| Designers, contractors and homeowners must understand
the connection between the crawlspace of a raised floor system
and the living space above. Every effort should be made to keep
moisture out of the crawlspace. Provisions should also be made
to maximize drying of any moisture that enters a crawlspace.
Local climatic conditions will dictate the specific design and
construction details for a particular raised floor system.
The subject of moisture control
in crawlspaces is an area of ongoing research. Most crawlspaces
today are constructed as unconditioned and vented systems, with
building codes mandating minimum ventilation requirements. As
an alternative, conditioned and unvented crawlspace systems
are sometimes used and also recognized in the codes.
Unconditioned and Vented Crawlspaces
Figure 3 provides design detail considerations
for unconditioned and vented crawlspaces. Important elements
for the best moisture control include:
HVAC systems of many houses built over crawlspaces deliver
conditioned air through ducts located in the crawlspaces. Whenever
possible, ductwork should be located within the floor cavity (e.g.
between joists, or between or through trusses or I-joists). When
that is not possible, adequate clearance between the bottom of the
ductwork and the ground should be provided to maintain proper
ventilation. All ductwork should be meticulously sealed to avoid
unnecessary energy losses from air leaks. Penetrations for plumbing,
wiring and air ducts should also be sealed to minimize air exchange
between the crawlspace and the living space. Ductwork should also be
insulated to prevent condensation on the ducts. In addition,
insulation should be carefully installed to the underside of the
floor within the floor cavity. Never vent moisture or heat-producing
sources (e.g. clothes dryers, kitchen or bath vents) into the
- Ground drainage
- Ground cover (vapor/gas retarder)
- Insulation installed within the floor cavity
- Plumbing located within the floor cavity, or well-insulated
- Air distribution ducts within the floor cavity or the interior of the structure
Building code requirements for ventilation openings through
foundation walls are intended to reduce moisture levels in the
crawlspace. Section 1203.3 of the 2003 International Building
Code sets forth the underfloor ventilation openings and
cross ventilation requirements for enclosed crawlspaces, such
as within stem wall foundations. Open pier-and-beam foundations,
commonly used with raised floor systems, already create a fully
Generally, building codes mandate that the minimum net area of
ventilation openings required are not less than one square foot for
each 150 square feet of crawlspace area. When an approved vapor
retarder covers the underfloor ground, the minimum vent opening area
can be decreased to one square foot for every 1500 square feet of
crawlspace area. Vent openings are placed to provide cross
ventilation of the underfloor space. These vent openings should be
screened to inhibit pest entry into the crawlspace (see Pest
Management). They also should not allow rain water or
runoff to enter into the crawlspace.
Conditioned and Unvented Crawlspaces
Conditioned and unvented crawlspaces are only recommended when
mechanical systems distribute conditioned air within the underfloor
area. A conditioned and unvented crawlspace typically has insulated
walls and can be thought of as a short basement. This type of
crawlspace is designed to communicate with the living space.
It should be dry, temperate, and have good air quality. Conditioned
air spaces should not be ventilated with outdoor air.
Conditioned and unvented crawlspace systems should have a continuous
ground cover sealed to insulated perimeter walls and any supporting
piers. Care should be taken with all air-sealing construction
details. This is necessary to minimize the unintentional introduction
of unconditioned air, reducing the possibility of condensation on
cold surfaces. In addition, extra care should be taken to prevent
moisture from being trapped in the crawlspace. Any moisture that
does get into the crawlspace should be remediated immediately.
For more information about proper construction of closed crawlspaces, click here.
Ground Cover (Vapor/Gas Retarder)
Draining storm water away from the foundation, preventing standing
water beneath the crawlspace, and making provisions to remove
excess moisture entering the crawlspace, are all important elements
needed to provide a dry, trouble-free raised floor system. Control
of ground moisture is also essential. One of the best ways to
control this moisture is through the use of a ground-applied
Exposed soil in crawlspaces and under porches and decks should
be covered with an approved vapor retarder. A ground cover that
retards transmission of water vapor from the soil into the crawlspace
provides an effective way to prevent moisture and humidity problems.
It should have a permeance of no more than 1.0 perm, complying
with ASTM E1745, to resist alkali and other chemicals
that can be contained in soils. It should also be rugged enough
to withstand foot and knee traffic. The most commonly used ground
cover material is a 6-mil (0.006 inch) polyethylene.
Before installation of the ground cover, the crawlspace floor
should be smooth and free from sharp rocks and construction
litter. Exact installation details will vary depending on the
primary function of the ground cover (i.e. moisture control
or radon gas control). For any crawlspace system, it is important
to avoid standing water on top of the ground cover.
For unconditioned and vented crawlspaces, the edges of the cover
should be overlapped 4" to 6". The cover does not
need to extend up the face of the foundation wall, and no sealing
is required. If the control of radon or other soil gases is
not of primary importance, the ground cover may be cut in several
low spots to provide drainage if needed.
Conditioned and unvented crawlspaces should have a continuous
ground cover over all crawlspace soil. The ground cover should
be sealed at the joints, as well as sealed to the perimeter
wall and any piers. A thin layer of concrete added over the
ground cover provides a better seal and further inhibits the
entry of rodents.
In areas where radon gas is a concern, care should be taken
to vent radon away from the building. By its very nature, an
open pier-and-beam foundation readily dissipates radon gas.
In enclosed, continuous wall foundations, the components of
a passive, sub-membrane depressurization system are readily
installed during construction. The soil within the crawlspace
should be covered with a continuous layer of 6-mil polyethylene
(minimum) soil-gas retarder. In addition, enclosed crawlspaces
should be provided with tightly sealed pipes vented to the exterior
of the building in accordance with the code. For more details,
see Radon Reduction in Wood Floor and Wood Foundation Systems
from the American Wood Council at www.awc.org.
Additional Moisture Control Requirements
In geographic areas where experience has demonstrated a need
for more protective moisture control measures, the preceding
general requirements should be modified to meet local climatic