If you properly insulate your crawl space,
in addition to air sealing and controlling
moisture, you will save on energy costs and increase your home's comfort.
How to insulate a crawl space depends on whether its ventilated or unventilated.
Traditionally, crawl spaces have been vented to prevent problems with moisture;
most building codes require vents to aid in removing moisture from the crawl
space. However, many building professionals now recognize that building
an unventilated crawl space (or closing vents after the crawl space dries
out following construction) is the best option in homes using proper moisture
control and exterior drainage techniques. There are two main reasons for
this line of thinking:
Ventilation in the winter makes it difficult to keep crawl spaces
Warm, moist outdoor air brought into the crawl space through foundation
vents in the summer is often unable to dehumidify a crawl space. In
fact, this moist outdoor air can lead to increased moisture levels
in the crawl space.
Insulating an unventilated crawl space
If you have or will have an unventilated crawl space, then your best approach
is to seal and insulate the foundation walls rather than the subfloor. The
advantages of insulating the crawl space are as follows:
You can avoid the problems associated with ventilating a crawl space.
Less insulation is required (around 400 square feet for a 1,000-square-foot
crawl space with 3-foot walls.)
Piping and ductwork are within the conditioned volume of the house
so they don't require insulation for energy efficiency or protection
Air sealing between the house and the crawl space is less critical.
The disadvantages of insulating a crawl space include the following:
The insulation may be damaged by rodents, pests, or water.
A radon mitigation system will require ventilation of the crawl space
to the exterior. Not planning for radon-resistant construction may
necessitate air sealing the floor to mitigate the radon through ventilation.
The crawl space must be built airtight, and the air
barrier must be maintained.
The access door to the crawl space must be located inside the home
through the subfloor unless an airtight, insulated access door in
the perimeter wall is built and maintained.
Steps for installing crawl space wall insulation
Review plans for this method of foundation insulation with pest control
and local building officials to ensure code compliance.
Eliminate or seal the foundation vents.
Ensure that combustion furnaces and water heaters located in the
crawl space are sealed-combustion units equipped with a powered combustion
Seal all air leaks through the exterior wall during and after construction,
including the band joist.
Locate the crawl space access inside the home or install an access
through the perimeter that will remain airtight after repeated use.
Install rigid foam board or batt insulation –
exterior foam, interior foam, or interior batt – to achieve
complete insulation coverage. Insulate the band joist with batt insulation,
as well as the crawl space access if it's located in the wall.
Install a continuous termite shield between the band joist and masonry
foundation wall that covers the wall insulation and extends completely
outside (or leave a 2- to 4-inch insulation gap at the top for termite
Install a supply outlet in the crawl space, relying on the leakiness
of the floor to provide the return air path.
Steps for installing underfloor insulation
During the early phases of construction, the builder should inform
all subcontractors (plumbing, electrical, HVAC, etc.) that they need
to keep the space between the floor joists as clear as possible. Run
drain lines, electrical wiring, and ductwork below the bottom of the
insulation so that a continuous layer of insulation can be installed.
For freeze protection, supply plumbing may be located within the insulation.
The best approach is to run supply plumbing together in a few joist
spaces. The insulation can be split and run around the plumbing.
Seal all air leaks between the conditioned area of the home and the
crawl space. High-priority leaks include holes around bathtub drains
and other drain lines, plenums for ductwork, and penetrations for
electrical wiring, plumbing, and ductwork (including duct boot connections
at the floor).
Insulation batts with an attached vapor
barrier are typically used to insulate framed floors. Obtain insulation
with the proper width for the joist spacing of the floor being insulated.
Complete coverage is essential. Leave no insulation voids. The batts
should be installed flush against the subfloor to eliminate any gaps,
which may serve as passageways for cold airflow between the insulation
and subfloor. The batts also should be cut to the full length of the
joist being insulated and slit to fit around wiring and plumbing.
Insulate the band joist area between the air ducts and the floor
as space permits. Use insulation hangers (wire staves) spaced every
12-18 inches to hold the floor insulation in place without compressing
the insulation more than 1 inch.
The orientation of the vapor barrier depends on the home's location
or climate. In most of the country, the vapor barrier should face
upward. However, in certain regions of the Gulf states and other areas
with mild winters and hot summers, it should face downward.
Insulate all ductwork in the crawl space.
Insulate all hot and cold water lines in the crawl space unless they
are located within the insulation.
Close crawl space vents after ensuring that the crawl space and all
the construction materials are dry.
For insulating truss floor systems, it's better to install netting or foam
board insulation to the underside of the floor trusses. Then, fill the space
created between the netting or insulation and subfloor with loose-fill
Insulating a ventilated crawl space
Here are some guidelines to follow for insulating a ventilated crawl space:
Carefully seal any and all holes in the floor above ("ceiling" of
the crawl space) to prevent air from blowing up into the house.
Insulate between the floor joists with rolled fiberglass. Install
it tight against the subfloor. Seal all of the seams carefully to
keep wind from blowing into the insulation. Also, adequately support
the insulation with mechanical fasteners so that it will not fall
out of the joist spaces in the years to come. DO NOT just rely on
the friction between the fiberglass and wood joists to secure it in
Cover the insulation with a house-wrap or face it with a vapor barrier.
Install a polyethylene vapor retarder, or equivalent material, over
the dirt floor. Tape and seal all seams carefully. You may also cover
the polyethylene with a thin layer of sand or concrete to protect
it from damage. Do not cover the plastic with anything that could
make holes in it, such as crushed gravel. Be sure the headroom of
the crawl space meets local code regulations if you are considering
pouring a concrete slab.
The orientation of the vapor barriers depends on the home's location
As mentioned above, when properly insulating a crawl space, you also have
to consider moisture control measures and air sealing. A crawl space vapor
barrier or retarder applied in existing vented crawl spaces will control moisture
and protect the insulation and floor joists.
Finally, you need to consider radon resistance or control when installing
any type of foundation.