R-Value Calculator

R-value measures thermal resistance — higher R means better insulation and slower heat flow. In layered assemblies, total R is approximately the sum of each layer's published R-value when layers are continuous and thermal bridges are ignored. U-factor is conductance: U = 1/R_total, in BTU/h·ft²·°F — lower U means less heat transfer through the assembly.

Building codes specify minimum R for walls, floors, and attics by climate zone. This calculator adds up to four layers you enter — drywall, cavity insulation, continuous exterior board, sheathing, air films if labeled with an R-value, etc. — for a quick envelope estimate before comparing to code tables or energy program requirements.

Windows and doors are rated in U directly; opaque walls are often discussed in R. Converting wall R to U lets you compare opaque envelope performance to fenestration specs on the same scale. Energy modeling software applies correction factors for framing fraction — this tool does not.

Worked example with defaults: R-13 cavity insulation plus R-5 continuous exterior board sums to R-18 total. U-factor = 1 ÷ 18 ≈ 0.056 BTU/h·ft²·°F. Many climate zones require higher attic R than wall R — run separate calculations for each assembly rather than assuming one number covers the whole house. Attic R-49 with R-18 walls is common in cold zones; the attic entry here would use four layers if you model decking, batts, and vent space separately.

Optional layers left at zero are ignored. Use layer fields for each distinct material — do not combine unlike materials into one guessed R unless the manufacturer publishes a combined assembly rating. Spray foam and fiberglass listings assume proper installation thickness; compressed batts lose R. Sheathing, gypsum, and siding each add small R values that stack in real walls — include them when you are comparing to a published assembly chart.

Air gaps and convection loops reduce real performance compared to labeled R. Continuous insulation over framing reduces thermal bridging that cavity-only R sums miss. Vapor control and air sealing matter as much as R for moisture and comfort — high R with leaky construction still wastes energy. Infrared photos and blower-door tests reveal gaps that paper R-values alone cannot show.

Limits: not a substitute for REScheck, energy modeling, or manufacturer assembly tests. Metal framing, furring, and fastener bridges lower effective R. Consult IECC or local amendments for compliance — this is an arithmetic sum only, not a code determination.