Exterior Wall Insulation Calculator: R-Value & U-Value

This calculator estimates the thermal performance of an exterior wall assembly by computing its R-value (thermal resistance) and U-value (thermal transmittance) from the materials you specify. You enter the thickness and R-value per inch of each layer (or select common materials), and the calculator returns the total R-value and U-value, which are the key numbers used in building energy codes and heat-loss calculations.

Formula

The total R-value of a wall assembly is the sum of the R-values of each layer:

R_total = R₁ + R₂ + … + R_n

where each layer’s R-value is its thickness (in inches) multiplied by its R-value per inch (in hr·ft²·°F/Btu·in). The U-value is the reciprocal of the total R-value:

U = 1 / R_total

Units: R-value is in hr·ft²·°F/Btu (imperial) or m²·K/W (metric). U-value is in Btu/hr·ft²·°F (imperial) or W/m²·K (metric). To convert R (imperial) to R (metric), multiply by 0.1761. To convert U (imperial) to U (metric), multiply by 5.678.

Worked Example

Using the default inputs:

• Layer 1: fiberglass batt, R-value per inch = 3.5, thickness = 3.5 in → R = 3.5 × 3.5 = 12.25
• Layer 2: plywood sheathing, R-value per inch = 1.0, thickness = 0.5 in → R = 1.0 × 0.5 = 0.50
• Layer 3: exterior siding (e.g., vinyl), R-value per inch = 0.25, thickness = 0.75 in → R = 0.25 × 0.75 = 0.1875

Total R = 12.25 + 0.50 + 0.1875 = 12.94 hr·ft²·°F/Btu

U-value = 1 / 12.94 = 0.0773 Btu/hr·ft²·°F

This matches the calculator’s output for the default values.

How to Use This Calculator

Enter the R-value per inch and thickness for each layer of your wall assembly, from interior to exterior. Common R-values per inch (imperial):

• Fiberglass batt: 3.1–3.5
• Rock wool batt: 3.0–3.3
• Closed-cell spray foam: 6.0–6.5
• Open-cell spray foam: 3.5–3.7
• Extruded polystyrene (XPS): 5.0
• Expanded polystyrene (EPS): 3.6–4.0
• Polyisocyanurate: 5.6–6.0
• Plywood: 1.0–1.1
• OSB: 0.9–1.0
• Gypsum drywall: 0.5–0.6
• Concrete: 0.08–0.12
• Brick: 0.11–0.20
• Wood (softwood): 1.0–1.2

For air films and air gaps, use approximate R-values: interior air film = 0.68, exterior air film = 0.17, unventilated air gap = 1.0–1.5. Add these as extra layers if you want a more accurate whole-assembly value.

Reading the Result

The total R-value tells you how well the wall resists heat flow—higher is better. The U-value tells you how much heat passes through per square foot per hour per degree Fahrenheit difference—lower is better. Building codes in the U.S. typically specify minimum R-values for walls by climate zone (e.g., R-13 to R-21 in most zones). The U-value is used in heat-loss calculations: Q = U × A × ΔT, where Q is heat flow in Btu/hr, A is area in ft², and ΔT is the temperature difference in °F.

Common Mistakes and Limitations

• Unit mix-ups: R-value per inch is often quoted as R-3.5 per inch, but some sources give R-value for a specific thickness (e.g., R-13 for 3.5-inch batt). Make sure you use per-inch values here.
• Ignoring thermal bridging: Wood studs have much lower R-value than insulation (about R-1.25 per inch). A wall with studs every 16 inches has a whole-wall R-value roughly 15–25% lower than the insulated cavity alone. This calculator gives the clear-wall value, not the framed-wall value.
• Compression: Squeezing batt insulation reduces its R-value. If you compress a 3.5-inch batt into a 2.5-inch cavity, you get less than the rated R-value.
• Moisture: Wet insulation loses R-value significantly. This calculator assumes dry materials.
• Air leakage: Real walls leak air, which bypasses insulation. This calculator only covers conduction, not convection or infiltration.

