Properly controlling moisture in your home will improve the effectiveness
of your air sealing and insulation efforts, and vice versa. Thus, moisture control contributes to a home's
overall energy efficiency.
The best strategy for controlling moisture in your home depends on your
climate and how your home is constructed. Before deciding on a moisture
control strategy for your home, it is important to understand how moisture
moves through a home.
How moisture moves through a home
Moisture or water vapor moves in and out of a home in three ways:
Of these three, air movement accounts for more than 98% of all water vapor
movement in building cavities. Air naturally moves from a high pressure
area to a lower one by the easiest path possible – generally through
any available hole or crack in the building envelope. Moisture transfer
by air currents is very fast (in the range of several hundred cubic feet
of air per minute). Thus, you need to carefully and permanently air seal
any unintended paths to control air movement.
- With air currents
- By diffusion through materials
- By heat transfer
The other two driving forces – diffusion through materials and heat
transfer – are much slower processes. Most common building materials
slow moisture diffusion to a large degree, although they never stop it completely.
Insulation also helps reduce heat transfer or flow.
The laws of physics govern how moist air reacts within various temperature
conditions. The study of moist air properties is technically referred to
as "psychrometrics." A psychrometric chart is used by professionals to determine
at what temperature and moisture concentration water vapor begins to condense.
This is called the "dew point." By understanding how to find the dew point,
you will better understand how to avoid moisture problems in your house.
Relative humidity (RH) refers to the amount of moisture contained in a quantity
of air compared to the maximum amount of moisture the air could hold at
the same temperature. As air warms, its ability to hold water vapor increases;
this capacity decreases as air cools. For example, according to the psychometric
chart, air at 68ºF (20ºC) with 0.216 ounces of water (H2O) per pound of
air (14.8g H2O/kg air) has a 100% RH. The same air at 59ºF (15ºC)
reaches 100% RH with only 0.156 ounces of water per pound of air (10.7g
H2O/kg air). The colder air holds about 28% of the moisture that
the warmer air does. The moisture that the air can no longer hold condenses
on the first cold surface it encounters (the dew point.) If this surface
is within an exterior wall cavity, wet insulation and framing will be the
In addition to air movement, you also can control temperature and moisture
content. Since insulation reduces heat transfer or flow, it also moderates
the effect of temperature across the building envelope cavity. In most U.S.
climates, properly installed vapor diffusion retarders can be used to reduce
the amount of moisture transfer. Except in deliberately ventilated spaces,
such as attics, insulation and vapor
retarders work together to reduce the opportunity for condensation in
a house's ceilings, walls, and floors.
To effectively control moisture in your home, you need to first consider
your climate when exploring your moisture control options.
Moisture control strategies
These typically include the following areas of a home:
- Crawl space
- Slab-on-grade floors
Moisture control in basements
To effectively insulate your
basement for energy efficiency and to create a comfortable space, you
need to properly control moisture in your basement.
Most basement water leakage results from either bulk moisture leaks
or capillary action. Bulk moisture is the flow of water through
holes, cracks, and other discontinuities into the home's basement walls.
Capillary action occurs when water wicks into the cracks and pores of porous
building materials, such as masonry blocks, concrete, or wood. These tiny
cracks and pores can absorb water in any direction – even upward.
The best approaches for preventing these problems will depend on your local
climate, type of insulation, and style
of construction. However, the following general rules apply to most basement
designs for creating a water-managed foundation system (see corresponding
Keep all untreated wood materials away from earth contact
Provide drainage, such as gutters, to conduct rainwater away from
Slope the earth away from all sides of the house for at least 5 feet
at a minimum 5% grade (3 inches in 5 feet). Establish drainage swales
to direct rainwater around
Add a sill gasket to provide air
Install a protective membrane, such as caulked metal flashing or
EPDM-type membrane, to serve as a capillary break that reduces wicking
of water up from the masonry foundation wall. This membrane can also
serve as a termite shield on top of foam
Damp-proof all below-grade portions of the foundation wall and footing
to prevent the wall from absorbing ground moisture by capillary action
Place a continuous drainage plane over the damp-proofing or exterior
insulation to channel water to the foundation drain and relieve hydrostatic
pressure. Drainage plane materials include special drainage mats,
insulation products, and washed gravel. All drainage planes should
be protected with a filter fabric to prevent dirt from clogging the
intentional gaps in the drainage material
Install a foundation drain directly below the drainage plane and
beside the footing, not on top of the footing. This prevents water
from flowing against the seam between the footing and the foundation
wall. Surround a perforated 4-inch plastic drainpipe with gravel and
wrap both with filter fabric
Underneath the basement's slab floor, install a capillary break and vapor retarder, consisting
of a layer of 6- to 10-mil polyethylene over at least 4 inches of
Consult a qualified builder, basement designer, basement
waterproofing contractor, and/or insulation contractor in your area
for specific basement moisture control measures concerning your climate,
type of insulation, and construction style.
