Rigid Insulation Overview

Most flat roofing systems require some kind of rigid insulation under the new roofing system. The biggest reason is that all flat roofing systems require a substrate between the structural decking and the new roof system. The second and more obvious reason is for higher R-values, which building owners are asking for, and building code throughout New England is requiring.

The most common types of flat roofing rigid insulation are below. For more information, click on the name of the insulation.

Polyisocyanurate Rigid Insulation “ISO”

Polyisocyanurate Insulation

R-Value per inch6
CostMid Range, but easily the best value.
StrengthMedium
Typical UseAny (Roof Recover, Re-roof, etc)
AdvantagesHigh R-Value per inch (6), Less than 1% water absorption rate, lightweight, holds up well over time, strong enough for most roof applications
DisadvantagesHigher price than EPS or Fiberboard
Can you taper it?Yes

HD Coverboard

R-Value per inch5
CostHigh
StrengthHigh
Typical UseAny (Roof Recover, Re-roof, etc)
AdvantagesStrong 100psi product that resists punctures and crunching, High R-Value per inch (5), less than 1% water absorption rate, lightweight, holds up well over time
DisadvantagesExpensive
Can you taper it?Yes (add a layer on top of tapered polyisocyanurate)

Expanded Polystyrene Rigid Insulation “EPS”

Flat EPS Coverphoto

R-Value per inch4
CostInexpensive
StrengthPoor
Typical UseRoof Recover or Flute Fill on metal roofs
AdvantagesLow Cost, Highly Customizable
DisadvantagesHighly flammable, absorbs more water than polyisocyanurate, breaks apart or crushes easily.
Can you taper it?Yes

Fiberboard Rigid Insulation

Fiberboard Insulation

R-Value per inch4
CostInexpensive
StrengthPoor
Typical UseRoof Recover
AdvantagesLow Cost
DisadvantagesSoaks up water, breaks apart, can shrink, find its way into the building
Can you taper it?No

Tapered Rigid Insulation

Tapered Insulation

Tapered Insulation Summary

R-Value per inchVaries Based on Product.
CostVery High.
StrengthVaries Based on Product.
Typical UseEliminating water on a ponded roof.
AdvantagesSignificantly reduces ponding, typically has greater R-value, follows advantages of product used.
DisadvantagesSignificantly higher materials cost, significantly higher labor cost, follows disadvantages of product used.
Products that can be tapered