Rigid Insulation R Value
The main advantage that rigid insulation provides over other types is its higher thermal resistance, or R value. Traditional loose-fill insulation, such as fiberglass or cellulose, has an R value between R-2.0 and 3.5 per inch of thickness. Rigid insulation R value, by comparison, usually ranges from R-4.0 to 6.5, and may get as high as R-8.7, depending on the insulation material.
Rigid insulation is usually made up of fibrous materials or plastic foams. The fibrous materials, such as wood, cane, or vegetable fibers, are mixed with fillers and binders to form insulation boards. The boards are usually coated or infused with asphalt to make them more resistant to moisture. These boards are inexpensive than other types of rigid insulation, but they also have much lower R values than the plastic foams.
Plastic foam rigid insulation come in several different varieties, each with its own strengths and weaknesses. The most popular types are:
- Molded expanded polystyrene (abbreviated MEPS) is made by bonding polystyrene beads together to form rigid boards. The boards are available in two different densities: low density boards have an R value of R-3.7 per inch, while higher density boards have an R value of R-4.0 and offer greater resistance to moisture.
- Extruded expanded polystyrene (abbreviated XEPS) is formed by combing solid polystyrene crystals, additives, and a blowing agent in an extruder. These ingredients are melted into a liquid and poured into a press, where they are formed into insulation boards. These boards come in various densities, with R values running from R-4.7 to 5.0 per inch. Extruded polystyrene boards can stand up to high pressure, but they degrade when exposed to light.
- Polyisocyanurate is a closed-cell material infused with a low-conductivity gas that increases the rigid insulation’s R value to around R-5.6 per inch. This material is resistant to both fire and moisture, and is typically used as roofing insulation.
- Polyurethane also contains a low-conductivity gas, but is unfortunately prone to gradual deterioration. To slow this process, polyurethane rigid boards are often coated with plastic or foil facings. These boards have the highest rigid insulation R value, running from R-7.1 to 8.7 per inch. Polyurethane boards are also used to add structural strength to walls and ceilings.