Types of Above-Deck Roof Insulation
By Roof Online Staff • Updated June 3, 2023
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.](https://roofonline.com/wp-content/uploads/Roof-test-cut-showing-layers-of-insulation.jpg)
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. |
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. |