In this section several typical crawl space wall sections are illustrated and described. Figure 3-14 shows a vented crawl space. Figures 3-15 through 3-18 show unvented examples. Figures 3-15 and 3-16 show exterior insulation solutions, while Figures 3-17 and 3-18 show interior insulation. Included in this group of illustrations are variations in construction systems and approaches to insulating the rim joist area.
The challenge is to develop integrated solutions that address all key considerations without unnecessarily complicating the construction or increasing the cost. There is no one set of perfect solutions; recommended practices or details often represent trade-offs and compromises. The purpose of this section is to show and describe a variety of reasonable alternatives. Individual circumstances will dictate final design choices.
Figure 3-14. Vented Crawl Space with Concrete Walls and Insulated Ceiling
Figure 3-14 illustrates a vented crawl space with a concrete foundation wall. The insulation consists of foil-faced polyisocyanurate attached to the bottom of the floor joists, with all joints sealed and taped to act as an air barrier, and to prevent moisture accumulation in the wood structure. Batt insulation is placed between the floor joists over the rigid foam.
Figure 3-15. Concrete Crawl Space Wall with Exterior Insulation. Click here for an animation.
Figure 3-15 illustrates a poured concrete crawl space with exterior rigid insulation. The rigid insulation aligns with rigid foam sheathing over the 2 x 4 wood frame wall above grade. Rim joist insulation is provided by the exterior foam sheathing, and additional rigid insulation on the inside of the rim. The crawl space floor consists of a thin “rat slab” cast over perimeter (or full-coverage) rigid insulation.
Figure 3-16. Concrete Masonry Crawl Space Wall with Exterior Insulation
Figure 3-16 illustrates a concrete masonry foundation with exterior insulation. Like Figure 3-15, the rigid insulation aligns with rigid foam sheathing over the 2 x 4 wood frame wall above grade. The floor consists of a vapor retarder sealed to the wall, placed over a granular drainage material.
Figure 3-17. Concrete Crawl Space Wall with Interior Insulation. Click here for an animation.
Figure 3-17 illustrates an unvented crawl space with a concrete foundation wall. Rigid insulation is placed vertically on the interior, and cut to fit between the joists and cover the rim. Framing and paperless drywall provide fire protection and a finished surface. All seams in the foam and the sheetrock should be sealed to prevent air exchange with the foundation wall. Note that cutting, fitting, and sealing rigid insulation to fit between floor joists is labor-intensive. Spray foam can be used at the rim as an alternate. The crawl space floor is below the level of the surrounding grade, and a dual perimeter drainage system is shown.
Figure 3-18. Concrete Crawl Space Wall with Interior Spray Foam Insulation
Figure 3-18 shows an interior insulation approach, executed with spray foam instead of rigid foam panels. First, the vapor retarder is laid over the gravel bed and sealed to the wall. Then foam is applied from the crawl space floor to the bottom of the subfloor above, encapsulating the sill and rim joist. The rim joist is also insulated by the exterior rigid foam on the above-grade walls, minimizing its exposure to cold temperatures and decreasing the likelihood of moisture damage. In this case, a code-approved fire-resistant coating like intumescent paint is applied to protect the foam from ignition.