I'm a general contractor in Williston, Vt.; my company does a
mix of new construction and remodeling. Last fall I received a
call from a prospective customer looking for a contractor to
replace the siding on his house. He seemed concerned that the
work be done as soon as possible, despite the fact that winter
was fast approaching.
The reason for his anxiety became clear when I visited the
site. The exterior of the house — which was located in a
subdivision where prices start at around $600,000 — was a
disaster. Behind the clapboards, the housewrap was stained and
degraded, and the surface of the OSB below was black with mold
and thoroughly rotted in places.
These problems, the owner told me, had come to light a few
weeks before when, intending to make a small repair, he removed
a section of rotted corner board. Noticing some damage to the
OSB skin of the home's SIPs, he started poking around with a
screwdriver. When the screwdriver reached all the way into the
home's interior, he pulled off more trim and an area of
clapboards. He was horrified at what he found, and hired a
consulting engineering company to come take a look. The
engineer determined that the home's entire south-facing
faade was severely water-damaged and in need of
Although he hadn't inspected the rest of the house, the owner
decided to replace all the siding. Given that the home was only
10 years old, it was a safe bet that the same shoddy
workmanship that led to failure on its south face would be
Proceeding With Caution
Fixing the house would be a huge job, and it would be
complicated by Vermont's looming winter and the site's
1,800-foot elevation, which was exposed to strong prevailing
winds. But I was most concerned about the mold. I'd read about
the seven-figure judgments brought against contractors in mold
lawsuits, and — like any other builder — I was
worried about the potential health risk to my crew.
With some prospective clients, I might have just steered clear
of the job. But in this case, I decided that the owner was
simply interested in repairing his home, and he seemed to
understand the precautions I would need to take and the costs
We agreed that I would hire an architect to design a repair
detail and an environmental company to clean up the mold. The
contract also stipulated that while I would do my best to keep
the estimate within the budget, I couldn't make any guarantees,
because most of the home was still covered with siding and I'd
seen only part of the damage (see Figure 1).
Figure 1. Judging by the exterior, one
would never imagine what was going on behind the siding of this
10-year-old house. The north elevation (before the siding was
removed) is shown here.
What Went Wrong?
It appeared that a number of factors had led to the siding
Incorrect flashing. To begin with, step flashings and window
head flashings had been installed incorrectly — they were
on top of the housewrap instead of tucked underneath (Figure
2). Also, at door and window openings, the housewrap had been
cut out in a big X, with little effort made to seal the corners
— a typical method when the house was built.
Figure 2. Incorrect and sloppy flashing
contributed to water damage behind the siding. The step
flashing over this bay window, for example, was installed on
top of the housewrap rather than being tucked beneath
No back-priming. In addition, the 6-inch cedar bevel siding
wasn't back-primed (Figure 3). It's possible that, at the time
of construction, back-priming wasn't specified in housewrap
installation instructions. Later, after researchers had shown
that the tannins in cedar could break down housewrap's water
resistance, manufacturers began to require that wood siding be
back-primed (see "Can Moisture Beat Housewrap?," Notebook,
Figure 3. Brown stains from water-soluble
extractives in the unprimed clapboards clearly indicate the
movement of water. The damage was much worse on the southern
exposure, where solar vapor drive played a role.
Solar vapor drive. Another factor in the failure was the home's
southern exposure. Although we repaired and re-sided the entire
home, we found that damage to the OSB sheathing was limited
largely to the southern side. This, according to architect Doug
Viehmann, who designed the repair details for the job, was the
result of solar vapor drive. Liquid water and water vapor
always move from a warmer to a cooler surface, so when the sun
heats the siding, it causes moisture to move upward in the
capillary spaces — where the siding overlaps —
toward the cooler sheathing. Because the tannins in the cedar
allowed the water to move through the housewrap, the sheathing
became soaked and began to rot.
The Prescribed Fix
Viehmann specified 30-pound asphalt felt for the
weather-resistive barrier. Felt is an ideal choice: Not only
does it absorb water and slowly release it when drying
conditions are present, but it actually becomes more
vapor-permeable as it gets wet. This means that water and water
vapor are less likely to get trapped behind the felt.
