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 has the following advantages:
- Requires less insulation (1,350 square feet of wall insulation for a 36 x 48-foot basement with 8-foot walls, compared with 1,725 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.
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
Source: Energy Efficiency and Renewable Energy, U.S. Department of Energy