Rim Joist Insulation

Rim joists are the pieces of lumber attached to the ends of floor joists to provide stability and close off the floor system. In many homes–both old and new–they remain uninsulated. Some updated building codes require rim joist insulation. Even new home rim joist insulation often needs improvement or replacement.

Rim Joist Insulation

Why Insulate Rim Joists?

The R-value of softwood lumber is around R-1.4 per inch. Some of the types of material used for rim joists include oriented strand board (OSB), plywood, and dimensional lumber. Thicknesses vary between 1” and 1 ¾”. All provide very poor thermal resistance without added insulation.

Old, warped, rotted, and/or uninsulated rim joists can allow more air leakage than all of the house windows combined. Many possible gaps meet at the rim joists. Wall sheathing, subfloor, sill plate, and the rim joist itself can all leak air. Warm air rises allowing cool air to be pulled into the house–cooling the basement and floor.

Many jurisdictions require rim joists to be insulated. Some–like North Dakota–do not. Rim joist insulation is often overlooked–or ignored. Rim joists are difficult and/or expensive to insulate in multi-story buildings after they are finished. Or in basements where the ceilings are drywalled.

Types of Rim Joist Insulation

Choosing the proper rim joist insulation will eliminate moisture problems, provide a good R-value, and save the labor and expense of future repairs. The rim joist’s low R-value can cause condensation to accumulate on the sill plate, subfloor, adjacent joists, and the rim joist itself–causing mold and mildew growth and promoting wood rot.

Spray Foam

Closed cell spray foam has an R-value of R-6.5 per inch. It provides a waterproof barrier and eliminates condensation. Spray foam expands to fill all gaps and cracks because it adheres to everything it touches. It is the best choice for insulating rim joists.

The advent of DIY spray foam kits allows homeowners to insulate rim joists without calling a contractor. Applying foam is a quick and easy one-step process.


  • Does not retain water. No mold or mildew will grow on the rim joist.
  • Not a pest food source.
  • Expands to fill the space. Seals around nails and pipes, and fill gaps.
  • Creates an air seal.
  • R-value of R-6.5 per inch.
  • Class 1 fire rated.


  • Most expensive product option.
  • Can produce toxic chemicals. Wear a hazmat-type suit, respirator, and goggles. Has an unpleasant odor while curing. It is a good idea for family members to leave until the foam is cured.
  • Can be messy for inexperienced installers.
  • Finding a contractor in more remote areas can be difficult.

Rigid Foam Boards

Insulating rim joists with rigid foam boards is usually the best option. It combines high R-value with reasonable cost and a DIY-friendly installation. Rigid foam options include:

  • Expanded polystyrene (EPS). R-3.5 per inch. Can absorb moisture over time.
  • Extruded polystyrene (XPS). R-5.0 per inch. Most popular product.
  • Polyisocyanurate (ISO). R-6.5 per inch. Foil faced. Fire retardant. Most expensive rigid foam. R-value may degrade over time.

All rigid foam can be cut to size and installed tightly against the insides of the rim joists. Installation is easier if the pieces are cut about ⅛” smaller in both directions. Use spray foam insulation in a can or acoustical caulking to seal gaps around the foam. Acoustic caulking never dries and continues to adhere even as the wood expands and contracts.

Rigid foam can be a little challenging to install around nails, pipes, and wiring. Cut the pieces to get them installed; then fill any holes or gaps.


  • Water resistant. Limits moisture buildup and mold and mildew growth.
  • Up to R-6.5 per inch.
  • Lightweight. Easy to cut and install.
  • Two inches thick or more acts as a vapor barrier when sealed. EPS does not act as a vapor barrier.


  • Two steps include installing the foam–then sealing the edges.

Fiberglass Batts

Fiberglass batts are the most used insulation in North America. It is inexpensive, readily available, and easy to cut and install. Many people insulate rim joists by filling the cavity with fiberglass.

Fiberglass will provide R-3.5 per inch if installed properly and it remains dry. The tighter the fiberglass–the lower the R-value. Wet fiberglass loses its insulation value. Fiberglass does not provide an air barrier. Warm moist air can get through the material and create condensation on the rim joist. Moisture promotes mold and mildew growth. Fiberglass holds moisture against the rim joist.

Fiberglass alone does not provide good rim joist insulation. It can be used to fill the cavity after spray foam or rigid foam is installed against the rim joist as an air and moisture barrier. This system saves money and can get to higher R-values without stacking or spraying a lot of foam into the cavity.


  • Inexpensive
  • Easy DIY project.
  • R-value of R-3.5 per inch.


  • Not a good option for rim joist insulation.
  • Holds moisture against the rim joist–promoting mold, mildew, and rot.
  • Particles can cause itchiness and breathing problems.

Mineral Wool Insulation

Lava rock is the main ingredient of mineral wool insulation. It does not absorb moisture. It is a dense rigid product that restricts–but does not eliminate–airflow. Mineral wool has an R-value of R-3.0 to R-3.3 per inch. Mineral wool insulation costs up to 50% more than fiberglass.


  • R-3.0 – R-3.3 per inch.
  • Will not absorb moisture. Reduces mold and mildew growth. Does not support rot.
  • Easy to cut and install.
  • Rigid enough to support spray foam sealant for gaps.


  • Expensive.
  • Lower R-value.
  • Does not provide an air barrier.

Effective rim joist insulation can only be achieved using a foam product. Spray foam or rigid foam board insulation.

Installed properly, they provide high R-values, moisture barriers, and fairly easy DIY installation.