To effectively insulate your
crawl space for energy efficiency and to create a comfortable home,
you need to properly control moisture in your crawl space.
A crawlspace is susceptible to moisture and deterioration problems because
of contact with the earth. The best approaches for preventing these problems
will depend on your local climate and the style of your home's construction.
However, the following general guidelines for creating a water-managed foundation
system apply to most crawl space designs:
Keep all untreated wood materials away from the earth.
Provide rain drainage, such as gutters, to conduct rainwater away
from the house.
Slope the earth away from the house for at least 5 feet at a minimum
5% grade (3 inches in 5 feet). Establish drainage swales to direct
rainwater around the house.
Add a sill gasket to provide air sealing.
Install a protective membrane, such as an EPDM-type membrane, to
serve as a capillary break that reduces wicking of water from the
masonry wall. This membrane, in addition to metal flashing, can serve
as a termite shield.
Damp-proof the below-grade portion of the foundation wall to prevent
the wall from absorbing ground moisture by capillary action.
Install drainage plane material or gravel against the foundation
wall to relieve hydrostatic pressure and channel water to the foundation
Provide a foundation drainage system at the bottom of the footing,
not on top, when the foundation floor (interior grade) is below the
exterior grade. Surround a perforated 4-inch drain pipe with gravel,
and cover them with filter fabric.
Install 6-mil polyethylene vapor diffusion barrier across the crawl
space floor to prevent soil moisture from migrating into the crawl
space. Overlap and tape all seams by 12 inches. Seal the polyethylene
6 inches up the crawl space walls. As an option, pour two inches (51mm)
of concrete over this to protect the polyethylene from damage.
Slab-on-grade foundation moisture and air leakage control
To maximize your home's energy efficiency and to protect the foundation,
you should use the following moisture and air leakage control techniques
when installing slab-on-grade floors:
Keep all untreated wood materials away from the earth.
Install well-designed guttering and downspouts that are connected
to a drainage system, which diverts rainwater completely away from
Add a sill gasket membrane between the slab and bottom plate to provide
Install a protective membrane (such as rubberized roofing material
or ice-dam protection membranes) to serve as a capillary break that
reduces wicking of water up from the foundation. This membrane can
also serve as a termite shield.
Install a foundation drain directly beside the bottom of the footing.
The foundation drain assembly includes a filter fabric, gravel, and
a perforated plastic drain pipe typically 4 inches in diameter. Locate
the drain beside the footing, not on top, to avoid water flowing against
the seam between the footing and the foundation wall, and to prevent
wicking from a web footing through the stem wall.
Install a capillary break and moisture barrier under the slab floor,
consisting of a layer of 10-mil polyethylene vapor diffusion retarder
placed over at least 4 inches of gravel.
Moisture control in walls
It is a myth that installing vapor barriers is the most important step for
controlling moisture in walls. Vapor barriers only retard moisture due to
diffusion, while most moisture enters walls either through fluid capillary
action or as water vapor through air leaks.
All climates require these moisture control steps:
Causes of rain leaks through exterior walls include improper installation
of the following:
Install a polyethylene ground cover on the earth floor of houses
with crawl spaces and slope the ground away from the foundations of
Install a continuous vapor barrier, if your climate needs one (see
map on this page) that has a perm rating of less than one.
Place a termite shield, sill gaskets, or other vapor-impermeable
membrane on the top of the foundation wall. This action will prevent
moisture from wicking into the framed wall from the concrete foundation
wall by capillary action.
To enhance protection against rain penetration, create a drainage plane
within the wall system of the home.
Weatherstripping or caulking around joints in the building exterior
(such as windows, doors, and bottom plates)
Wind-driven rain can also penetrate the exterior finish
AND MOISTURE CONTROL