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”
| 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
| 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”
| 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
| 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 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 | |