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 inch 6
Cost Mid Range, but easily the best value.
Strength Medium
Typical Use Any (Roof Recover, Re-roof, etc)
Advantages High R-Value per inch (6), Less than 1% water absorption rate, lightweight, holds up well over time, strong enough for most roof applications
Disadvantages Higher price than EPS or Fiberboard
Can you taper it? Yes

HD Coverboard

HD Coverboard

R-Value per inch 5
Cost High
Strength High
Typical Use Any (Roof Recover, Re-roof, etc)
Advantages Strong 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
Disadvantages Expensive
Can you taper it? Yes (add a layer on top of tapered polyisocyanurate)

Expanded Polystyrene Rigid Insulation “EPS”

Flat EPS Coverphoto

R-Value per inch 4
Cost Inexpensive
Strength Poor
Typical Use Roof Recover or Flute Fill on metal roofs
Advantages Low Cost, Highly Customizable
Disadvantages Highly flammable, absorbs more water than polyisocyanurate, breaks apart or crushes easily.
Can you taper it? Yes

Fiberboard Rigid Insulation

Fiberboard Insulation

R-Value per inch 4
Cost Inexpensive
Strength Poor
Typical Use Roof Recover
Advantages Low Cost
Disadvantages Soaks 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 inch Varies Based on Product.
Cost Very High.
Strength Varies Based on Product.
Typical Use Eliminating water on a ponded roof.
Advantages Significantly reduces ponding, typically has greater R-value, follows advantages of product used.
Disadvantages Significantly higher materials cost, significantly higher labor cost, follows disadvantages of product used.
Products that can be tapered