Does this account for studs, rafters, or other thermal bridges?

No. It focuses on the insulation layer itself. Framing members conduct more heat than insulation, so the effective R-value of a whole assembly is usually lower than the cavity insulation R alone. Use this tool for quick thickness estimates, then consult assembly R-value tables or an energy professional for whole-system performance.

How do I find the correct R per inch for my product?

Check the insulation’s packaging, product data sheet, or manufacturer website. R per inch can vary by product type, density, and installation method—for example, dense-pack cellulose vs. loose-fill or open-cell vs. closed-cell spray foam.

Can I use this to prove code compliance?

No. While it can help you plan, code compliance is based on approved methods and documentation such as labeled R-values, installed depths, and sometimes blower-door results for air tightness. Always follow your local code official’s requirements.

What if my required thickness is greater than my framing depth?

You may need to add continuous insulation, build up the framing (for example, with raised-heel trusses or scabbed-on extensions), or choose a higher-R-per-inch product. Use the calculator to explore how those choices change the required thickness.

construction calculator

Insulation R-Value Calculator

Estimate insulation thickness needed to hit a target R-value.

Results

Required thickness (in): 10.86
Total R-value: 38.00
Coverage area (sq ft): 500.00

How to use this calculator

Check local energy code or your design goals to determine the target R-value for the space you are insulating—for example, R-38 for an attic in many climates or R-21 for certain wall assemblies.
Look up the R per inch for the insulation product you are considering (fiberglass batts, blown cellulose, spray foam, rigid foam board, etc.). If you are not sure, use a representative value from manufacturer literature.
Enter the area you plan to insulate in square feet (optional but helpful for calculating coverage and material counts).
Enter your target R-value and the insulation’s R per inch.
Review the required thickness in inches, along with the total R-value output, and compare it to your framing depth and construction details.
If the thickness does not fit your framing cavity or assembly, experiment with different products or multiple layers to see how they change the required thickness.

Inputs explained

Area (sq ft): The surface area you plan to insulate, in square feet. For a rectangular attic or wall section, multiply length by width. This field is mainly for pairing thickness with coverage when you plan material quantities.
Target R-value: The total R-value you want to achieve for the insulation layer, often dictated by local energy code or your comfort/efficiency goals (for example, R-38 for many attics, R-13 to R-23 for various wall assemblies).
Insulation R per inch: The thermal resistance provided per inch of thickness for the insulation type you are considering. Common values might be around R-3 to R-3.7 per inch for fiberglass batts, R-3.5 to R-3.8 for blown cellulose, and higher for spray foams and rigid boards. Always check the product label or technical data.

How it works

Each insulation product has a rated thermal resistance per inch—often listed as R/inch on packaging or in product data sheets.

To hit a target R-value for a layer of insulation, you divide the target R-value by the product’s R per inch to get the required installed thickness.

Thickness (inches) = Target R-value ÷ R per inch.

We report that required thickness and echo back the total R-value as a simple check. The optional area input is carried through as coverage so you can pair thickness with square footage for planning.

This model focuses on the insulation layer itself and does not attempt to model framing thermal bridging or multi-layer assemblies—that’s better handled with detailed energy modeling.

Formula

The calculation is straightforward:\n\n1. Required thickness (in) = Target R-value ÷ R per inch.\n2. Total R-value = Required thickness × R per inch (a consistency check).\n3. Coverage area is passed through from your input for pairing with manufacturer coverage tables.

When to use it

Estimating how deep loose-fill or batt insulation needs to be blown or installed to hit a code-required attic R-value.
Checking whether a proposed wall or roof assembly using a given insulation product can reach the desired R-value within available cavity depth.
Comparing different insulation types (fiberglass vs. cellulose vs. spray foam) by plugging in their R-per-inch values and seeing how thickness changes.
Providing homeowners or clients with a tangible sense of what “R-38” or “R-21” actually means in terms of inches of material.

