How to Insulate Rim Joists

Joists and other obstructions make it difficult to effectively insulate rim joists.

Uninsulated or poorly insulated rim joists can result in serious heat loss and even frozen pipes. Spray in place foam insulation is a very effective solution but it is costly. Carefully cut rigid insulation can effectively insulate this difficult area at low material cost.

Things You'll Need

• Sharp utility knife
• Framing square
• Rigid insulation
• Expanding foam sealant
• Caulk
• Dry wall
• Wood Battens

Instructions

1. Insulating the rim joist of typical houses is very difficult. Fiber insulation is easily tucked into the spaces between the joists. However, without an effective vapor retarder, fiber insulation provides no barrier to moisture migration. Moisture will condense at the back of the cavity. This will wet wood components providing the opportunity for mold, mildew and decay.

2. Carefully cut and fit rigid insulation such as expanded or extruded polystyrene provide good resistance to air and moisture penetration. Caulking the edges with standard caulk or expanding foam sealant provides good insulation and minimizes moisture intrusion.

3. Expanded polystyrene or bead board is the lowest cost rigid insulation commonly available. Expanded polystyrene has R-4 per inch. Four inches results in R-16. This is adequate for rim joists even in cold climate areas. Two layers of 2" material can be used since 4" insulation is not commonly available.

4. Carefully measure the distance between the floor joists. It is critical that the insulation fit tightly into the joist cavity. Using a sharp utility knife and a straight edge score both sides of the insulation. It can then be snapped off along the scored lines.

5. 

   Run a bead of caulk around the four back corners of the joist cavity. Carefully fit the cut insulation into the joist cavity. The bottom edge of a three pound mall can be used as a battering ram to tap the tightly fitting pieces into position.

6. A cardboard template can be used to get a tight fit around pipes and other obstructions. Fit the template first and then use it to mark and cut the insulation. Expanding foam can be used to fill any gaps.

7. 

   Expanded polystyrene is flammable. It must be covered with drywall to protect it from possible flame sources. The drywall can be glued to the foam with appropriate construction adhesive or held in position with small wooden cleats nailed to the joists as shown here.

Tips & Warnings

• Expanded polystyrene is somewhat compressible. A three pound mall can be used as a battering ram to force slightly oversized pieces into position.

• A tight fit is important to prevent moisture penetration and thermal shorts.

• Expanding foam sealant can be used to fill any gaps.

• Expanded polystyrene is flammable. Be sure to cover it with an approve thermal barrier like gypsum wall board.