Roof R-Value 101: Important Facts and Guidelines

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R-Value Formula
R-Value Formula
3.5 inches composite polyiso insulation board. 3 inches regular polyiso + 1/2 inch high density polyiso cover board. Total roof R-value = 19.5.
3.5 inches composite polyiso insulation board. 3 inches regular polyiso + 1/2 inch high density polyiso cover board. Total R-value = 19.5.

Roof R-Value Basics

What is R-Value?

R-value is used as a measurement of the effectiveness of thermal insulating materials.

An insulating material’s resistance to conductive heat flow is measured or rated in terms of its thermal resistance, or R-value (defined as ft²·°F·hr/BTU; or square feet · degrees Fahrenheit · hour/BTU; or area · temperature difference · time/heat loss).

The higher the R-value, the better a material performs as a thermal insulator.

R-value depends on the type of material, its thickness, and its density.

Some types of insulation will have different R-values for the same thickness due to differences in density, and some will even increase in R-value per inch as the total thickness increases.

What is Included in the R-Value of a Roof System?

Building insulation, of course, is what we are normally referring to when we say “thermal insulating materials”. Low-slope (“flat”) roof systems almost always include a layer (or layers) of building insulation. This is the primary source of the roof’s total R-value.

In a commercial roof system the R-values of other components, such as cover boards and certain foam adhesives can often be included when calculating the total R-value of the system. Energy codes permit this.

The roof deck, most underlayment, most roof coverings/membranes, and air films cannot be included when calculating the R-value of the system. Spray polyurethane foam is one roof covering than can be included, because it doubles as building insulation.

How to Find the R-Value of a Roof

To calculate the total roof R-value, find the total thickness in inches of each type of insulating material within the roof system.

For each different material, multiply the thickness in inches by the standard R-value per inch for that material.

Add the results for all the materials together to get the total R-value for the roof assembly.

It’s fairly simple.

Again, remember that when you’re calculating roof R-value for energy code compliance, the R-value of roof membranes, shingles, tiles, air films, and roof deck materials probably can’t be included in your calculation of total roof R-value for code purposes.

Always check your local code to be sure, but these parts of the roof are almost always excluded. Generally only the R-value of actual insulation products can be used.

Minimum R-Value Requirements

Building and energy codes almost always specify a required minimum R-value for exterior walls and roof assemblies with above-deck insulation. These minimum R-value requirements vary by location and climate zone.

The minimum required R-value in your local jurisdiction is something that you need to take into account when planning a roofing project.

The energy code R-value requirements can be different from state to state and even city to city. When new energy codes are adopted, the required R-value is often increased.

The R-value you need for the new roof system will be a big part of figuring out the type and thickness of the insulation you will have to install.

R-Value for Pitched Roofs

Steep-slope (or “pitched”) roofs will almost never include insulation as a part of the roof assembly. For pitched roofs, energy codes are concerned with ceiling or attic insulation R-value, not roof R-value.

While roof coverings such as asphalt or wood shingles do have a very small as-installed R-value, this is not used when calculating the attic/ceiling R-value as required by code.

The insulation which provides the R-value in pitched-roof structures will be underneath the roof deck and therefore considered attic insulation, or it will be part of the ceiling assembly. See the links below for building code information about R-values and residential construction.

Check with the Manufacturer!

Since R-value is so often used to calculate the amount and type of insulation required for a job, it’s an important part of the product data provided by insulation manufacturers, and can normally be found on the technical data sheets at their websites.

When looking at these data sheets, you should remember that although the standard R-value for a given material is per inch, manufacturers often publish the total R-value of a product, reflecting the actual thickness of that particular product.

Always consult the manufacturer’s data sheets for the R-value of the actual product being used.

1. GeneralR-Value of Various Materials: Here’s a handy roof R-value table that covers most types of roof coverings, building insulation, and a few other materials. You should, of course, see the manufacturer-provided technical data sheet for the R-value of specific products.

2. General: For a very good introductory overview article on roof R-value, see “R-Values” over at the website of the EPS Industry Alliance. Fairly short and not too technical.

3. General: See Roof Online’s Cool Roofs page for more about roofs and heat.

4. GeneralLTTR (Long-Term Thermal Resistance): “Long-Term Thermal Resistance” is an archived article from 2003 on that does a very good job explaining how LTTR is defined and how it came to be incorporated in the standards governing closed-cell insulation (i.e., polyisocyanurate, extruded polystyrene, and sprayed polyurethane foam). Again, the article is from 2003, and parts of it may not reflect the current state of affairs.

5. GeneralPolystyrene and Polyisocyanurate: “Thermal Drift of Polyiso and XPS” by Martin Holladay is an exceptionally well-written article from 2016. Very easy to follow, objective, and the author efficiently conveys a lot of information about closed-cell foam insulations. Available at Green Building Advisor, a great site that we recommend.

6. Building CodesInternational Residential Code: “Section N1102 (R402) Building Thermal Envelope” is a must-read for residential construction. “N1102.1.3 (R402.1.3) R-value computation” may be what you’re looking for in particular. Always check with your local building authority to confirm which codes are in effect in your area. (Links go to the UpCodes website.)

7. Energy Codes: “Minimum Insulation R-value Requirements Non-Residential, Above Roof Deck” is an extremely handy table for state and local jurisdictions in the United States. The document was updated in 2021 and is available on the Carlisle SynTec website. Always double-check with your local authority to confirm which code is currently in effect in your area. 

8. Polyisocyanurate Insulation: “Measuring the R-value of Polyiso Roof Insulation“, at the website of PIMA (Polyisocyanurate Insulation Manufacturers Association), discusses Long Term Thermal Resistance (LTTR), the ASTM standards underlying the R-values for polyisocyanurate insulation, and provides R-values for various thicknesses of polyiso, which increases in R-value per inch as total thickness of the installation increases.  

9. Polyisocyanurate Insulation: “New Polyisocyanurate R-values” is a 2-page 2016 Industry Issue Update from the National Roofing Contractors Association (NRCA). This Industry Issue Update explains why the NRCA recommends using a significantly lower R-value for polyiso than PIMA does. Recommended reading for anyone specifying polyiso insulation. 

10. Polyisocyanurate Insulation: “PIMA Performance Bulletin: Measuring the R-value of Polyiso Roof Insulation” disagrees with the NRCA’s polyiso R-value and explains why. From April 2016.