Rigid foam insulation is a type of insulation that comes in pre-made panels. It can be used to insulate the roof, foundation, or outer walls of a building. This type of insulation is a good choice for buildings with limited space for insulation that still need high thermal resistance. Rigid panel insulation is made from fibrous materials (fiberglass, rock and slag wool) or from plastic foam.
The effectiveness of insulation is due to resistance value (R value). R value is the insulation’s ability to resist the flow of hot and cold air. The higher the R value the greater the temperature protection. This value varies based on the climate in which the insulation is installed.
The R value of rigid foam insulation ranges from R3 to R6.5. This is nearly twice the R value attributed to other types of insulation of the same thickness. Rigid foam insulation is installed as an unbroken sheet on the outside of the building, so it increases the overall R value of the structure.
The R value of the rigid foam insulation is dependent on the material used to make the panels. It can be made of expanded polystyrene, extruded polystyrene, or polyisocyanurate. The most commonly used of these is expanded polystyrene. It is the least expensive but also has the lowest R value. It also has no facing, so it is not water resistant.
The most expensive of the three, polyisocyanurate insulation also has the highest R value. Construction crews start with liquid foam which they spray on to a backing to create rigid foam insulation panels. If foil facing is used, the insulation can create a water-tight barrier for construction.
Extruded polystyrene panels are the middle range of rigid foam insulation. Their cost and R value fall halfway between the other two varieties. These panels can be faced or unfaced. Faced panels form a moisture resistant barrier for construction.
Structural insulated panels (SIPs), also called stressed-skin walls, use the same concept as in foam-core external doors. They can be used to insulate ceilings, floors, walls, and roofs. The panels usually consist of plywood, oriented strandboard, or drywall glued and sandwiched around a core consisting of expanded polystyrene, polyurethane, polyisocyanurate, compressed wheat straw, or epoxy. Epoxy is too expensive to use as an insulator on its own, but it has a high R-value (7 to 9), high strength, and good chemical and moisture resistance.
Advantages
Strong – Able to bear loads, including external loads from precipitation and wind
Acoustic insulator
Impermeable to moisture
Easily cut
Reduces the need for stick-frame in construction
No formaldehyde, CFCs, or HCFCs in manufacturing
Accurate R-values and lower energy costs
Disadvantages
More expensive than other types of insulation
Thermal bridging at splines and lumber fastening points unless a thermally broken spline is used (insulated lumber)
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