(Also see "Energy Codes".)
R-value is used as a measurement of the effectiveness of thermal insulating materials. The higher the R-value, the better a material performs as a thermal insulator. Building insulation, of course, is what we are normally referring to when we say "thermal insulating materials", but in a roof system the R-values of other components, such as underlayment, cover boards, and even some adhesives should be included when calculating the total R-value of the system.
Building and energy codes almost always specify a required minimum R-value for roof assemblies and exterior walls, so the minimum required R-value in your local jurisdiction is something that you need to take into account when planning a building project. 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.
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 greater the effectiveness of the insulation. The 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. Again, always consult the manufacturer's data sheets for the R-value of the actual product being used. Calculating the R-value of an assembly with different materials is usually as simple as adding the R-values of the individual layers.
1. General: Insulation R-Value: Here's a handy insulation R-value table that covers most types of building insulation and a few other building materials. You should, of course, see the manufacturer-provided technical data sheet for the R-value of specific products.
3. General: LTTR (Long-Term Thermal Resistance): "Long-Term Thermal Resistance" is an archived article from 2003 on Buildings.com 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.
4. General: Polystyrene 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.
5. 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 is from February of 2018 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.
6. Polyisocyanurate Insulation: this page, "LTTR/QualityMark", 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.
7. 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.
8. 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.