Back to Basics: R-Value and U-Value

NIA’s Metal Building Laminator Committee

This article was written by a task force from NIA’s Metal Building Laminator Committee. The member companies that make up this committee are: • Bay Insulation Systems, Inc. (www.bayinsulation.com) • DAW Construction Group, LLC (www.dawcg.com) • The E.J. Davis Company (www.ejdavis.com) • International Insulation Products, LLC (www.iiproducts.com) • L&L Insulations (www.llinsulation.com) • Silvercote, A Service Partners Company (www.silvercote.com) • Therm-All, Inc. (www.therm-all.com) You can learn more about metal building insulation at www.insulatemetalbuildings.org. Read the newly revised NIA Certified Faced Insulation® Standard (Version 404.2–2024) at www.insulation.org/mblstandard. To learn more about NIA’s Metal Building Laminator Committee, or to join, email membership@insulation.org. For questions or comments about any of the articles, please email publications@insulation.org.

September 1, 2024

The R-value of an insulation product is a thermal resistance rating that tells us how well a material resists heat flow. The higher the R-value, the better the insulation restricts heat transfer. Most insulation manufacturers use the ASTM C518 test to measure a material’s resistance to heat flow. Established by the American Society for Testing and Materials (ASTM), the ASTM C518 test is a standardized method that assesses the steady-state thermal transmission properties of a material using a heat flow meter apparatus. In addition to serving as an essential tool for many industries, ASTM C518 is a building code requirement for insulation.

R-values for metal building insulation range from R-8 to R-30. The most common fiberglass metal building insulation R-values include:

  • 2” (R-8, before lamination)
  • 3.4” (R-10)
  • 3.7” (R-11)
  • 4.3” (R-13)
  • 5.3” (R-13)
  • 6.3” (R-19)
  • 8.0” (R-25)
  • 9.25” (R-30)

It should be noted that thicknesses are nominal, as manufacturers produce different thicknesses to meet R-values. Commercial energy code requirements dictate the minimum R-value necessary for all new construction, additions, and alterations performed in the United States.

Once the R-value is determined, designers can use this data to better understand how a material will perform in an assembly. Heat flow through a material is driven by the temperature difference on each side of the material or assembly. Most construction types and code tables leverage the R-value table as a guide to code compliance. However, it is important to remember that the R-value table should not be mistaken as a reference for the total performance of an assembly. Metal-framed walls, wood-framed walls, and metal building walls and roofs undergo testing that determines the U-value of the entire assembly. The result of that testing can be found in the U-value table.

A U-value, also called a U-factor, measures heat transfer, or loss, through the assembly over time. While the R-value measures a single product’s ability to resist heat, the U-value measures the entire assembly’s resistance to heat transfer. The lower the U-factor, the better the insulation. Mathematically, R-values and U-values are reciprocals of one another: U-value = 1/R-value, R-value = 1/U-value. ASTM C1363 is the standard test method of U-values and includes a hot box apparatus. See Figure 1 and Figure 2.