Rigid insulation commonly used in home construction consists of “bead” or “extruded” boards made from polystyrene.
Closed cell polystyrene is used to make rigid boards that are normally sandwiched between two layers of foil. Also, a high-density semi-rigid board is made from compressed fibreglass insulation with a paper-type coating on both sides.
It is difficult to use rigid insulation between studs. Unless the studding system of your home is quite accurate, every piece of insulation has to be individually cut to fit between the studs. Then caulking compound has to be used to ensure a tight seal between the edge of the stud and the rigid insulation.
But, the biggest disadvantage of using such a method is that the studs are poor insulators. You may have a high insulation value between the studs, but the studs will allow cold to penetrate the home. For that reason, I recommend a combination of rigid insulation and fibreglass or mineral-fibre batts. Such batts are installed between studs and rigid insulation is installed over the studs. The rigid insulation can be installed either inside or outside the stud.
High-density urethane insulation with a foil coating is best installed on the inside. Foil, being a metal, is an absolute vapour barrier. If foil-backed, rigid insulation is used on the inside of the home and joins between the sheets of rigid insulation are taped, you then have a very good insulation barrier.
If the same insulation is used on the outside of the home, I recommend that you cut it into two-by-eight-foot sheets and install it horizontally. This allows for moisture to escape, which finds its way into the wall. When installing foil-backed insulation on the outside of the home, it is important that you have a good vapour barrier on the outside.
“Bead” board is made by pressing small beads of polystyrene together. Bead boards usually come in four-by-four sheets and in various thickness.
“Extruded” board usually has a higher R-value than bead board. That doesn’t mean it’s a better insulator. In this case, it just takes less thickness to provide the same R-value. If you are not concerned about thickness, you should check the price of both types and see which provides the better insulation value at the lowest cost.
Polystyrene insulation board can be installed inside or outside the home over the framing or on both sides of the home. I built a double off-set two-by-four stud system in my home and used two inches of extruded polystyrene on the inside and the outside of the house. The total effective R-value of the wall of that particular house was R-52.
When installed on the outside, any exterior finish can be used over a rigid insulation, including stucco, wood, brick or aluminum or vinyl siding.