Wall Assembly R-Value Calculator

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

Assembly size, framing, insulation, and openings

Start with the assembly length and height/width, then enter insulation, stud size, stud spacing, and approximate windows/doors. Advanced settings are available for sheathing, drywall, air films, and comparison targets.

Visual guide

Which measurement goes where?

This calculator estimates the full assembly from length, height, cavity insulation, framing, sheathing, drywall, continuous insulation, and optional windows or doors.

Wall / assembly length

ft

Measure the gross length of the wall, roof section, or assembly being estimated.

Wall height / assembly width

ft

For walls, use wall height. For roof or floor sections, use the section width.

Cavity insulation R-value

R

Use the labeled R-value of the insulation in the stud cavity. Common examples are R-13/R-15 in 2x4 walls and R-21 fiberglass or R-23 stone wool in 2x6 walls.

Stud / rafter depth

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.

Stud / rafter spacing

in o.c.

Use the on-center spacing of the framing. This is more intuitive than entering a raw framing percentage.

Number of windows

windows

Use zero if the assembly has no windows. This helps estimate the full system, not just the insulated wall area.

Average window size

sq ft

Approximate area per window. A 3 ft × 4 ft window is about 12 sq ft.

Continuous foam / insulated sheathing

R

Continuous insulation goes over the framing and helps reduce thermal bridging through studs.

Advanced settings

Exterior door area

sq ft

Optional. Add door area if you want the full system estimate to include exterior doors.

Window / door R-value

R

Approximate effective R-value for windows and exterior doors. Many windows are far lower than insulated walls.

Extra framing at corners/openings

%

Adds allowance for corners, headers, plates, blocking, and extra framing around windows/doors.

Drywall / interior finish

R

Approximate planning value for gypsum drywall or similar interior finish.

Plywood / OSB sheathing

R

Approximate planning value for structural sheathing outside the studs.

Wood framing R-value per inch

R/in

Approximate softwood framing value. Wood varies by species and moisture content.

Interior/exterior air films

R

Planning allowance for surface air films. Leave this alone unless you have a reason to change it.

Optional comparison target

R

Use this only as a planning comparison. Code-required R-values depend on location, assembly type, and code version.

Buying and sizing notes

Center-cavity insulation is not the same as whole-wall performance because wood framing bypasses some of the insulation.
Windows and doors can pull the full-system R-value down sharply because they are usually much lower R-value than insulated wall areas.
Continuous exterior insulation or insulated sheathing is usually more effective than simply increasing cavity R-value because it covers the framing.
Air sealing, moisture control, vapor strategy, flashing, and installation quality can matter as much as labeled R-value.

Whole-wall R-value explained

A wall with R-21 cavity insulation is not automatically an R-21 wall. Studs, sheathing, drywall, continuous insulation, windows, and doors all affect the full system value.

Cavity vs assembly

Cavity insulation is only the insulated space between studs. Whole-wall R-value includes the framing path and other layers too.

Stud spacing matters

Closer stud spacing usually means more wood in the wall, which lowers the whole-assembly R-value compared with the center-cavity value.

Windows change the average

Even good windows usually have much lower R-value than insulated wall sections, so a wall with many openings can test lower overall.

Continuous insulation helps

Rigid foam or insulated sheathing outside the framing can improve the assembly because it covers the studs instead of only filling the cavities.

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
stud spacing and extra framing allowance

What it does not include

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

Common mistakes

Treating cavity insulation R-value as the full wall R-value
Ignoring windows and exterior doors
Ignoring continuous insulation and thermal bridging
Using code minimums without checking climate zone requirements

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.

What is continuous insulation?

Continuous insulation is insulation installed outside the framing, such as rigid foam or insulated sheathing. It helps reduce thermal bridging through studs.

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.

Is R-21 a good 2x6 wall assumption?

R-21 fiberglass is a common 2x6 wall batt starting point. The calculator still estimates whole-assembly performance, which will usually be lower than the cavity label because of framing and openings.

Does R-23 mineral wool exist?

R-23 stone wool/mineral wool products exist for 2x6 cavities, but they should be treated as a product-specific option rather than the generic default for every 2x6 wall.

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