Rigid foam board insulation should not be used interchangeably for all applications and must be installed correctly for building envelope purposes as well as fire & environmental safety reasons.
There are basically three types of this petroleum based product, having differing insulation values and colours. Generally, these products are used on walls, floors, ceilings, roofs & foundations and have a higher insulation value than cellulose & fibreglass insulation. With properly sealed seams, it can provide a better air barrier too. Some foam is closed cell while other is open cell which affects its ability to absorb water.
What most people do not realize is that all foam plastics such as rigid foam board insulation should be covered with a fire rated / thermal material such as 1/2 inch gypsum board. In addition, some contain harmful toxins (and or are not thought to be a “green” building product) that can be released into the air.
Here is a brief overview on some of the traits of rigid foam board insulation:
Expanded polystyrene (EPS)
- Least expensive
- Vapour permeable
- R-value: 3.6-4.2 per inch
- Uses: insulated concrete forms and structural insulated panels (SIPS)
- When applied in wall assembly / sheathing it should be over the air barrier
- Foam plastics, including Expanded polystyrene (EPS), are considered combustible and should be protected from high heat sources. A protective barrier or thermal barrier may be required as specified in the appropriate building code.
Extruded polystyrene (XPS)
- Water resistant, can act as a vapour retarder
- Uses: side walls, floors, ceilings, below grade waterproofing & roof systems
- R-value: 5 per inch
- Blowing agent and flame retardant contain toxins, and not thought to be green builder friendly.
- Foam plastics, including Extruded polystyrene (XPS), are considered combustible and should be protected from high heat sources. A protective barrier or thermal barrier may be required as specified in the appropriate building code.
Polyisocyanurate / polyiso (ISO)
- Most expensive
- Water absorbent
- R-value 6-6.5 per inch
- Faced product typically with foil
- Uses: side walls, roofs, vapour barrier, ductwork
- Green builder friendly
- No recommend for below grade applications
- Foil facing means no vapour barrier recommended on exterior walls.
- All construction materials, including foam plastics materials such as polyiso insulation, must provide a suitable margin of fire safety. Among all foam plastics, polyiso possesses the highest level of inherent fire resistance due to its unique structure of strong isocyanurate chemical bonds. A protective barrier or thermal barrier may or may not be required as specified in the appropriate building code.
If you’re not sure what type if rigid foam board insulation you have or you’re planning work to your home, consult a professional and install to the manufacturer’s specifications and local building code.