Moisture Myths

May 15, 2004

Each year, American homeowners
spend millions of
dollars attempting to fix or
prevent moisture-related
problems. Too often, their
efforts don’t fix the problem.
In some cases, these efforts actually
make matters worse. So says Anton
TenWolde, a physicist and researcher
who has been studying moisture in
buildings for more than 20 years.
According to TenWolde, many generally
accepted moisture-control practices
in the United States are based on limited
or no research, but mostly on tradition
among homebuilders and others.
“We spend very little on housing
research in the United States. Several
countries, including Canada, and even
smaller nations like Denmark, the
Netherlands, and Sweden, invest more
than the United States in research into
home-building technology,” he says.
TenWolde, a native of the
Netherlands, holds degrees in physics
and engineering from the University of
Delft in the Netherlands and the
University of Wisconsin at Madison. He has
been a physicist at the USDA Forest Service
Forest Products Laboratory (FPL) in
Madison, Wisconsin, since 1980. He is currently
project leader of the Building
Moisture and Durability research unit
Since 1999, the FPL has been home to
the Advanced Housing Research Center,
through which much housing-related
research is managed and coordinated.
Several ongoing projects at FPL study moisture-
related issues. For example, FPL’s
research-demonstration house, a full-size,
four-bedroom house built in 2000, is
equipped with scores of sensors embedded
in the walls and elsewhere to track the
movement of moisture and water vapor in
the walls and foundation. Other projects
evaluate sealants and wood preservatives
as well as adhesives.
The problem of basing home construction
or repairs on unproven building practices
is amplified because some of those
traditional practices have become part of
building codes around the country,
TenWolde says.
2 6 • I N T E R F A C E O C T O B E R 2 0 0 4
O C T O B E R 2 0 0 4 IN T E R F A C E •
According to TenWolde, building codes
historically deal with safety issues such as
fire prevention, electrical safety, or structural
standards. Building codes sometimes
go beyond safety when they try to deal with
moisture. “And moisture is one area where
current building codes get it wrong, especially
when they apply standards that might
make some sense in northern Maine or
Minnesota to Florida or Texas,” TenWolde
TenWolde identifies five main ideas
about home construction and maintenance
that are widely misunderstood or downright
incorrect. He calls them “The Five Myths of
Myth One concerns so-called vapor
barriers or vapor retarders. “A vapor
retarder, normally installed only on exterior
walls, is intended to slow the diffusion of
moisture from an area of higher humidity to
one of lower humidity. Such barriers are
ineffective if there is any air movement,
which is almost always the case in woodframe
construction. An air barrier, to be
effective, needs to envelop the entire house
– ceiling and floor as well as walls,”
TenWolde says. “Problems caused by diffusion
are very rare; moisture problems
caused by moving air are much more common.”
In warm, humid climates, a vapor barrier
can do harm. Nonetheless, practically all
building codes require vapor barriers or
retarders. (Vapor barriers originated in the
1930s, partly based on research conducted
at FPL.) A more effective approach to controlling
moisture intrusion would be to
make the house as airtight as possible and
provide good drainage around the house,
according to TenWolde.
Myth Two is that attics need to have
lots of ventilation. Again, venting requirements
are not based on rigorous scientific
research. TenWolde explains that attic venting
originally arose as a moisture-control
strategy for cold climates. Other purported
benefits, such as longevity of the shingles,
arose later. It is widely believed that
increased attic venting will prolong the life
of roofing shingles by cooling them. But
research shows that venting has very little,
if any, effect on shingle temperature. The
most important issue in shingle temperature
appears to be the color of the shingles.
Light-colored shingles reflect sunlight and
don’t get as hot as dark shingles.
One possible real benefit of attic venting
in climates with large snowfalls is to reduce
snow melt on the roof to avoid the formation
of ice dams. But, according to TenWolde, a
more effective – and energy-efficient – way
to control snow melt in almost all climates
in the United States would be to use air barriers
and insulation to prevent heat from
entering the attic.
Myth Three is that new homes are built
“too tightly” and that walls have to
“breathe.” That is the reason often given for
the presence of mold in newly built houses.
TenWolde cites recent research in Canada
that revealed that houses that leaked air
had as much, or in some cases more, mold
than tight houses.
“It takes very little air movement to
accomplish drying, and even a house with
good air barriers usually will permit
enough movement to permit moisture to
escape, unless there is massive water
entry. Uncontrolled air movement
may actually cause moisture
problems and certainly can cost
money in air conditioning and
heating,” he says.
