Article #1368 - Advantages and Disadvantages of Finishing Basements During Initial Construction of the Home
The 2000 and 2003 Editions of the International Energy Conservation Code (IECC) require basement walls to be insulated if the basement is considered part of the heated and/or cooled living space (conditioned space). If the basement is initially designed to be unfinished, insulation is required in the basement ceiling. The 2003 IECC requires floors over unheated spaces or basement walls that define the conditioned space (Section 502.2.3.3 or 502.2.3.6) to meet the applicable overall thermal transmittance factor (U-factor) or the minimum R-value based on the prescriptive specifications on an individual component basis.
If the basement is unconditioned, the floor above the basement (basement ceiling) must be insulated and meet all the requirements of the IECC for floors over an unconditioned space. If the basement is considered part of the conditioned building envelope, the basement walls must be insulated and meet all the requirements of the IECC for basement walls. The requirements in the code vary depending on location and climate conditions. Requirements in the IECC include some of the following: Insulation Installation (Section 102.4), Moisture Control (Section 502.1.1), and Caulking and Sealants (Section 502.1.4.2).
Many homes are being constructed with unfinished basements to reduce initial costs. In most cases, the homeowner eventually finishes the basement for additional living space by installing basement wall insulation. Because most basements are eventually occupied, the advantages and disadvantages of conditioning the basement should be thoroughly reviewed prior to permitting and construction.
Table 1 provides a list of advantages and disadvantages of basement wall insulation compared with basement ceiling insulation.
Table 1: Compared to Basement Ceiling Insulation
| Advantages of Basement Wall Insulation | Disadvantages of Basement Wall Insulation |
|---|---|
| More easily achieves continuous thermal and air leakage boundaries because basement ceilings are typically penetrated with electrical wiring, plumbing, ductwork, and doors | Costs may exceed those for insulating the basement ceiling depending on the materials and approach selected |
| Requires little, if any, increase in the size of heating and cooling equipment - the heat loss and air leakage through the basement ceiling is similar to that through the basement''s exterior walls | Installing insulation improperly may cause moisture and mold problems |
| Eliminates the need for insulation of the piping and ductwork to provide energy efficiency or to protect against freezing because they are located within the house''s conditioned volume | May require non-invasive termite detection systems, such as termite baits, in termite-prone areas |
| Eliminates the requirement of insulating the basement ceiling |
Basement wall insulation can be installed on either the exterior or interior of the walls (see Figures). Moisture must be addressed in conditioned basements. (According to the 2003 IECC, Section 502.1, the design shall not create conditions of accelerated deterioration from moisture condensation.) From a building science perspective, the preferred method is insulating the wall on the exterior with rigid insulation suitable for below-grade installations, such as extruded polystyrene or rigid fiberglass. Table 2 lists several advantages and disadvantages of exterior and interior basement wall insulation.
Several of the following advantages and disadvantages in Table 2 have been generated from other written sources. For more detailed information on basement insulation techniques, refer to the Sources/Citations listed below.
Table 2: Basement Wall Insulation Choices
| Exterior Basement Wall Insulation | Interior Basement Wall Insulation |
|---|---|
| Advantages | Advantages |
| Minimizes thermal bridging and reduces heat loss through the basement walls | Reduces installation costs significantly in an existing building compared with exterior insulation which requires digging up and exposing the exterior basement wall |
| Protects the basement wall from the effects of the freeze-thaw cycle in extreme climates | Offers a wider choice in materials because many types of insulation can be used |
| Reduces the potential for condensation on interior basement wall surfaces | Eliminates the threat of insect infestation within the insulation |
| Protects the exterior damp proofing coating from damage during back-filling | |
| Can serve as a capillary break to moisture intrusion | |
| Disadvantages | Disadvantages |
| Is expensive to install in an existing building unless the wall is already going to be exposed to install a perimeter drainage system | Often requires a fire-rated covering because many insulation types release toxic gases when ignited and paper facings do not meet building code flame spread requirements |
| Is often susceptible to insect infestation | Does not protect the exterior damp proofing coating like exterior insulation |
| May be more difficult to install because contractors may not be familiar with proper detailing procedures | May become saturated by moisture weeping through the foundation walls if the perimeter drainage is poor |
| Requires careful attention to infiltration sealing of the interior wall |