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Construction glossary · Envelope and finishes

What is r-valuein construction?

R-value is the measure of an insulation material's resistance to heat flow, expressed per inch of thickness or for a whole assembly, where a higher number means more resistance. In construction it is how the spec defines required thermal performance and how the estimator matches products to the assembly. The key distinction is nominal R-value of the insulation versus the whole-assembly effective R-value after thermal bridging.

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Updated August 2026 · Reviewed by the Ruh construction team

Units hr·ft²·°F/Btu2x6 batt R-19 to R-21Higher R less heat flowSeries walls R-values add

R-value thermal resistance scale

R-13 (2x4 batt)R-19 (2x6 batt)R-21 (2x6 HD)R-30 (attic)R-38 (cold attic)R-49 (zone 6)R-30 typical attic
The definition

R-value, explained.

R-value quantifies thermal resistance: how well a material or assembly resists conductive heat flow. It is the inverse of U-factor, so an assembly with R-20 has a U-factor of 0.05. Insulation products carry a nominal R-value, often stated per inch (polyiso around R-6 per inch, XPS around R-5, fiberglass batt around R-3.1), and a wall or roof carries an assembly R-value that the energy code checks. The catch for an estimator is that the assembly value is not the simple sum of the labels, because studs and joists are thermal bridges that conduct heat around the cavity insulation. That is why specs increasingly pair continuous insulation (ci) with cavity batt: the ci layer runs unbroken and is credited near its full R-value, while the cavity layer is derated for framing. When a spec writes a requirement like R-10 ci plus R-19 cavity, you are reading both the product to carry and the assembly target it has to hit.

How it is measured

R-value is read off the spec and the wall and roof types, not measured in the field by the estimator. For materials it is stated per inch on the product data, so thickness times R-per-inch gives the board or batt value. For assemblies the energy code wants the whole-wall or whole-roof effective R-value, calculated by a parallel-path method that weights the insulated cavity against the framing using a framing factor (often around 25 percent for 2x6 at 16 inch on-center). On the drawings you find the requirement in the wall and roof type legends and the division 07 spec, and in the energy compliance documents (the COMcheck or the energy code tables). The estimator's job is to carry products whose stacked R-values meet that assembly target.

Worked example

A worked r-value example.

Worked example

Take a spec calling for R-10 continuous insulation plus an R-19 cavity in a 2x6 wall. The nominal stack reads R-10 + R-19 = R-29, but that overstates the wall because the studs bridge the cavity. Run the cavity layer as a parallel path: with a 25 percent framing factor, the insulated path is R-19 and the framing path is about R-6.9 for the 5.5 inch stud. The cavity layer's effective value is 1 divided by (0.75 / 19 + 0.25 / 6.9) = about R-13.2. The continuous R-10 runs unbroken, so it adds in series: R-13.2 + R-10 = about R-23.2 whole-wall effective, against an R-29 nominal. That gap is exactly why the spec adds continuous insulation. The estimator confirms the assembly target against the energy compliance documents before selecting the board and batt to carry.

How Ruh handles it

How Ruh handles r-value.

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R-value: frequently asked questions

Why is the assembly R-value lower than the sum of the labels?+

Because the studs and joists conduct heat around the cavity insulation. The wood frames are a thermal bridge at a much lower R-value than the batt, so the whole-wall effective number drops below the simple sum. Continuous insulation helps because it runs unbroken over the framing.

What is the difference between R-value and U-factor?+

They are inverses. R-value measures resistance to heat flow and U-factor measures conductance, so U equals 1 divided by R. Specs and energy code tables use both, so you convert between them. A higher R or a lower U both mean a better-insulated assembly.

Does the estimator calculate the assembly R-value?+

Usually the requirement comes from the energy compliance documents and the spec, and the estimator carries products that meet it. But knowing the parallel-path math lets you catch when a nominal stack will not hit the whole-assembly target, which is the kind of gap that bites at submittal review.

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Related terms

Rigid insulationBatt insulationEIFS (exterior insulation finish system)House wrap (WRB)

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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.

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