Calculate total R-value, check building code compliance, and compare insulation materials.
Single Material
Layered Assembly
Find Thickness
Please enter a valid R/inch value.
Enter a positive thickness.
Add up to 5 insulation layers. Total R-value = sum of all layers.
Enter your target R-value and material to find the required thickness.
Enter a valid target R-value.
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Total R-Value
Property
Value
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Total Assembly R-Value
Layer
Material
Thickness
R/in
R-Value
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Required Thickness
Property
Value
How to Use This R-Value Calculator
Choose a tab based on your task: Single Material calculates the R-value for one insulation product at a given thickness; Layered Assembly adds up R-values across multiple layers (great for walls with sheathing + batt + rigid foam); Find Thickness works backward — enter a target R-value and get the required inches or centimeters of a specific material. Select your climate zone and building component to see whether your result meets 2021 IECC code minimums.
Why This Matters
Insulation is the single biggest lever most homeowners have over their energy bills. A poorly insulated attic in Climate Zone 5 (think Chicago or Denver) can lose 25–30% of a home's heating energy right through the ceiling — that's hundreds of dollars per winter. The Department of Energy recommends R-49 to R-60 for attics in those zones, yet many older homes sit at R-11 or R-19 from insulation installed in the 1970s.
Builders use R-value to specify products and bid jobs. An architect designing a high-performance wall assembly needs to sum the contributions of exterior rigid foam, cavity insulation, and even the wall sheathing itself to hit code. A DIY homeowner adding blown-in cellulose to an attic needs to know exactly how many inches to add on top of existing insulation. Real estate investors use R-value assessments to estimate upgrade costs before purchasing a property.
Getting this right also affects moisture control. Closed-cell spray foam at R-6.5/inch doubles as a vapor barrier; open-cell foam at R-3.7/inch does not. Choosing the wrong product for the right R-value can lead to condensation and mold — so knowing exactly what each material does is critical.
How It's Calculated
Basic formula: R-Value = R per inch × Thickness (inches) Layered assembly: R-Total = R₁ + R₂ + R₃ + ... (layers in series add directly) Reverse (Find Thickness): Thickness = Target R ÷ R per inch
R-value measures thermal resistance — how strongly a material resists heat flow. A higher number means better insulation. Each material has a characteristic R-value per inch (sometimes listed as RSI in SI units; 1 R = 0.176 RSI). When insulation layers are stacked in series (which is always the case in a wall assembly), their R-values add directly — no complex math required.
Code minimums in this calculator are based on the 2021 IECC (International Energy Conservation Code) prescriptive requirements by climate zone and component. Local jurisdictions may adopt earlier versions or add amendments, so always verify with your local building department.
Tips & Common Mistakes
Don't compress batts. Fiberglass batt compressed to fit a smaller cavity loses R-value proportionally. A 6-inch R-19 batt stuffed into a 5.5-inch cavity performs closer to R-15.
Account for framing. Wood studs have R-1.25/inch, much lower than cavity insulation. Real-world "whole-wall" R-value is 10–15% lower than cavity R-value alone due to thermal bridging.
Polyiso degrades in cold. Polyiso foam boards lose effective R-value in very cold temperatures. In Climate Zones 6–8, derate polyiso to ~R-5.5/inch rather than the rated R-6.5.
Air sealing comes first. Even R-60 attic insulation underperforms if air leaks bypass it. Seal all penetrations before adding insulation depth.
Check existing insulation. Before adding more attic insulation, measure what's there. Adding R-19 over existing R-11 gives R-30, not R-19 extra.
Frequently Asked Questions
What R-value do I need for my attic?
It depends on your climate zone. Zone 1–2 (Florida, Hawaii, southern Texas): R-30 minimum. Zones 3–4: R-38 to R-49. Zones 5–8 (most of the northern US and Canada): R-49 to R-60. The DOE recommends erring on the higher side — the incremental cost of adding more cellulose is small compared to lifetime energy savings.
Can I mix different insulation materials?
Yes, and it's very common. A typical high-performance wall might have R-13 fiberglass batt in the cavity plus R-5 rigid foam on the exterior — totaling R-18 continuous. Use the Layered Assembly tab to add them up. Just make sure the materials are compatible from a moisture and fire perspective.
What's the difference between R-value and U-factor?
R-value is thermal resistance (higher = better insulation). U-factor is thermal transmittance — essentially 1 ÷ R-value. Windows are typically rated by U-factor because they're thin and multiple layers (glass, gas, frame) make R-value less intuitive. A U-0.30 window equals approximately R-3.3.
Does insulation R-value degrade over time?
Most insulation materials (fiberglass, mineral wool, cellulose) retain their rated R-value for decades if kept dry. Polyiso foam can see modest long-term aging. Spray foam is generally stable. The biggest enemy of any insulation is moisture — wet insulation can lose 50%+ of its R-value and may not recover fully.