Roofing system 4


Rigid, or Board-stock insulation, typically has sufficient compressive strength to support the roof membrane and the loads placed upon it. In addition to supporting the roof membrane, rigid insulation can provide other functions for the roof system such as a uniform surface for membrane application and improved hail resistance. Rigid insulation is commonly used to achieve slope in low-slope applications where the deck does not provide the necessary slope. Tapered insulation typically provides 1/4″ to 1/2″ of slope. Insulation should typically be applied in two layers with offset joints to minimize thermal bridging.

The following common types of rigid insulation boards are available:

Perlite: This is an open-cell low R-value insulation (R-2.78 per inch) that is commonly used as a cover board (see Note below). It has good fire resistance, but when exposed to water, it loses compressive resistance, turns to mush, and can be easily compressed. Half-inch thick boards have a greater percentage of organic material content than do 3/4″ or thicker boards. Hence, when hot asphalt is applied over 1/2″ boards, the potential for the development of blisters in built-up and hot-applied modified bitumen membranes is increased. For these reasons, perlite is generally not recommended.

Polyisocyanurate: There have historically been issues with aging of polyisocyanurate affecting R-value. Nevertheless, polyisocyanurate is a high R-value insulation (R-5.6 per inch thickness in cooling conditions and R-5.0 per inch thickness in heating conditions using NRCA’s “in-service” recommendation, or approximately R-5.7 for one inch thickness using the Long-Term Thermal Resistance (LTTR) method for determining resistance).

Polyisocyanurate is one of the plastic foam insulations. It is widely used in low-slope roof systems. Polyisocyanurate insulation is inherently more fire-resistant than polystyrene insulation. It always comes with facers, which are thin sheets on both faces of the insulation, because facers are necessary in the production process. Note that the foam insulation can compress and facers can delaminate when subjected to heavy traffic, therefore a cover board is always recommended. Also, facers act as vapor retarders, which may or my not be desirable.

Consider using the 25 psi product instead of the standard 20 psi. If subjected to a leak condition, polyisocyanurate will absorb moisture, lose its insulating value, and will have to be replaced in correction, unlike polystyrene, which can often be reused. Although more fire-resistant than polystyrene, it should be verified with the manufacturer if a thermal barrier (see next paragraph) is required. Polyisocyanurate is less expensive than extruded polystyrene.

Polystyrene: There are two types of polystyrene insulation: expanded polystyrene (sometimes referred to as EPS or bead-board) and extruded polystyrene (sometimes referred to as XPS). The two types have distinctly different properties. Polystyrene is one of the plastic foam insulations and should be used with caution where hot roofing materials are employed. The International Building Code requires a fire separation layer called a “thermal barrier” with polystyrene insulation used over a steel deck. This is usually a 1/2 inch sheet of gypsum directly above the deck.

Polystyrene boards should not be in direct contact with PVC membranes, otherwise the polystyrene will leach plasticizers out of the PVC. A suitable separator needs to occur between polystyrene and PVC.

Expanded Polystyrene (EPS): EPS is sometimes referred to as “molded expanded polystyrene” or “bead-board.” This is moderate R-value insulation (from slightly less to slightly more than R-4 per inch, depending upon density). The low-density product is relatively inexpensive. Solvent-based adhesive and hot asphalt disintegrate EPS. Hence, if either of these is used, a suitable cover board needs to be installed over the EPS. EPS can also be decomposed or melt at high temperatures. Therefore, EPS should not be used underneath a black membrane unless a suitable cover board is installed between the EPS and the membrane. The National Roofing Contractors Association (NRCA) recommends expanded polystyrene insulation intended for use as rigid board roof insulation have a minimum density of a nominal 1.25 pounds per cubic foot, that complying with ASTM C578, type VIII, having a minimum density of 1.15 pounds per cubic foot. EPS cells are filled with air. Therefore, unlike the other plastic foam insulations, EPS does not thermally age (i.e., loose R-value over time). EPS is not very resistant to water vapor; when exposed to water vapor drive, EPS can absorb a considerable amount of moisture.

Extruded Polystyrene (XPS, sometimes EXPS or XEPS): This is a high R-value insulation (R-5 per inch for products with a minimum compressive resistance of 25 psi, R-4.6 per inch for products with a minimum compressive resistance of 15 psi). Generally products with an R-value of 5.0 per inch thickness are used in roof applications. Some manufacturers offer XPS made from recycled XPS.