Figure 3-6. Potential Locations for Crawl Space Insulation
Another important factor to consider when managing moisture in a crawl space is the way it is to be insulated. Crawl spaces can be insulated at the exterior walls, or vented and insulated at the crawl space ceiling (Figure 3-6). Insulation not only plays a role in the thermal efficiency of a home but also in the way that moisture behaves. Cooler surfaces in a crawl space can cause moisture from the air to condense on the surfaces. For unvented crawl spaces, the best approach is to treat the crawl space as a short basement, placing insulation on the exterior or interior surface of the crawl space walls. Research has shown that closed crawl spaces with wall insulation have better energy and moisture performance than wall-vented crawl spaces with ceiling insulation (Dastur et al. 2005).
A key question in the design of an unvented crawl space is whether to place insulation inside or outside the wall. In terms of energy use, there is not a significant difference between the same amount of insulation applied to the exterior versus the interior of a concrete or masonry wall. However, the installation costs, ease of application, appearance, and various technical concerns can be quite different.
Rigid insulation placed on the exterior surface of a concrete (Figure 3-6a) or masonry wall has some advantages over interior placement in that it can provide continuous insulation with no thermal bridges, protect structural walls at moderate temperatures, and minimize moisture condensation problems (Figure 3-7). If the exterior insulation extends up over the rim joist and its R-value is high enough, the joists and sill plates can be left open to inspection from the interior for termites and decay. On the other hand, exterior insulation on the wall can be a path for termites and can prevent inspection of the wall from the exterior. If needed a termite barrier should be installed through the insulation where the sill plate rests on the foundation wall. This option is shown in all drawings that depict exterior crawl space foundation insulation. Vertical exterior insulation on a crawl space wall can extend as deep as the top of the footing and, if desired, be supplemented by extending the insulation horizontally from the face of the foundation wall. Insulation that is exposed above grade must be protected with a coating to prevent physical damage and degradation. Such coatings include fiber cement board, parging (stucco type material), treated plywood, or membrane material (Baechler et al. 2005).
Figure 3-7. Crawl Space with Exterior Insulation
Exterior wall insulation must be approved for below-grade use. Typically, three products are used below grade: extruded polystyrene, expanded polystyrene, and rigid mineral fiber panels. (Baechler et al. 2005). Extruded polystyrene (nominal R-5 per inch) is a common choice. Expanded polystyrene (nominal R-4 per inch) is less expensive, but it has a lower insulating value. Below-grade foams can be at risk for moisture accumulation under certain conditions. Experimental data indicate that this moisture accumulation may reduce the effective R-value as much as 35%-44%. Research conducted at Oak Ridge National Laboratories studied the moisture content and thermal resistance of foam insulation exposed below grade for fifteen years; moisture may continue to accumulate and degrade thermal performance beyond the fifteen-year time frame of the study. This potential reduction should be accounted for when selecting the amount and type of insulation to be used (Kehrer, et al., 2012, Crandell 2010).
Rigid fiberglass and rigid mineral wool panels (R-4 per inch) do not insulate as well as extruded polystyrene, but are the only insulations that can provide a drainage space for foundation walls because of their porous structure. Use of these materials as a drainage space only works if effective footing perimeter drains are present.
Interior crawl space wall insulation (Figure 3-6b) is more common than exterior, primarily because it is less expensive since no protective covering is required, and can present a reduced hazard of termite infestation. On the other hand, interior wall insulation may be considered less desirable than exterior insulation because it (1) increases the exposure of the wall to thermal stress and freezing, (2) may increase the likelihood of condensation on sill plates, band joists, and joist ends, (3) often results in some thermal bridges through framing members, and (4) usually requires installation of a flame resistant cover. Interior insulation is not recommended on non-core filled masonry block walls, due to an increased risk of moisture accumulation within the assembly. In addition, interior insulation should not be used if a positive capillary break is not present between the top of the foundation wall and wood framing system due to the potential for moisture accumulation in wood framing materials.
Materials that are resistant to moisture damage are recommended for use in contact with concrete foundation components. Rigid foam plastic or high-density spray polyurethane foam are the two materials recommended to insulate the interior side of walls in unvented crawl spaces (Figure 3-8). In areas not prone to termite infestation, rigid foam should be installed and sealed at the rim joist to prevent entry of moist air into the wood structural components. This air barrier is especially critical in cold climates, and when exterior insulation is not installed. Batt insulation should only be used at the rim joist where access is required for termite inspections. Expanded or extruded polystyrene rigid foam insulation should be used to cover the walls and be attached with mechanical fasteners. A three-inch wicking gap should be left between the wall insulation and the ground, and a three inch termite inspection gap or continuous termite shield should be present at the top of the wall and the sill plate (Marshall 2008). An ignition barrier or fire barrier will likely be required, based on code jurisdiction and occupancy.
Figure 3-8. Crawl Space Interior Insulation with EPS or XPS Semi-Permeable Insulation on Inside Wall
It is possible to eliminate the ignition barrier requirement. This has been done by using foil-faced polyisocyanurate insulation panels, which are rated for exposure in basements and crawl spaces in some jurisdictions. Note however that the unperforated foil facing is completely vapor-impermeable, and very little drying will occur through it. Many jurisdictions will also allow high-density polyurethane foam to coat the rim and sill area (but not the entire wall) with no additional fire protection.
Interior insulation retrofits carry additional risks: capillary breaks may not be present, either at the top of the wall or between the foundation and the framing; insulating on the interior will tend to increase moisture accumulation in the framing in that case. A capillary break may not be present between the footing and the wall, potentially increasing the presence of moisture due to capillary wicking. Since waterproofing and drainage systems are often not present or not working on older houses, bulk water penetration is possible. For description of a robust retrofit interior insulation strategy see Ueno (2011).
Insulation placed horizontally around the crawl space floor perimeter can provide additional thermal protection for sealed crawl spaces with interior or exterior insulation on the foundation walls. However, it may also create additional paths for termite entry. In cold climates, insulation of the entire floor area to prevent heat loss may be desirable.
In a vented crawl space, the insulation is always located in the ceiling (Figures 3-6e and 3-9). There are two recommended approaches to crawl space ceiling insulation:
Figure 3-9. Vented Crawl Space with Insulation in the Ceiling
These systems are the only ones capable of preventing mold and decay due to high humidity conditions that may occur in the crawl space in most climates (Lstiburek 2008). Impermeable floor finishes like vinyl flooring and some types of ceramic tile must be avoided to allow the floor to dry upward into the home.
In addition to more conventional interior or exterior placement covered in this handbook, there are several systems that incorporate insulation into the construction of the concrete or masonry walls. These include (1) rigid foam plastic insulation cast within concrete walls, (2) polystyrene beads or granular insulation materials poured into the cavities of conventional masonry walls, (3) systems of concrete blocks with insulating foam inserts, (4) formed, interlocking rigid foam units that serve as a permanent insulating form for cast-in-place concrete, and (5) masonry blocks made with polystyrene beads instead of aggregate in the concrete mixture, resulting in significantly higher R-values. However, the effectiveness of systems that insulate only a portion of the wall area should be evaluated closely because thermal bridges around the insulation can impact the total performance significantly.
For more information visit Water Managed Foundations and Insulating Foundations within the Building America Solution Center.