A properly insulated basement can help reduce your energy costs. However, basement walls are one of the most controversial areas of a house to insulate and seal. You need to carefully consider the advantages and disadvantages, not to mention moisture control.

 

Advantages and disadvantages

In most cases, a basement with insulation installed in the exterior basement walls should be considered a conditioned space. Even in a house with an unconditioned basement, the basement is more connected to other living spaces than to the outside. This connection makes basement wall insulation preferable to insulating the basement ceiling.

 

Compared to insulating the basement ceiling, insulating basement walls have the following advantages:

 

Requires less insulation (1,350 square feet of wall insulation for a 36 × 48-foot basement with 8-foot walls, compared with 1,725 square foot ceiling)

More easily achieves continuous thermal and air leakage boundaries because basement ceilings typically include electrical wiring, plumbing, and ductwork.

Requires little, if any, increase in the size of the heating and cooling equipment. The heat loss and air leakage through the basement ceiling is similar to that through the exterior walls of the basement.

 

There are some basement finishing systems available that have the insulation built into the wall panels.

 

These are some other advantages of insulation on exterior basement walls:

 

Minimizes thermal bridging and reducing heat loss through the foundation

Protects the damp-proof coating from damage during backfilling

Serves as a capillary break to moisture intrusion

Protects the foundation from the effects of the freeze-thaw cycle in extreme climates

Reduces the potential for condensation on surfaces in the basement

Conserves room area, relative to installing insulation on the interior

 

The disadvantages of basement wall insulation include the following:

 

Costs may exceed those for insulating the basement ceiling, depending on materials and approach selected

Installation is expensive for an existing building unless a perimeter drainage system is also being installed

Many exterior insulation materials are susceptible to insect infestation

Some contractors are unfamiliar with proper detailing procedures that are critical to performance

If surrounding soil contains radon gas, the house will require a mitigation system underneath the basement floor.

 

Adding insulation to the interior of the foundation is often more cost effective for an existing building. Interior insulation has the following advantages and disadvantages.

 

Advantages:

 

Interior insulation is much less expensive to install than exterior insulation for existing buildings

Almost any insulation type can be used, giving a wider selection of materials

The threat of insect infestation is eliminated

The space is isolated from the colder earth more effectively than when using exterior methods

 

Disadvantages:

 

Many insulation types require a fire-rated covering since they release toxic gases when ignited

Interior insulation reduces usable interior space by a few inches

It doesn't protect the damp-proof coating like the exterior insulation

If the perimeter drainage is poor, the insulation may become saturated by moisture weeping through the foundation walls

Superior air-sealing details and vapor diffusion retarders are important for adequate performance

 

Types of basement insulation

Once you have determined the insulation R-value you'll need for either adding insulation to an existing basement or new home construction, you can choose the type of insulation, including the following:

 

Blanket (batt and roll) insulation

Concrete block insulation (new construction)

Foam board insulation

Insulating concrete forms (new construction)

Loose-fill insulation (good choice for finished basements)

Sprayed foam insulation (good choice for finished basements)

 

The installation of basement insulation will depend on the type you choose and the best way to control moisture in your particular climate.

 

Other considerations

When insulating any part of your home, you also need to consider proper air sealing and moisture control. Moisture control in basements is particularly important since they are notorious for problems with water intrusion, humidity, and mold.

 

Finally, you need to consider radon resistance or control when installing any type of foundation.