Tips & cautions

Always cross-check your target R-values with the current version of your local energy code or building program; prescriptive R-values can change over time and vary by climate zone.
Use the manufacturer’s published R-per-inch values for the specific product and density you plan to install—generic values can be a good starting point but should not be used for final design.
Remember air sealing and moisture management: good R-value is important, but controlling air leaks and condensation risk can be just as critical for comfort and durability.
For attic retrofits, consider settling or fluff in blown products; installers often blow slightly above the rated depth to account for future settling to the target R.
This calculator looks only at a single insulation layer’s R-value and thickness; it does not model framing thermal bridges, sheathing, drywall, or layered assemblies.
Real-world R-values can differ from labeled values due to installation quality, compression, gaps, and moisture. Treat this as a planning tool, not a field measurement.
It does not address code details such as continuous insulation requirements, U-factor tradeoffs, or climate-specific dew point and condensation control strategies.
Area is used purely as a coverage reference; for precise material counts, you’ll need to combine thickness with manufacturer coverage rates and packaging information.

Worked examples

Example 1: Attic upgrade to R-38 with fiberglass batts

Target R-value = 38, insulation R per inch = 3.5.
Required thickness = 38 ÷ 3.5 ≈ 10.86 inches.
This suggests you need just under 11 inches of fiberglass batt or blown insulation to reach R-38 under ideal conditions.

Example 2: 2×6 wall cavity with higher-R insulation

Target R-value = 21, insulation R per inch = 4.2 (a higher-density batt or foam product).
Required thickness = 21 ÷ 4.2 = 5 inches.
A nominal 2×6 cavity (about 5.5 inches deep) can fit this thickness with a little room; you could hit R-21 within the stud bay.

Example 3: Comparing two attic insulation options

Option A: cellulose at R-3.7 per inch; Option B: fiberglass at R-3.2 per inch; target R-value = 49.
Option A thickness = 49 ÷ 3.7 ≈ 13.24 inches.
Option B thickness = 49 ÷ 3.2 ≈ 15.31 inches.
You can see that cellulose would reach R-49 in a shallower layer compared with the fiberglass product.

Deep dive

Use this insulation R-value calculator to estimate how many inches of insulation you need to hit a target R-value based on your chosen product’s R per inch.

Enter your target R-value and insulation R per inch, plus the area you plan to insulate, to get required thickness and coverage that you can use when planning attic, wall, or crawlspace upgrades.

Ideal for builders, energy auditors, and homeowners who want a quick way to translate code R-values into real-world insulation depth before ordering materials or talking to contractors.

FAQs

Does this account for studs, rafters, or other thermal bridges?: No. It focuses on the insulation layer itself. Framing members conduct more heat than insulation, so the effective R-value of a whole assembly is usually lower than the cavity insulation R alone. Use this tool for quick thickness estimates, then consult assembly R-value tables or an energy professional for whole-system performance.
How do I find the correct R per inch for my product?: Check the insulation’s packaging, product data sheet, or manufacturer website. R per inch can vary by product type, density, and installation method—for example, dense-pack cellulose vs. loose-fill or open-cell vs. closed-cell spray foam.
Can I use this to prove code compliance?: No. While it can help you plan, code compliance is based on approved methods and documentation such as labeled R-values, installed depths, and sometimes blower-door results for air tightness. Always follow your local code official’s requirements.
What if my required thickness is greater than my framing depth?: You may need to add continuous insulation, build up the framing (for example, with raised-heel trusses or scabbed-on extensions), or choose a higher-R-per-inch product. Use the calculator to explore how those choices change the required thickness.

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This insulation R-value calculator provides simplified thickness estimates based on user-entered R-value targets and R-per-inch ratings. It does not model full wall or roof assemblies, thermal bridging, moisture, or code compliance in detail and is not a substitute for stamped engineering, energy modeling, or local code review. Always verify insulation design with manufacturer specifications, current building codes, and qualified professionals before construction.