Rigid Board Insulation

Alias(s): Polyisocyanurate, polyiso board
R-value per Inch: 6 - 7
Weaknesses: Does not prevent radiant heat transfer (The primary source of heatflow in and out of your structure). Expensive, r-value diminishes slightly over time especially if the foil face is removed. It will absorb moisture if wet, degrades in sunlight. In cold weather, warm inside air containing water vapor can get past the wall finish and insulation, condensing inside the colder wall cavity. In hot, humid climates the same thing can happen, just in the reverse direction. Humid outdoor air in the summer can condense inside cool, air conditioned wall cavities. If enough of this happens and the water cannot escape, wood rot, mold, and other moisture-related problems can occur.

When insulating a foundation you need to consider that, although insects don't eat foam board, they can easily tunnel through it. Insect burrows reduce the r-value and structural integrity of the insulation. For these reasons, some manufacturers treat their foam products with an insecticide, usually a borate compound. Many building jurisdictions also mandate treating the earth around the building with insecticides. These jurisdictions may also want an inspection area several inches wide and all around the foundation of a house kept bare of insulation board.