An insulated basement or crawl space provides many benefits year-round, and the rim joist (or band joist) is often a focal point in energy-saving initiatives. Installing insulation at the rim joists enhances energy efficiency and acts as an effective air barrier. Given its significance in maintaining a secure and well-insulated structure, prioritize sealing these gaps. Enhancing this thermal barrier on these building components will contribute to a more comfortable living space and increase overall energy efficiency.
If your floors feel cold during winter, the rim joist is one place to check.
What Is a Rim Joist and Where is it Located?
A rim joist runs along the edge of your floor system where it meets the outside wall. It sits on the sill plate at the top of the concrete foundation and holds the ends of the floor joists. In many homes, especially older ones, this part is uninsulated, leaving gaps where outside air and moisture can get in.
These gaps lead to drafts, mold growth, and extra load on your HVAC system.
What insulation should I use?
- Easiest: A layer of fiberglass insulation is easy to cut and install. A layer of fiberglass batt insulation also boasts a lower material cost than other common types of insulation.
- Next level up: Rigid foam boards, used in conjunction with caulk or a spray foam sealant offer better air sealing and are resistant to mold growth. There are many options of rigid foam available, but contractors recommend the R-10 extruded polystyrene, which is sold as insulation boards that are 2-inches thick.
- Most energy efficient: Closed-cell spray foam insulation offers high R-value, creates an airtight seal, and is moisture resistant. Closed-cell foam is impermeable insulation. It provides superior energy efficiency, and minimizes heat loss or gain in your crawl space.
How to Add Insulation
Regardless of the foam type you choose to insulate this tight space, these steps help to outline the insulation process:
1. Clean and seal gaps. Remove dirt and debris from the joist area. Use caulk or expanding foam to seal gaps between the concrete foundation, sill plate, and framing.
2. Install insulation:
- Fiberglass batts: Cut to fit between each floor joist. Push into place. If needed, support with wire or tension rods to keep them from sagging.
- Rigid foam boards: Cut to size and glue to the rim joist using construction adhesive. Seal edges with caulk or tape.
- Spray foam insulating material: Apply a 1-inch layer across the surface of the rim joist and into the corners. This forms a tight air seal and moisture-resistant layer.
3. Attach a vapor barrier or continuous air barrier. If you used faced fiberglass insulation for rim joists, the vapor layer is already built into the product. For rigid foam panels, consider installing an extra layer, like house wrap or an additional moisture barrier to prevent moisture buildup and enhance air tightness. Seal the edges of the insulation with tape or caulk to create a tight seal.
4. Cover and finish. Depending on local building codes or preferences, you may choose to cover the insulation and finish your basement crawl space with drywall. If you're creating a more energy efficiency in an unfinished crawlspace, skip the drywall. It's not necessary.
FAQ
Should I insulate rim joists in a crawl space?
Yes. Crawl space rim joists are a major source of air leaks and heat loss. Spray foam or rigid foam works well in tight or damp areas.
Does insulation stop mold?
Insulation helps when paired with good air sealing and moisture control. Closed-cell spray foam and rigid foam insulation both resist mold growth better than fiberglass.
Can I use foam board insulation in a basement?
Yes. Foam board insulation (like XPS) is a good choice for basement walls and rim joists. Just be sure to seal it well to prevent air and moisture leaks.
