 
A
Vapor Retarder is Necessary!
(The term "vapor barrier" has been commonly used to indicate
materials which inhibit moisture movement. Most of these materials do permit the
passage of small amounts of moisture; therefore the term "vapor barrier" is not
totally accurate and "vapor retarder" is more appropriate.)
The MOST frequent cause of moisture problems
in a new home is moisture
trapped within the structure during construction and/or a continuing source of
excess moisture from the basement, crawl space, or slab. These moisture source(s) can cause problems with wood flooring. A properly placed vapor
retarder can prevent or reduce problem moisture from entering the home.
CONCRETE SLAB CONSTRUCTION
Strip flooring and related
products should be protected from moisture migration through a slab. Proper on
grade or above grade construction requires that a vapor retarder be in place
beneath the slab. ALWAYS
perform appropriate moisture
tests to determine suitability of the slab before delivering wood products.
Also, a vapor retarder equivalent to 4- or 6-mil polyethylene should
ALWAYS
be installed on top of the slab to further
protect the wood products.
"Use a moisture retarding membrane, such as 6-mil polyethylene, under concrete
slabs to block moisture from the soil. Use three to four inches of coarse sand
or gravel under the membrane to act as a water capillarity break in all but dry,
well-drained soils"
Reprinted with permission from Controlling Moisture in Homes
(Washington, D.C. Home Builders Press, National Association of Home Builders,
1987), p. 27.
"Wood Flooring...To give added assurance that moisture does not reach the
finished floor a vapor retarder must be used on top of each slab. Where this is
placed will depend on the type of nailing surface and/or type of wood flooring
used."
FROM: Forest Products Laboratory USDA Forest Service, Madison
Wisconsin AGRICULTURE HANDBOOK 72 (Revised September 1987)
"REDUCING MOISTURE AT THE SOURCE....Install a 4-inch base course of washed
gravel or crushed rock under the slab. Place a vapor retarder such as 6-mil
polyethylene film between the base course and the slab..."
FROM: University of Illinois at Urbana-Champaign, Small Homes
Council- Building Research Council, MOISTURE CONDENSATION F6.2 (Revised
1990)
WOOD JOIST CONSTRUCTION
For a new home with wood joist
construction after the roof, windows and doors are installed, place a
polyethylene film over the crawl space earth as soon as possible. Cover the
earth 100%, overlap sheets, turn up at foundation walls, and weight down to
avoid dislocation.
"REDUCING MOISTURE SOURCES... Most homes with problems of excess moisture
have wet basements or crawl spaces. Some researchers have estimated that as much
as ten times the normal moisture production in a house can be contributed by a
wet crawl space.
To keep basements and crawl spaces dry, the soil in contact with the
foundation must be kept dry...Every crawl space must have a ground cover. This
is a sheet or membrane, usually of 6 mil thick polyethylene, which covers all of
the exposed soil. The joints may be simply lapped. The edges should extend to
the walls. Crawl spaces should be inspected regularly (after every heavy rain,
for example) to ensure water does not collect on top of the ground cover."
FROM: University of Illinois at Urbana-Champaign, Small Homes
Council-Building Research Council, MOISTURE CONDENSATION F6.2 (Revised
1993)
"SOURCES OF MOISTURE...Dampness within a crawl space can usually be avoided if
the lot is graded correctly and a moisture control device, such as a plastic
ground cover, is used....Using a vapor-barrier-type of ground cover which is not
susceptible to damage by fungi - such as polyethylene film, either 4 or 6 mils
thick. Grade and smooth the ground before installing the ground cover. Turn up
the ground cover 4 to 6 inches on the wall of the crawl space. Hold the
polyethylene in place against the wall with sand or bricks. Where more than one
piece of polyethylene is needed, lap the edges 4 to 6 inches -- sealing is not
necessary. The use of wide rolls of polyethylene reduces the number of edge
joints."
FROM: University of Illinois at Urbana-Champaign, Small Homes
Council-Building Research Council, CRAWL-SPACE HOUSES F4.4 Volume 4 Number
2
"Crawl spaces below the floor of basement-less houses and under porches should
be ventilated and protected from ground moisture by a soil or ground cover... A
soil cover, preferably 6-mil polyethylene, is normally recommended under all
conditions to protect wood framing members from ground moisture..."
FROM: Forest Products Laboratory, USDA Forest Service,
Madison, Wisconsin AGRICULTURE HANDBOOK 72, (Revised September
1987)
"CRAWL SPACES The principal source of moisture in crawl spaces is the soil.
Moisture from the soil can create excessive humidity that may condense on floor
framing members.
In problem-prone areas, a soil cover should always be used. The soil cover
should be a heavy, vapor retarding material that is tear and puncture resistant.
The material should be laid on the soil with all joints lapped 8-12 inches and
held in place with bricks or other weights... While roll roofing was used as a
soil cover for many years, 6-mil polyethylene has now become the preferred
material..."
Reprinted with permission from Controlling Moisture in Homes
(Washington, D.C.: Home Builder Press, Natl Assn of Home Builders, 1987), p.
23.
A "vapor retarder" is a material which has a permeance of less than 1
perm. 6-mil polyethylene (.06 perm rating) or equivalent materials are now used
as vapor retarders. (Ordinary asphalt saturated roofing felt or building papers
are not good vapor retarders.)
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