Myth Four: Crawl spaces
need to be vented. To
TenWolde, venting crawl
spaces is just as dubious a
practice as venting attics.
Venting crawl spaces is marginally
effective in dry climates but can
be harmful in wet or warm, humid
climates. The best way to control moisture
in crawl spaces is to use site grading,
downspouts, and soil covers to prevent
water from entering the crawl space.
TenWolde’s Myth Five is the belief that
building codes actually address residential
moisture problems. “Building codes address
only vapor ‘barriers’ and venting attic and
crawl spaces. These are only two ways of
controlling moisture and not very effective
ones at that,” TenWolde notes. “Most real
moisture damage in buildings is caused by
water entering the building through leaks or
poor flashing details. The most effective
practices for controlling moisture are related
to proper installation of windows, flashing,
site grading, foundations, rain
absorption, roof overhangs, and wholehouse
ventilation and humidity control.”
TenWolde’s “Five Myths” reflect the fact
Researchers at the USDA
Forest Service Forest
Products Laboratory
study moisture movement
within a home using this
three-year-old, 2,300-
square-foot house built on the
lab’s grounds. Scores of
moisture sensors are connected to
computers. The roof is shingled with a
prototype made from recycled plastic and
wood fiber.
that there is considerable confusion and
misunderstanding surrounding moisture
problems. In an attempt to resolve some of
those differences and publicize the latest
research findings, the FPL has joined with
industry-related organizations to establish
a Residential Moisture-Management
Network. The network will evaluate existing
research and develop uniform recommendations
for dealing with moisture.
The USDA Forest Service Forest
Products Laboratory was established in
1910 with the mission of conserving and
extending the country’s wood resources.
Today, FPL’s research scientists explore
ways to promote healthy forests and clean
water and improve papermaking and recycling
processes. Through FPL’s Advanced
Housing Research Center, researchers also
work to improve homebuilding technologies
and materials. Additional information about
FPL and its research activities is available
George N. Couch is a public affairs specialist with the USDA
Forest Service’s Forest Products Library in Madison,
George N. Couch
2 8 • I N T E R F A C E O C T O B E R 2 0 0 4
Roofing Demand Approaches
7 Billion Square Meters
Worldwide demand for roofing materials is forecast to rise 2.7 percent per year
through 2008 to nearly seven billion square meters, with a value of almost $48 billion
(U.S.). Gains will be supported by an upswing in residential and nonresidential
building construction in the developing countries of Asia and Eastern Europe.
Demand for roofing in China is forecasted to advance 6.2 percent per year through
2008. Rising incomes in other developing countries in Asia, such as Malaysia and
Thailand, will bolster residential applications of roofing materials in conjunction
with the construction of new housing. Eastern Europe is also expected to enjoy
growth in demand for roofing materials in excess of the global average. Growth in
demand for roofing in industrialized countries will be much more muted, with
expansion projected at less than two percent annually through 2008.
These trends are forecasted in World Roofing, a new study from Freedonia
Group, Inc., Cleveland, OH.
Bituminous roofing constituted the largest product segment in 2003.
Bituminous shingles dominate in the U.S. and Canadian steep-slope applications,
and these two countries account for more than three-quarters of worldwide
demand for the product. Tile roofing was the second largest product segment in
2003, with clay tiles and concrete tiles each with demand in excess of 600 million
square meters.
© 2004 by The Freedonia Group, Inc.
% Annual Growth
1998 2003 2008 03/98 08/03
Worldwide 5482 6084 6965 2.1 2.7
North America 2278 6473 2665 1.7 1.5
Western Europe 918 920 985 1.4
Asia/Pacific 1540 1905 2405 4.3 4.8
Rest of the World 745 786 910 1.1 3.0
The “BE&K School of Industrial
Construction: It’s a Girl Thing” is a
week-long day camp program founded
to help raise awareness among
young women of the opportunities
provided by careers in construction.
The camp, in Carrollton, GA, west of
Atlanta, offers high school girls
training in jobsite safety, welding,
carpentry, and electrical crafts. It is
led by women construction professionals
from the BE&K Construction
company of Birmingham, Alabama,
and was launched in 2000 with just
nine girls. Each year since then,
attendance has doubled.
The American Society of Civil
Engineers (ASCE) is conducting a
study to examine why technicallyinclined
girls do not enter the engineering
field. Women now account
for 20% of registered architects. A
television documentary about
women in engineering and construction
will air in February 2006.
— Associated Builders
and Contractors