What, exactly, is R-value?
If you’ve ever purchased insulation or even walked by it in a big box store, you probably know that it’s labeled with an R-value. But what does that mean? How is it measured? How do you know which R-value you need?
We’ve got all of that, plus more, in this blog, chock full of information about R-value.
The “R” in R-value stands for resistance to heat flow. Simply put, the higher the R-value – or the larger the number – the greater a product’s insulating power.
As the first standardized method for measuring resistance to heat flow, R-value was first proposed in 1945 at Penn State. It became more broadly accepted and used in the 1970s when the Federal Trade Commission issued its R-value Rule, which requires home insulation manufacturers, professional installers, new home sellers and retailers to provide R-value information, based on the results of standard tests, to help inform consumers.
J.R. Babineau, Senior Research Manager, Building Science, Research and Development for Johns Manville, explained the test behind determining R-value of insulation.
There are many ways to measure R-value, but at its simplest: “We take a piece of insulation and put it between two metal plates. One is hot and the other is cold – for insulations in the U.S., those temperatures are a stable 100°F and 50°F,” Babineau said. “Then we measure how much power it takes to keep the hot plate hot, because of course heat wants to flow and travel through the insulation to the cold plate. That’s thermodynamics, with hot going to cold.”
From there, the amount of energy needed to keep the hot plate at its steady temperature is measured, and the thermal conductivity can be calculated. The material’s thickness is divided by its thermal conductivity, determining the R-value.
“The less extra energy we have to put into the plate, the better the insulation, because it’s resisting the heat flow,” Babineau said.
The U.S. Department of Energy, building energy codes and EPA’s ENERGY STAR® all recommend home insulation R-values based on a climate zone map. Within each climate zone there are recommended R-values for different portions of a home (attics, walls, crawlspaces, etc.).
“Climate zone maps for the energy code are driven by heating degree days and cooling degree days,” Babineau said. A home in Florida, for example, requires less insulation than a home in Minnesota.
The EPA estimates that homeowners can save an average of 15% on heating and cooling costs (or an average of 11% on total energy costs) by adding insulation in attics, floors over crawlspaces and basements, as well as air sealing. Older homes nowadays likely need more insulation than when they were originally built, as codes and requirements have changed over the years.
Determine how much insulation you need and get product recommendations with the Johns Manville Insulation Calculator. Fill in a little info and you’ll get the recommended R-value for your project and details on how much JM insulation you’ll need, plus details on where to buy product.
Improving your home’s insulation offers a big return on investment. For the 2025 tax year, the Energy Efficient Home Improvement Credit through the Inflation Reduction Act is available to homeowners.
Install insulation (or other energy efficiency upgrades) and receive a tax credit equal to 30% of the cost of insulation materials installed, up to $1,200. In addition to the tax credit, homeowners will see additional savings by lowering their energy bills month after month, year after year.
Johns Manville has a webpage dedicated to the Inflation Reduction Act. Check it out for examples, resources and more information.
