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The following table lists the types of insulation that are used in low-slope roofing (or found in existing roofs). These insulation materials usually take the form of 4’ x 4’ or 4’ x 8’ rigid boards. They are normally secured with adhesives or fastener systems, but they may be loose-laid in ballasted roof systems, which depend on the weight of the ballast to hold the entire system in place.

These types of insulation are used in above-deck insulation applications, which means that they are installed over the roof deck, and are considered a part of the roof system or roof assembly. Steep-slope or pitched roof systems will almost never include insulation; in steep-slope construction, the insulation will be installed below the roof or above the ceiling.

This is the hole where a roof core sample was taken on an old built-up roof. You can see a layer of expanded polystyrene and gypsum board in the first roof system (at the bottom), and a layer of yellow rigid board fiberglass insulation in the second roof application (above).

This is the hole where a roof core sample was taken on an old built-up roof. You can see a layer of expanded polystyrene and gypsum board in the first roof system (at the bottom), and a layer of yellow rigid board fiberglass insulation in the second roof application (above).

Types of Above-Deck Roof Insulation
Insulation Type R-Value per Inch Typical Colors Notes
Cellular Glass 3.4 Charcoal, Gray Also called "foam glass". Extremely fire-resistant.
Fiberglass (Rigid Board) 4.3 - 4.5 Orange, Yellow Also called "glass wool". Commonly found in older built-up roofs. Not common today.
Gypsum Board 0.90 - 1.12 Gray, White Used as a cover board or thermal barrier.
Lightweight Insulating Concrete 0.90 - 1.49 Gray "LWIC". Can be sloped and shaped during application. Much softer than structural concrete.
Mineral Wool 3.8 Gray, Tan, Yellow Also called "mineral fiber", "rock wool", and "stone wool". Very fire-resistant. Can have high recycled content.
Perlite Board 2.64 Brown, with whitish beads throughout More fire-resistant than regular fiberboard. Often used as a cover board.
Phenolic Foam Board 6.7 - 7.5 Orange NOT used in roofing any more. When it got wet it caused severe corrosion in steel roof decks. Many lawsuits resulted.
Polyisocyanurate 5.7 Yellow Extremely popular in commercial roof installations due to ever-increasing energy code R-value requirements.
Polyisocyanurate,
High Density
4.8 Yellow Much harder than regular polyiso. Typically used as a cover board.
Polystyrene, Expanded
(EPS)
3.85 - 4.5 White Also called "Styrofoam" or "beadboard".
Polystyrene, Extruded
(XPS)
5.0 - 5.5 Blue, Green, Pink, White, Yellow Very water-resistant. Common in green roof and protected membrane roof assemblies.
Polyurethane Foam, Closed Cell 4.9 - 7.1 Yellow, Orange Spray Polyurethane Foam. Spray-applied. Considered a roof system in its own right. Requires a protective silicone coating.
Wood Fiberboard 2.5 Brown Currently is typically used only as a cover board.