Compare polyiso board thickness, R-values, insulation brands and tapered systems for TPO and modified-bitumen roofs.
The insulation below a flat roof affects heat flow, roof height and drainage. This guide compares above-deck insulation for low-slope roofs. R-value measures resistance to heat flow: a higher rating means more thermal resistance under the stated test conditions.
What goes under the membrane?
GAF’s EverGuard TPO adhered-system cutaway illustrates the roof as separate layers. The membrane sheds water, insulation provides thermal resistance, and the deck supports the assembly. Cover boards, tapered insulation, vapor control and attachment depend on the selected system.
Illustration: GAF. View the manufacturer’s roof-system specifications.
Polyiso thickness and R-value
These are published LTTR ratings for Johns Manville ENRGY 3.E flat boards. LTTR means long-term thermal resistance. Use the exact product data sheet rather than multiplying every board thickness by six.
| Board thickness | Published R-value |
|---|---|
| 1 inch | R-5.7 |
| 1.5 inches | R-8.6 |
| 2 inches | R-11.4 |
| 2.2 inches | R-12.6 |
| 2.6 inches | R-15.0 |
| 3 inches | R-17.4 |
| 3.5 inches | R-20.5 |
| 4 inches | R-23.6 |
How multiple layers add up
The examples below add the nominal board ratings above. Total thickness excludes the membrane, cover board and any extra tapered insulation. These are comparison examples, not a universal energy-code requirement.
| Two-layer example | Total insulation thickness | Added board R-value |
|---|---|---|
| 1.75 + 1.75 inches | 3.5 inches | R-20.0 |
| 2.2 + 2.2 inches | 4.4 inches | R-25.2 |
| 2.6 + 2.6 inches | 5.2 inches | R-30.0 |
| 3 + 3 inches | 6 inches | R-34.8 |
| 3.5 + 3.5 inches | 7 inches | R-41.0 |
The complete roof’s effective thermal performance can differ because of fasteners, joints, temperature and moisture. Layer layout and attachment should be specified together. Board R-value alone does not establish whole-building energy performance.
Insulation brands to compare
| Manufacturer / family | Material | What to specify |
|---|---|---|
| GAF EnergyGuard | Polyiso | Exact board, facer, thickness and approved membrane assembly. |
| Johns Manville ENRGY 3.E | Polyiso | LTTR rating, flat or tapered layout and compressive grade. |
| Carlisle InsulBase | Polyiso | Product configuration, attachment and system compatibility. |
| Elevate ISOGARD | Polyiso | Exact regional product, facer and published rating. |
| Insulfoam | EPS and graphite-enhanced EPS options | Density, product rating and roof assembly approval. |
| ROCKWOOL TOPROCK DD | Stone wool | Board thickness, loading and compatible membrane system. |
These are product families available for comparison. The specified roof system determines which insulation, adhesive, cover board and membrane can be combined. Availability and manufacturer warranty eligibility are confirmed for each project.
Polyiso, EPS, XPS or stone wool?
| Insulation type | Why consider it? | What to check |
|---|---|---|
| Polyiso | High thermal resistance in a relatively thin roof build-up. | Exact LTTR rating, facer and temperature-dependent performance. |
| EPS / graphite EPS | An alternative rigid foam available in different densities and tapered layouts. | Product-specific R-value, compressive strength and membrane compatibility. |
| XPS | Rigid extruded foam used in specified roofing assemblies, including some protected-membrane designs. | Use a roof-rated product and approved assembly; a general-purpose foam board is not automatically suitable. |
| Stone wool | A noncombustible insulation material with acoustic benefits. | ROCKWOOL TOPROCK DD lists R-3.8 per inch at 75°F mean temperature. Check product and system requirements. |
| Gypsum cover board | Provides a durable substrate in approved systems. | Primarily a cover board, not a substitute for the main thermal insulation. |
| High-density polyiso cover board | Combines a cover-board function with its own thermal rating. | Use its stated R-value; do not substitute the rating of ordinary polyiso insulation. |
A noncombustible insulation board does not make every complete roof assembly fireproof. Compare tested assemblies and the building’s requirements, not a single material claim.
Tapered insulation: slope plus insulation
Tapered boards vary in thickness to direct water toward drains or scuppers. Crickets help redirect water around obstructions. The thinnest point, average thickness and thermal design are different measurements.
| Ask your roofer | Why it matters |
|---|---|
| What is the minimum thickness at drains? | The low points may have less insulation than the high points. |
| Is continuous flat insulation included below the taper? | Separates the thermal base layer from the drainage layout. |
| What are the maximum heights? | Affects curbs, flashings, edge details and transitions. |
| How was the R-value checked? | An average tapered thickness is not automatically proof of compliance. |
Insulation below the roof deck
Batts and spray foam below the deck require a different assembly review. Air leakage, vapor control and condensation must be considered before combining above-deck and below-deck insulation. More insulation is not a reason to trap wet material in an existing roof.
What should your estimate say?
| Include in the quote | Clear description |
|---|---|
| Product | Manufacturer, board name, facer and strength grade. |
| Thermal build-up | Number of layers, thickness of each layer and stated R-value. |
| Drainage | Flat base, taper plan, crickets, drains and minimum thickness. |
| Protection and attachment | Cover board, fastening or adhesive, and approved assembly. |
| Existing conditions | Wet-insulation removal, deck repairs and height constraints. |
Barnes N’ Roofs can compare suitable insulation assemblies alongside your TPO or modified-bitumen roof estimate. The right target depends on the building, applicable requirements, existing conditions and the approved roofing system.
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Product references
Johns Manville thickness and LTTR table
GAF EnergyGuard polyiso data sheet