On top of the felt, Viehmann specified vertical 1x3 strapping
16 inches on-center, and on top of the strapping, back-primed
A-grade cedar claps. To deal with the extra 3/4-inch build-out,
he suggested a simple backband around the existing trim —
a clean detail that allowed us to leave the windows in place
and keep most of the trim intact (Figure 4).
Figure 4. A 2-inch-wide backband applied
to casings and corner boards made it possible to install the
new clapboards over a 3/4-inch air space without having to
reset the windows.
In retrospect, despite daytime high temperatures in the teens
and one of the largest snowfalls on record, winter was not a
bad time to tackle this project (see sidebar, next page).
Overall, conditions were dry and windy, which helped the shell
air out. We started in mid-November, stripping off all the
siding on the south side and measuring moisture levels in
numerous locations with a Delmhorst J-2000 moisture meter
Figure 5. The author's crew used a
hand-held moisture meter to mea-sure the moisture levels in the
OSB-covered SIPs panels. Readings were highest around windows
— which had been poorly flashed — and closer to
grade, where water leaking in from above would have
accumulated. Levels were lowest on the second floor under the
We then called in an environmental contractor to clean the mold
from the OSB. A crew of three workers — all wearing HEPA
respirators, rubber gloves, and disposable coveralls —
scrubbed the mold with a biocide and sanded down the swollen
sections of OSB. This service cost about $3,000 for about 1,000
square feet of wall area.
While waiting for the walls to dry, we replaced any rotted trim
and primed the clapboards in the garage (Figure 6). After about
a week, the wettest sections of wall were down to about 10
percent moisture content, so we started the re-siding.
Figure 6. While the members of the mold
remediation company did their work, the carpenters primed and
painted the new clapboards in the garage (left), and replaced
rotten trim, making sure to prime every cut (above).
Patching the panels. Some sections of the OSB looked
more like compost than lumber. It's fortunate that the home was
built with a structural timber frame and wasn't relying on the
SIPs panels as the primary structural component; otherwise, we
might have had bigger problems with gravity loads in areas
where the OSB had rotted away. The panels also have to provide
important lateral strength to the frame, given the home's wind
exposure, but the engineer concluded that even with the damaged
areas there was still plenty of lateral capacity.
Using utility knives and chisels, we removed the damaged
material and made patches from new OSB, gluing them in place
with polyurethane spray foam (Figure 7). We also filled in any
gaps around windows.
Figure 7. Heavily damaged sections of the
SIPs panels were patched with OSB inserts secured with
Next, we patched most of the trim on the home's lower portion
and replaced the water table entirely. The original drip cap
was finger-jointed wood, which in my experience never holds up.
We ripped it out and replaced it with one made from clear
stock, then covered the new cap with an aluminum cap.
Installing the Rain Screen
Following the architect's design, we applied a 2-inch-wide
3/4-inch backband around all the existing window and door trim
and along the edges of the frieze and corner boards (Figure 8).
Then, after applying the strapping, we installed the preprimed
clapboards. We used a 100 percent acrylic latex primer, and
made sure to field-prime all cuts before installation (Figure
Figure 8. Spruce strapping over 30-pound
felt provides a 3/4-inch air space and drainage plane, so that
the new clapboards can dry when wetted and infiltrating water
can drain freely (left). Flashings were either tucked beneath
the felt or sealed to the felt with Vycor (above left). Note
the backband detail around the window trim (above
Figure 9. With the trim build-out in
place, installing the prestained cedar clapboards was
straightforward (top). The crew used pneumatic stainless steel
ring-shank siding nails, and painted every cut end
This project has made me question the use of plastic housewrap.
I'll probably use felt in the future when the choice is mine
— even though it's harder to work with. I've always
preferred plywood to OSB, anyway; it holds up better when wet,
so I'll also encourage future clients to use plywood sheathing,
despite its higher cost.
Not counting the architect's fee, the repair cost my client a
little more than $80,000. What amazed me most on this job was
that from outward appearances you could hardly tell there was
anything wrong with the house. It leaves me wondering how many
other homes out there have similar problems, but the owners
have no idea.
Mike Goldfield owns Goldfield Construction in