carpentry forum, and Bill
Robinson, a general contractor in Arroyo Grande, Calif., are
regular presenters at JLC Live.