How to Install Polyisocyanurate Rigid Foam Insulation

Polyisocyanurate foam board is a closed-cell form of rigid insulation used in residential and commercial properties. The plastic polymers are combined with a low-conductivity gas to make them bind together, hence, the term closed cell. They often come faced with foil as a vapor and air barrier. Without foil, the boards have an average R-value – the unit of a material’s effectiveness at preventing thermal penetration, with the higher the number the better -- of between 5.0 and 8.0. Foil facing raises the value by approximately 2.0.

Woodens struts
Polyisocyanurate boards
Construction glue
Utility knife
Facing
Drywall

Instructions

- 1
  Frame out the wall, ceiling or floor if it does not already have a frame. This is so that you can cover the boards with the required fire-resistant material after installation. Use 2-by-2 wooden struts to frame the wall, creating a framework onto which drywall or similar material can be stapled.
- 2
  Cut the foam boards to size to fit snugly between the struts of the frame. Use a utility knife to score the board, then bend it.
- 3
  Affix the boards directly to the walls, ceiling or floor with construction-grade adhesive. Avoid nails or screws, as these create voids in the insulation into which air and moisture can penetrate.
- 4
  Cover the boards with a facing or membrane if they did not come fabricated with a foil facing. The covering acts as a vapor barrier, preventing moisture from penetrating the insulation, which can reduce its effectiveness.
- 5
  Place a fire-resistant barrier over the foam boards. Polyisocyanurate remains stable over a large temperature range but, like other foams, can ignite if exposed to sufficiently high temperatures. If it burns, the foam will emit noxious gases. Wallboard or drywall are commonly used to cover foam insulation for this reason.

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