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Mold and New Home Construction Guide
Friday, 14 April 2006
UNDERSTANDING AND MINIMIZING THE RISK OF THE GROWING PROBLEM OF MOLD IN NEW HOMES
Is the home you buy susceptible to Mold Growth?
"A California Builder's Guide to Reducing Mold Risk"
FREE 61-page PDF- April 12th, 2006
Database Development of Mold Incidences in California Homes
Literature Review Summary Report
The interviews with experts in the field provided an up-to-date sampling of the most relevant issues related to moisture intrusion in building assemblies. The (mostly peer reviewed) literature created a supporting foundation of information detailing backgroundon the moisture intrusion issues cited by the experts in the field, as well as other issues.
The major water intrusion issues cited in the 16 interviews include:
Bulk Water --
Improper lot grading and/or settling create slopes toward foundations and allow
water to intrude into slabs, crawlspaces or basements; irrigation systems
overspray water and repeatedly wet foundations and walls; and roof downspouts
are inadequately separated away from homes and deposit water near
foundations instead of to a properly sloped grade.
Roof Eaves --
Roof eaves lack sufficient extensions outward and permit rain, especially wind
driven rain, to soak exterior porous (brick, stucco, wood, etc.) facades.
Vapor Barriers --
Barriers are sometimes incorrectly applied or of the wrong type, depending on
climate, and allow moisture to be trapped or build up in walls. Walls need to be
able to breathe.
Window Detailing in Wall and Roof Penetrations --
o Builders and subcontractors lack sufficient training for proper installation of
windows (and flashing).
o Building codes lack standardized window installation practices.
o Standards are non-existent for installation of building membranes and
flashings with windows.
o Standards for windows, building membranes, and flashing are outdated
(post WW-II ASTM standards) and need to be modernized.
o Trade organizations for fenestration, building membranes, and flashing
are needed to foster these training, standard, and code activities. Interior Moisture/HVAC --
o Houses are too tight for natural ventilation to be effective. Mechanical
outside air (OA) ventilation is needed.
o Architects and engineers need to be educated on ventilation issues and
mechanical ventilation options.
o Oriented Strand Board (OSB) is often gets wet during construction and
then is not adequately dried before wall and roof construction is
o Some newer paints have much less tolerance to mold growth than paints
manufactured 20-25 years ago.
o DHW heaters, AC coils/drain pans, and ductwork, are placed in attics
where leaks can go unnoticed for some time until after substantial damage
o Baths and kitchens do not have properly sized exhaust fans that run until
moisture levels are reduced to desired humidity levels. Fan operation
needs to be automated (based on humidity level) to remove the human
factor. Most of these fans are too noisy and people are annoyed and limit
o Wet spray cellulose, if used, Is not allowed to dry sufficiently. Moisture
content needs to be checked before closing drywall.
o Moisture content in wood framing is often to high before closing in the
walls. Moisture content needs to be checked before closing drywall and
should not be higher than 19%.
o An homeowners manual needs to be developed that would address some
of the maintenance issues that could prevent mold incidences over the life
of a house.
o A builders manual needs to be developed that offers a "systems
approach" to ensuring quality and managing risk during construction. For
example, utilizing a series of checkpoints established by extensive
litigation and failure analysis background, specially trained inspectors can
check for specific, known anomalies in the building assemblies at greatest
risk of water damage and mold formation.
o Construction sequencing is a clear issue with poor scheduling leading to
unprotected building materials getting soaked with rain/snow on the job
o Elastomeric paints for stucco to limit cracking and water intrusion need to
LiteratureA total of 85 literature reviews were completed based on extensive input from the
project team and expert interviewees. A survey was also conducted using key words on
a variety of search engines, including major commercial and university library based
search engines. The following additional keywords to Table 1 were included in, but not
limited to, the search: moisture intrusion, residential, homes, and California. The search
produced many additional pieces of literature from a variety of sources from trade
journals and magazines to product and material organizations.
Some literature provided the scientific and engineering basis for moisture migration in
buildings, and ultimately the understanding to manage moisture in homes.
Many of the pieces of literature cited established the methods by which moisture travels:
Other literature detailed the points at which traveling moisture intrudes into buildings
and its assemblies. The following moisture intrusions problem areas were mentioned
frequently in the literature. Common moisture intrusion points are:
Penetrations in walls and roofs: windows, doors, skylights, piping through walls,
ductwork through walls and ceilings
Roofs with improper installation of flashing, or gutters not working properly
Surface or ground water pooling from improper grading, irrigation system
overspray, and improper placement of downspouts.
Interior moisture buildup due to over crowding, improper exhaust venting
(bathrooms, kitchen, and laundry), too low of a cooling temperature set point,
inadequate removal of moisture (insufficient dehumidification).
Attic/crawlspace moisture from leaky HVAC ducts, condensation on cooler
surfaces, improper ventilation of attic/crawl spaces.
Improperly installed vapor and air barriers improperly installed. Proper placement
of air barriers is important to allow building assemblies to dry out.
Infiltration of moist air when leaky HVAC ducts cause pressure differentials,
leading to outside air being drawn in to building, and oversized air conditioning
equipment leading to short cycling that reduces the AC equipment ability to
Potential solutions that were cited for moisture problems are:
Use materials that are not prone to moisture build-up and mold formation: no
vinyl wallpaper, etc.
Provide proper air barrier placement for the building assembly to dry out.
Provide eaves with at least 18 inches overhang, and do not start siding within 8
inches of the ground/soil.
Increase the cooling set point temperature, and allow the air to flow around the
entire space (well mixed conditions). Furniture can block airflow to corners,
leading to cold spots and moisture condensation on cooler, dark surfaces.
Provide supplemental dehumidification in addition to air conditioning.
Make sure that flashing is properly installed everywhere, particularly at doors and
Venting of crawlspaces depends on climate: hot humid climates do not
necessarily benefit from natural ventilation of crawlspaces.
There have been several field studies of moisture and mold in manufactured housing, but not asSee full pdf report:
much for single-family residences, especially in warmer climates dealing with summer cooling
moisture problems. Future researchers are encouraged to update this literature survey by searching
for the latest literature related to field work on moistureproblems in single family residences,
especially in cooling dominated locations. The existing field data for manufactured homes, and single family homes in heating dominated climates, is best summarized in the 1994 ASTM Manual 18 on Moisture Control in Buildings [R84] from the literature survey.
Database Development of Mold Incidences in California Homes