How to Insulate Over Horsehair Plaster With Foam Board

Plaster, a common wall covering material in older homes, is a hardened amalgam of portland cement, ground limestone, sand and aggregates such as horsehair or pig hair. The material is applied wet over strips of wooden boards called lathe, and soon hardens into a weighty and durable wall covering. Unfortunately, plaster cannot hold nails or screws, as the material crumbles when punctured. To insulate the walls with foam board, you must first install furring strips to the wall studs beneath the plaster wall.

Stud finder
Furring strips
Drywall screws
Screwgun
4-by-8-foot foam boards
Plastic washers
Duct tape or house wrap tape

Instructions

- 1
  Find the location of the vertical wall studs using the stud finder. Mark the location of the studs on the plaster wall with the pencil.
- 2
  Screw furring strips through the plaster wall into the wall studs. Space the screws every 12 to 16 inches. Do not over-tighten the screws or the furring strips will splinter and break.
- 3
  Place a foam board against at least two furring strips. If the furring strips are wider than four feet apart, you will need to install the foam boards horizontally, with the 8-foot length laying horizontally across the wall.
- 4
  Place a drywall screw through the plastic washer. Drill the screw and washer through the foam board into the furring strip. Place a new screw and washer every eight inches or so.
- 5
  Continue securing the foam boards in the furring strips.
- 6
  Cover the seams between the foam boards with duct tape or adhesive house wrap tape.

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Walls