How to Install Styrofoam SM Insulation

Styrofoam Brand SM insulation, an extruded polystyrene board, protects above- and below-grade foundation walls. Usually used against the exterior side of a residential basement or crawl space wall, Styrofoam Brand SM insulation has shiplap seams on each end. A shiplap seam uses inverted L-shaped edges to seal each joint. The SM insulation board's outer L-shaped shiplap seam overlaps the neighboring SM insulation board's inner L-shaped shiplap seam. Dow manufactures Brand SM insulation in nine different thicknesses, from one to four inches.

Electric chipping hammer
Drywall saw
Polystyrene-compatible adhesive caulk
Caulking gun
Foam mastic
Paint brush

Instructions

- 1
  Remove any ridges or sharp edges from the foundation's wall with an electric chipping hammer. Pressing Styrofoam Brand SM against a ridge or sharp edge breaks the insulation board's surface, reducing its insulating efficiency.
- 2
  Clean the dirt and dust from the foundation's wall, using a broom. Dirt and debris prevents the SM insulation board's adhesive from bonding with the foundation wall.
- 3
  Cut the outer shiplap edge from the first piece of SM insulation board with a drywall saw, using the inside edge of the L-shaped seam as a cutting guide.
- 4
  Run a 3/8-inch-wide S-shaped bead of polystyrene-compatible adhesive caulk on the back of the Styrofoam SM insulation board, using a caulking gun to apply the bead.
- 5
  Align the cutoff edge of the Styrofoam SM insulation with the corner of the foundation. Press the side insulation board with the polystyrene-compatible adhesive caulk against the foundation. Set a bubble level on top of the SM insulation and adjust the board for plumb, using the bubble level as a guide.
- 6
  Cover the inner shiplap edge of the Styrofoam Brand SM insulation installed on the foundation with a foam mastic, using a paint brush to apply the mastic. The mastic seals the shiplap seams.
- 7
  Apply an S-shaped 3/8-inch-wide bead of the polystyrene-compatible adhesive caulk to the back of the second piece of SM insulation board. Align its outer shiplap edge with the first piece of SM insulation's inner shiplap edge and press the second piece against the foundation wall. Adjust the second SM insulation board for plumb, using the bubble level as a guide. Coat the second SM board's inner shiplap edge with the mastic.
- 8
  Install all of the Styrofoam Brand SM insulation boards across the bottom of the foundation wall, using the same method used on the second piece. Cut the last piece to size with the drywall saw.
- 9
  Paint the top of the first row of SM insulation with the mastic, using the brush to spread the mastic on the insulation's top edge.
- 10
  Run each additional row of Styrofoam Brand SM insulation across the top of the previous row of Styrofoam Brand SM insulation, using the same installation method used on the first row.

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