rIGID FOAM INSULATIOn - polystyrene and polysocyanurate - uses, R-value, advantages and disadvantages
Rigid insulation foams include products such as extruded and extended polystyrene and polysocyanurate (polyso). They are largely available and mostly used on the outside (or on the inside) of external walls, floors and roof assemblies.
Rigid Foam Panels Size
Plastic foams - polystyrene and polysocyanurate (polyso) - come mostly in four-by-eight (4x8) and two-by-eight (2x8) panels, but their thickness and R-value (thermal insulation) vary significantly.
R-value and Density
Polyso boards provide excellent high thermal insulation (R-value) per inch: R-6 to R-8 per inch.
Polyisocyanurate is also available as a sprayed foam, and largely used in Structural Insulated Panels.
Rigid foam panels: Polystyrene and Polyso
- Polyso and extruded polystyrene have a high R-value.
- Excellent for exterior walls (insulation sheathing)
- Lightweight, easy to transport
- Water resistant
Like high-density polyurethane products, polyso loses a small part of its R- value over a period of years.
Extruded polystyrene has a higher R-value: about R-5 per inch of thickness (R- 1,7 per 50 mm, Metric system); the more common expanded polystyrene has R-3/R-4 per inch of thickness (very similar to insulation batts: mineral wool, fiberglass...).
Rigid Foam Uses
Foam boards are excellent to insulate the outside (or inside) of exterior walls, but they are also extensively used in roof assemblies and underfloor insulation.
They are typical insulation sheathing products - which means that they are used to prevent thermal bridging (heat loss and gain) through the framing of walls, floors and roofs, by providing a continous layer of insulation.
Foam Boards advantages
Polysocyanurate boards have the highest R-value of any common insulation material, and are easy to find in specialty and big retail stores; they are also easy to cut and fit.
Expanded polystyrene is inexpensive, water resistant and widely available.
Extruded polystyrene has an exceptional moisture resistance, and is a great material for foundation wall insulation.
Unfortunately, there is now enough evidence that extruded polystyrene is hazardous to the environment, due to its basic constituents - benzene and styrene.
Foam Boards Disadvantages & Protection
Polysocyanurate is relatively more expensive than other insulation products, and its R-value decreases a little over time.
Both polysocyanurate and the expanded and extruded polystyrene require covering with a fire barrier.
Installation also requires special adhesives, and proper protection from ultraviolet light or at temperatures above 165ºF/74ºC.
In foundation walls, polystyrene should be covered with a proper finish (a drywall finish, for fire and heat protection). Polyso, which is more fragile (and protected by factory-installed aluminum foil), should also be properly protected.
Safety and fire characteristics
Foam boards are not fire-resistant. They are flammable at temperatures above 160ºF (71Cº), combust spontaneaously at about 600ºC (315ºC) and melt at 300º F (150ºC).