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.

Figure 1. This PMR has of two layers of XPS. The top board has a factory-applied mortar surface.

A cover board is usually required with XPS, to provide a surface to adhere the membrane (XPS is not available with facers). XPS is very resistant to water vapor drive. However, as with EPS, XPS should not be exposed to solvent, hot asphalt, or very high temperature. But unlike EPS, in order to avoid membrane splitting, XPS should not be used below a built-up or modified bitumen membrane (even if a cover board is installed over the XPS).

XPS is the only insulation suitable for use above the roof membrane in protected membrane roof (PMR) systems (see section on this topic). However, boards intended for PMRs need to be specifically manufactured for this application. Some minor water absorption may occur in boards located above the membrane during the roof’s service life. To account for the R-value reduction due to the water absorption, it is recommended that the roof designer reduce the board’s initial R-value by 10%.

XPS can often be re-used when re-roofing, because it does not absorb water when there are the inevitable leaks, distinguishing it from the more commonly used polyisocyanurate insulation. That, combined with the required use of a cover board with XPS, which increases durability, is why the life-cycle cost of XPS insulation is often better than polyisocyanurate insulation.

XPS boards with extremely high compressive resistance are available for use in plaza decks where high compressive loads occur. 40 psi is recommended for light pedestrian loads, 60 psi is recommended for heavy pedestrian loads with light vehicular traffic, 100 psi is recommended for heavy vehicular traffic.

Mineral wool: R-4 (approximate) is not seen very often in low slope roofs, except as a replacement for fiberglass insulation between roof rafters in residential construction. Cover board or a rigid upper layer is required that is integral with the mineral wool (or sheathing in residential construction).

Composite boards: Composite boards typically consist of two layers of different types of insulation that are laminated together in a factory. The primary insulation is typically polyisocyanurate or EPS. The secondary layer is typically perlite, wood fiberboard, oriented strand board (OSB), plywood, or gypsum board. Composite boards made with OSB or plywood are commonly referred to as “nail base.” Some nail base products have a small ventilation cavity between the primary insulation and the OSB or plywood. OSB and fiberboard composites are not recommended, as they do not withstand incidental wetting.

A nail-base insulation product should be checked to make sure it possesses adequate compressive strength and shear strength to withstand the loads expected for the roof system. For vented nail-base insulation, the product should be checked to make sure the spacer material and distance between spacer blocks provides adequate compressive strength, and the bearing surface of spacers provides adequate shear strength.

With some composite boards, the secondary layer (which is typically the top surface) is superficially adhered to the primary layer. With these boards, it is important to mechanically attach the composite board rather than adhere it. Otherwise the secondary layer could easily detach. The designer should understand that joints between the boards and the fasteners will represent a path for thermal bridging, therefore composite insulation is recommended to be installed over an underlying layer of non-composite insulation. The top layer composite insulation may be used in lieu of a separate insulation cover board.

For all types of insulation, is recommended to use multiple layers, with staggered joints. Joints over 1/8″ wide are usually filled with spray foam insulation, especially if there is only one layer of insulation. Consider using maximum 4 by 4 foot boards to reduce the gaps caused by shrinkage of the insulation.