Comparisons: Insulation Types

Choosing insulation involves more than R-value. Consider:

• Fiberglass: Cheap, widely available, but needs careful installation to avoid gaps and compression.
• Rock wool: Slightly higher R per inch, fire-resistant, repels water, but costs more.
• Spray foam: Highest R per inch (closed-cell), also seals air leaks, but expensive and requires professional installation.
• Rigid foam boards: Good for exterior sheathing, add R-value without taking interior space, but need proper taping and rain-screen detailing.
• Cellulose: Blown-in, good for retrofits, but settles over time and can absorb moisture.

For exterior walls, a common high-performance approach is to put rigid foam on the exterior side of the sheathing to reduce thermal bridging, then batt or spray foam in the stud cavity.

Cost and Payback Considerations

While this calculator doesn’t compute cost, you can use the U-value to estimate annual heating/cooling savings. The heat loss through a wall is Q = U × A × ΔT (Btu/hr). Multiply by hours of heating season and divide by furnace efficiency to get fuel use. Typical payback for adding insulation to an existing wall is 5–15 years, depending on climate, energy prices, and installation cost. However, for new construction, upgrading from R-13 to R-20 often costs only a few hundred dollars and pays back in a few years.

Safety and Practical Tips

• When adding insulation, always follow local fire codes—some foams require a thermal barrier (e.g., drywall) over them.
• Wear gloves, long sleeves, and a dust mask when handling fiberglass or mineral wool.
• Do not cover electrical wiring with insulation that is not rated for direct contact (e.g., some spray foams are not rated for that).
• Ensure proper ventilation and vapor barriers in cold climates to prevent condensation inside the wall.
• If you’re insulating an existing wall, check for knob-and-tube wiring first—it must be replaced before insulation is added.

Frequently Asked Questions

What is a good R-value for exterior walls?

In the U.S., the International Energy Conservation Code (IECC) recommends R-13 to R-21 for wood-framed walls in most climate zones. For colder zones (6-8), R-20 to R-30 is recommended, often achieved by adding exterior rigid foam. Check your local code for exact requirements.

How do I convert R-value to U-value?

Simply take the reciprocal: U = 1/R. For example, R-20 → U = 0.05. If you have metric R-value (m²·K/W), divide 1 by that to get U in W/m²·K.

Does the calculator include air films and framing?

No, it only sums the layers you enter. For a more realistic whole-wall value, add interior/exterior air films (0.68 and 0.17) and account for framing by using an adjusted R-value (multiply clear-wall R by 0.75–0.85 for 16-inch stud spacing).

Can I use this for metal studs or steel framing?

Metal studs conduct heat much more than wood, creating significant thermal bridging. The calculator will overestimate the R-value. For steel studs, use a derating factor of about 0.5–0.6, or better, consult ASHRAE tables.

What’s the difference between R-value and U-value?

R-value measures resistance to heat flow—higher is better. U-value measures heat flow per area per temperature difference—lower is better. They are reciprocals. R-value is used for comparing insulation levels; U-value is used in heat-loss equations and window ratings.

How do I measure the R-value of an existing wall?

You can’t directly measure R-value without specialized equipment. Instead, identify the wall layers (e.g., by drilling a small hole or looking at an outlet box) and look up each material’s R-value per inch, then multiply by thickness and sum. This calculator does exactly that.

Related

• Building Square Footage Calculator: How to Measure
• Building Square Calculator: How to Find Area
• Buco Concrete Calculator: How to Use It

in r1 | Layer 1 R-value per inch | hr·ft²·°F/Btu·in | 3.5 | 0
in t1 | Layer 1 thickness | in | 3.5 | 0
in r2 | Layer 2 R-value per inch | hr·ft²·°F/Btu·in | 1.0 | 0
in t2 | Layer 2 thickness | in | 0.5 | 0
in r3 | Layer 3 R-value per inch | hr·ft²·°F/Btu·in | 0.25 | 0
in t3 | Layer 3 thickness | in | 0.75 | 0
out rv | Total R-value | hr·ft²·°F/Btu | (r1*t1)+(r2*t2)+(r3*t3) | 2
out uv | U-value | Btu/hr·ft²·°F | 1/((r1*t1)+(r2*t2)+(r3*t3)) | 4