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Insulation R-Value Calculator

Calculate achieved thermal resistance from insulation thickness, compare against code target R-values from R-13 walls to R-60 ceilings, and find how many additional inches you need for fiberglass, cellulose, spray foam, and rigid foam.

Insulation Details

in
R-30
R-10R-60

Live Results

Current Achieved R-Value

R-29.8

Target Status

Short by 0.2

Target R-30

Additional Thickness Required

0.1 in

Same material type

Material Performance Rate

R-3.14/in

Fiberglass Batt

9.5" installed · Target R-30 · Single-layer estimate

How to Use This Calculator

  1. Select insulation material. Choose fiberglass batt, blown-in, cellulose, spray foam, or rigid foam — each has a different R-value per inch.
  2. Enter installed thickness. Measure the actual installed depth, not just the nominal batt label thickness.
  3. Set target R-value. Use the slider to match your climate zone code requirement — walls R-13 to R-21, attics R-38 to R-60.
  4. Review deficit thickness. If achieved R-value falls short, the calculator shows how many additional inches you need to add.

Formula & Example

R-value measures thermal resistance — how well a material slows heat transfer. For a single homogeneous layer, total R is simply thickness multiplied by the material's R-value per inch. This calculator applies that relationship to one insulation layer at a time.

Real wall and roof assemblies are composite systems. Heat flows through multiple layers in series — exterior sheathing, framing cavity insulation, interior drywall, air films, and sometimes continuous exterior insulation. When layers are stacked with no parallel paths, their R-values add together: a 2×6 cavity filled with R-21 batts plus R-5 exterior rigid foam yields roughly R-26 for that portion of the wall cross-section.

Wood-framed walls also create parallel heat paths. Studs conduct heat more readily than cavity insulation, reducing the effective whole-wall R-value below the labeled batt rating. That is why continuous insulation (ci) on the exterior — uninterrupted by studs — delivers better real-world performance than cavity-only upgrades at the same nominal R.

Material R/in = Selected insulation constant

Current Achieved R = Layer Thickness × Material R/in

Target Deficit = MAX(0, Target R − Current Achieved R)

Additional Thickness = Target Deficit ÷ Material R/in

Composite R (series) = R₁ + R₂ + R₃ + …

U-factor (whole assembly) = 1 ÷ R_total

US & Canadian Climate Zone Insulation Recommendations

Prescriptive minimum R-values vary by climate zone under the IECC (US) and NBC/ provincial codes (Canada). Use these ranges as planning reference points — always verify against your local amended code and energy compliance path.

AssemblyTypical Prescriptive RangeNotes
Attic / CeilingR-38 to R-60Higher zones and cold climates trend toward R-49–R-60
Wood-Frame WallR-13 to R-212×4 cavities typically R-13–R-15; 2×6 cavities R-19–R-21
Floor (over unconditioned space)R-25 to R-30Crawlspace and cantilevered floor assemblies
Basement / Crawlspace WallR-10 to R-15Interior or exterior continuous insulation common in cold zones
Cathedral CeilingR-30 to R-49Limited depth — spray foam or high-density batts often required

Reference values only — confirm prescriptive or performance compliance requirements with your jurisdiction before ordering material.

Worked Example

R-13 fiberglass batt at 3.5 in depth in a 2×4 wall achieves R-13. To reach R-20 in the same cavity, add 1 in of R-7 rigid foam on the exterior, or upgrade to R-15 batt plus air-sealing.

Frequently Asked Questions

Why does continuous insulation outperform cavity-only upgrades?▾
Cavity insulation sits between studs, but studs themselves are thermal bridges — paths where heat bypasses the insulation. Adding continuous insulation (ci) on the exterior (or interior) covers the full wall area without interruption, reducing bridging and improving the effective whole-wall R-value. A wall with R-21 cavity batts plus R-5 exterior foam often performs closer to its labeled total R than R-21 cavity insulation alone.
How do open-cell and closed-cell spray foam compare?▾
Open-cell spray foam (roughly R-3.5/in) is lighter, expands more, and is vapor-permeable — good for interior applications where drying to the interior is desired. Closed-cell spray foam (roughly R-6.5/in) is denser, adds structural rigidity, and acts as a vapor retarder and air barrier in one pass. Closed-cell achieves higher R in limited depth (cathedral ceilings, rim joists) but costs more per inch. Both must be installed to manufacturer spec — off-ratio foam loses R-value and can shrink over time.
Do loose-fill materials settle and lose R-value over time?▾
Yes — blown-in fiberglass and cellulose can settle in open attic applications, reducing effective thickness and R-value unless installed to proper density or topped up over time. Cellulose is typically blown at higher density than loose fiberglass and settles less when installed correctly. Bag labels show R-value at a specific installed thickness — always measure actual depth in the field rather than assuming original install depth. This calculator uses nominal installed thickness; add margin if settling is a concern in your climate.
What R-value do I need in my climate zone?▾
IECC code varies by zone: walls typically R-13 to R-21, ceilings R-38 to R-60. Check your local amended code — many jurisdictions exceed federal minimums, especially in cold climates.
Can I mix insulation types to reach target R-value?▾
Yes. R-values are additive for layers — batt plus rigid foam, or blown-in over existing insulation. Ensure each layer is installed without gaps, compression, or moisture traps per manufacturer specs.

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