| Get to Know R-value |
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Design professionals routinely identify R-value or thermal performance as the most important factor when specifying insulation products. R-value is a measure of a product’s ability to resist heat flow and provides a means of comparing the effectiveness of various product options. A basic understanding of the details and science behind this important metric is key to helping professionals become more informed insulation users. The information below addresses common questions and answers on R-value and insulation materials.
What is R-value?R-value is a standardized rating used to report a material’s resistance to heat flow and is commonly associated with thermal insulation materials. A higher R-value means higher resistance to heat flow and greater insulating effectiveness. What factors determine a product's R-value?Several factors determine an insulation material’s R-value, including how the material is designed (open cell v. closed cell) and what constituent components are used to produce the material (foam v. fibers). Other factors that influence the R-value
of a specific material include age, temperature, quality of installation, and the standard used to determine R-value. For a deeper dive into these factors, click the buttons below. AGE TEMPERATURE INSTALLATION STANDARDSWhy is R-value important for insulation materials?R-value provides a means for users to compare the insulating effectiveness of various materials when the materials are evaluated using the same conditions. Generally, thermal resistivity is used to determine what thickness is necessary to achieve a required minimum or specified R-value. The required R-value is typically determined by a jurisdiction’s energy code or specified by the project’s design documents. The table below lists the thermal resistivity, or R-value per inch, of commonly used board insulation materials.
Once installed as part of the building envelope, insulation materials, or other materials that effectively resist heat flow, help keep our buildings warm in the winter and cool in the summer while using less energy and saving more money. Materials
that provide increased R-value per inch like polyiso insulation can deliver higher performance using standard construction practices when compared to alternatives. Other advantages include thinner building envelope assemblies and less
materials on jobsites.
How is polyiso's R-value determined?The thermal resistivity of insulation materials is determined using standardized test methods. These test methods are often prescribed in the relevant product standard or by the U.S. Federal Trade Commission’s R-value Rule. Additional standards
determine how to take age and temperature into consideration so that R-values are reported on an equal and comparable basis. The polyiso industry uses two primary procedures for determining thermal resistivity: (1) long-term thermal resistance (or LTTR) and (2) thermal resistance following a conditioning period (typically referred to as a 180-day or aged R-value).
Both procedures are appropriate for insulation materials with captive blowing agents (i.e., polyiso, extruded polystyrene (XPS), spray foam). What is the difference between an LTTR-value and an R-value?In simple terms, an LTTR-value is the predicted R-value of the product at an age of 5 years (which corresponds closely to the average thermal resistivity over a 15-year service life). The metrics both report the thermal resistivity of
the tested material, but at different times relative to the material’s age (5-years v. 180-days). More information on these standardized test methods and procedures can be found under the “Standards” button above. |
