Types of Above-Deck Roof Insulation

By Roof Online Staff • Updated June 3, 2023

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About Roof Insulation

The roof insulation referred to on this page is the insulation used in low-slope (“flat”) roofing applications.

These types of insulation are used in above-deck insulation applications, which means that they are installed over the roof deck, under the roof covering, and are considered a part of the roof system or roof assembly.

Roof insulation is a necessary component in low-slope roof installations. Having roof insulation of the proper type and thickness is how a roof system achieves the minimum R-value requirement specified by local energy codes.

Insulation R-value is given as a rating per inch of thickness of the material and it varies significantly by insulation type. Polyisocyanurate has around twice the R-value of perlite board, for instance, so one inch of polyisocyanurate will provide about the same R-value as two inches of perlite.

Energy codes have become more demanding over the past few years and R-value requirements have increased significantly. As a result, new roofs now require more insulation than ever before.

In order to meet these new R-value requirements without stacking up layers of roof insulation within roof systems to the point that it becomes impractical or technically ill-advised, the lower R-value types of insulation have mostly disappeared from new roof installations.

The insulation materials listed below 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 stone or pavers to hold the entire system in place.

Steep-slope or pitched roof systems will almost never include insulation; in steep-slope construction, the insulation will be installed below the roof deck or above the ceiling of the space below the attic (at the attic floor).

Layers of roof insulation visible in a roof core sample hole.
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 (near the top).

The following table lists the types of insulation that are used in low-slope roofing (or found in existing flat roof systems).

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.
High Density
4.8 Yellow Much harder than regular polyiso. Typically used as a cover board.
Polystyrene, Expanded
3.85 – 4.5 White Also called “Styrofoam” or “beadboard”.
Polystyrene, Extruded
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.
  1. Energy Codes and Roofs
  2. R-Value and Roofs
  3. R-Value Table
  4. Types of Roofing