In the past, basements and cellars were generally not considered part of a home’s living space and,
therefore, the best insulation for basement walls was to have no insulation at all. Many basements are now
included in the home’s living space and are likely to be temperature-controlled. Proper methods must be
utilized to ensure comfortable living conditions, radon reduction, and moisture control; substantial
energy cost savings may also be realized.
To determine the best insulation, several factors need to be taken into account. As with any project, the
budget is typically the limiting factor in the application method and material selection of insulation. Local
building code requirements must also be considered when making the decision to insulate a basement. In
addition, moisture control, current construction, and whether the basement will be used for personal
living space will play a role in determining the proper method.
Energy cost savings is a primary reason leading homeowners to the decision to insulate a basement.
Depending upon the home heating method, regional climate, and homeowner temperature
preferences, insulating correctly can lead to substantial yearly cost savings. The proper method for sealing
and insulating basement walls is one of the most debated topics in home construction and improvement;
the advantages and disadvantages of each method should be carefully considered.
Concrete or masonry wall foundations are common in modern home construction and, to some extent, are usually
below-grade. Depending upon the permeability of the concrete material used for the construction, the
lack of proper insulation can lead to damp, musty conditions, destructive mold growth, and building
material deterioration. Depending upon the soil conditions, radon penetration may also pose a
significant health risk to occupants. Furthermore, a substantial energy loss for the heating and cooling of the home can occur due to lack of insulation.
Some homeowners believe that simply insulating the ceiling will ensure adequate thermal and moisture
protection to the living space above. Fiberglass batt insulation with a paper face is often placed between
the floor joists. With this solution, the paper face acts as the moisture barrier and is designed to face the
un-conditioned space, such as the basement. Although this solution may help insulate against sound and
energy loss, it does not address the flow of moisture into the basement from the outside; moisture is one of
the most notorious issues with un-insulated basements. In both conditioned and un-conditioned
basements, the association of a basement to the living space above is greater than it is to the outside.
Because of this, wall insulation is preferable to insulating the ceiling.
Exterior Wall Insulation
Insulating the exterior walls is more easily done during initial construction. With an existing building, this
option may prove impractical due to the need for extensive renovation. Various exterior coatings are
available to produce a damp-proofing barrier for below-grade surfaces. This option is best used with
effective exterior water drainage systems that are located near the foundation. The soil around the building
and water guttering plays an important role in diverting moisture from the walls. For portions of the basement
wall above-grade, rigid foil-faced polyisocyanurate foam sheathing is a good solution on the exterior. It is
important to make sure that this foam insulation is fire-rated. This kind of insulation may be covered with a
protective layer, such as siding or stucco. Depending upon the thickness of the insulation used, it may
cause the house frame to be inset as compared to the foundation walls. This results in requiring the frame
to be expanded in order to overlap the foundation, which ads expense.
In-between Wall Insulation
Though uncommon, this solution requires placing rigid insulation, such as extruded foam sheets, in
between concrete slabs. This must be performed during a home’s initial construction. This allows
moisture trapped in the outer concrete layer to vent towards the exterior of the home and moisture in the
interior slab to vent towards the inside. This method is relatively expensive due to the complexity of
construction and provides relatively good thermal and moisture protection.
Insulating a wall internally is the most common method, and usually the most cost-effective. It is very important to use the right kind of insulation so that trapped moisture in the wall may vent properly. Common solutions are insulation blankets, rigid foam, spray foam, and paper-faced batt fiberglass insulation.
The use of insulation blankets for walls and fiberglass paper-faced batt insulation requires the wall to be
virtually free of moisture as the layer on the face of these types of insulation is vapor impermeable; in other
words, it blocks the venting of vapor to the inside and may trap it within the insulation. Although this kind of
insulation provides relatively acceptable thermal protection, the trapped moisture within will likely result
in mold growth, a musty smell, and poor air quality within the basement. Rigid semi-permeable foam, such
as extruded or expanded polystyrene insulation is a relatively expensive solution. It allows moisture
venting to the interior as well as good thermal protection when covered with gypsum board. Spray foam
insulation is a semi-permeable moisture barrier and provides good thermal protection if applied in the
appropriate thickness. This method, although effective, is expensive considering the amount needed to
cover the entire wall.
If the decision is made to add insulation to the walls, it is important to consider the varieties of insulation
available, along with the advantages and disadvantages of each. For effective insulation of a basement for
energy efficiency and comfort, it is very important to properly control moisture. The solution used will depend
greatly upon local climate, the budget of the home-owner, and style of the building construction.