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Home & Basement

Wall Assembly R-Value Calculator

Build a simple wall or roof assembly and estimate the whole-assembly R-value. This accounts for thermal bridging through studs instead of only adding up the center-cavity insulation.

Last updated: May 2026

Unit system Switch between U.S. customary and metric units. Your choice is remembered on other calculators.

Wall size, framing, insulation, and openings

Start with the wall area, insulation, stud size, stud spacing, and approximate windows/doors. Advanced settings are available for sheathing, drywall, air films, and comparison targets.

sq ft
Use the gross area of the assembly you want to estimate before subtracting windows or doors.
R
This is the insulation between the studs, such as fiberglass, mineral wool, cellulose, or spray foam.
in
Use the actual framing depth, not the nominal lumber size. A 2x4 is usually 3.5 in; a 2x6 is usually 5.5 in.
in o.c.
Use the on-center spacing of the framing. This is more intuitive than entering a raw framing percentage.
windows
Use zero if the assembly has no windows. This helps estimate the full system, not just the insulated wall area.
sq ft
Approximate area per window. A 3 ft × 4 ft window is about 12 sq ft.
R
Continuous insulation goes over the framing and helps reduce thermal bridging through studs.
Advanced settings
sq ft
Optional. Add door area if you want the full system estimate to include exterior doors.
R
Approximate effective R-value for windows and exterior doors. Many windows are far lower than insulated walls.
%
Adds allowance for corners, headers, plates, blocking, and extra framing around windows/doors.
R
Approximate planning value for gypsum drywall or similar interior finish.
R
Approximate planning value for structural sheathing outside the studs.
R/in
Approximate softwood framing value. Wood varies by species and moisture content.
R
Planning allowance for surface air films. Leave this alone unless you have a reason to change it.
R
Use this only as a planning comparison. Code-required R-values depend on location, assembly type, and code version.

Buying and sizing notes

How to use this estimate

What this includes

  • Cavity insulation
  • Wood framing thermal bridging
  • continuous foam or insulated sheathing
  • drywall and sheathing
  • optional windows and doors

What it does not include

  • Air leakage testing
  • moisture analysis
  • thermal imaging
  • local code compliance
  • Manual J or energy modeling

Common mistakes

  • Treating cavity insulation R-value as the full wall R-value
  • Ignoring windows and exterior doors
  • Ignoring continuous insulation and thermal bridging

Quick questions

Why is whole-wall R-value lower than the cavity insulation label?

Wood framing conducts more heat than cavity insulation, so heat can bypass some of the insulated cavity through studs, plates, headers, and other framing.

Should windows be included?

Include windows and exterior doors if you want the full system estimate for that wall area. Leave them at zero if you only want the opaque wall assembly.

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