✦ Insulation & Thermal Resistance

R-Value Calculator & Converter

Calculate thermal resistance (R-value), convert effortlessly between Imperial and Metric units, and determine the exact insulation thickness required for walls, attics, and crawlspaces.

Instant Bidirectional Thermal Converter

Type into any field below to convert seamlessly between US Imperial R-values, Metric RSI, and European/American U-factors.

Standard Building Code Insulation Benchmarks (ASHRAE 90.1 / IECC)

Building Component US Code (IECC) Imperial R-Value Metric RSI Metric U-Value
Wood Frame Wall (Zone 4–5) R-20 or R-13 + 5 ci R-20 3.52 m²K/W 0.28 W/m²K
Attic / Ceiling (Zone 4–6) R-49 to R-60 R-49 8.63 m²K/W 0.12 W/m²K
Floor over Unconditioned Space R-30 R-30 5.28 m²K/W 0.19 W/m²K
Basement Wall R-15 ci or R-19 cavity R-15 2.64 m²K/W 0.38 W/m²K
📊 Efficiency Curves

Interactive R-Value vs. Heat Loss Reduction Simulator

Slide the target R-value to see the percentage reduction in conductive heat loss compared to an uninsulated baseline (R-1.5).

Metric RSI Equivalent 3.52 m²K/W
Metric U-Value 0.27 W/m²K
Heat Loss Reduction 92.5% Cut
🗺️ Building Codes

US Climate Zone IECC Insulation Code Explorer

Select your US Climate Zone (IECC / ASHRAE 90.1) to view code-mandated minimum R-values for residential construction.

Attic / Ceiling R-60
Wood Frame Wall R-20 or R-13+5
Floors over Unconditioned Space R-19
🧱 Insulation Types

R-Value per Inch Comparison of Insulation Materials

Thermal resistance per unit thickness varies substantially across insulation products.

Insulation Material Imperial R-Value / Inch Metric Conductivity (λ) Key Applications & Properties
Closed-Cell Spray Foam (2 lb) R-6.5 – R-6.8 / in 0.021 – 0.023 W/mK Highest thermal resistance, serves as vapor and air barrier.
Polyiso / PIR Rigid Board R-6.0 – R-6.5 / in 0.022 – 0.024 W/mK Foil-faced rigid foam for continuous exterior wall & roof insulation.
Extruded Polystyrene (XPS) R-5.0 / in 0.029 W/mK Moisture-resistant foam board for below-grade foundation & slab edges.
Mineral Wool (Rockwool) R-3.8 – R-4.2 / in 0.035 – 0.038 W/mK Fireproof stone wool batts offering superior acoustic soundproofing.
Dense-Pack Cellulose R-3.6 – R-3.8 / in 0.038 – 0.040 W/mK Recycled paper fiber blown into closed wall cavities and open attics.
Fiberglass Batt & Blown R-3.1 – R-3.4 / in 0.042 – 0.045 W/mK Most cost-effective residential cavity insulation. Sensitive to compression.
⚠️ Insulation Traps

5 Common R-Value Misconceptions to Avoid

Avoid these frequent insulation mistakes to maximize return on thermal retrofit investments.

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1. Compressing Fiberglass Batts

Squashing an R-19 (6.25" thick) batt into a 3.5" (2x4) stud cavity reduces its trapped air volume, dropping its effective performance down to ~R-13.

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2. Relying on R-Value without Air Sealing

Fiberglass and mineral wool are air-permeable filters. Cold drafts blow straight through insulation fibers unless an airtight barrier is installed.

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3. Expecting Linear Bill Reductions

Increasing attic insulation from R-38 to R-60 only saves an additional 2%–4% on whole-house heating bills because the uninsulated walls and air leaks dominate.

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4. Ignoring Stud Framing Thermal Bridges

Solid wood framing conducts heat 3× faster than cavity insulation. Nominal R-21 fiberglass batts deliver only ~R-15 whole-wall performance.

🧰 Dedicated Calculation Engines

Explore the Heat Loss Calculator Suite

Access our complete family of specialized thermal engineering tools for rooms, whole houses, radiators, heat pumps, and insulation assemblies.

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Single Room

Room Heat Loss Calculator

Calculate precise heat loss for individual rooms to size replacement radiators.

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Whole Building

Whole House Calculator

Aggregate whole-building heat demand to size central heating boilers and heat pumps.

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Multi-Zone + CSV

Room-by-Room Calculator

Comprehensive multi-room inventory with cumulative heat loss schedules.

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ISO 6946

U-Value Calculator

Calculate composite U-values for multi-layer wall, roof, and floor assemblies.

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4-Way Converter

R-Value to U-Value Converter

Instantly convert between imperial R-values (hr·ft²·°F/BTU) and metric U-values.

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BS EN 442

Radiator Sizing Calculator

Size panel radiators for ΔT50, ΔT40, and low-temperature ΔT30 heat pumps.

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Combi & System

Boiler Sizing Calculator

Size condensing gas, oil, or LPG boilers with domestic hot water allowances.

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MCS & SCOP

Heat Pump Sizing Calculator

Determine design kW capacity and seasonal coefficient of performance (SCOP).

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80+ Cities Database

Design Temperature Lookup

Search regional 99.6% winter design outdoor temperatures across the globe.

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Blower Door & MVHR

Air Infiltration / ACH Calculator

Estimate ventilation and air leakage heat loss based on air changes per hour.

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Frequently Asked Questions

Find clear, expert answers to common questions about heat loss calculations, heating system sizing, U-values, and building thermal efficiency.

R-value and U-value are mathematical inverses. R-value measures thermal resistance (how much a material resists heat flow), while U-value measures thermal transmittance (how fast heat conducts through it). In metric units: U = 1 / RSI. In US imperial units: U = 1 / R_imperial. Higher R-values and lower U-values indicate superior insulation.
To convert Imperial R-value (h·ft²·°F/BTU) to Metric RSI (m²K/W), divide by 5.678263. For example, R-20 = 20 / 5.678 = 3.52 RSI. To convert RSI to Imperial R-value, multiply by 5.678263 (e.g. 5.0 RSI = R-28.4).
Closed-cell polyurethane spray foam and polyisocyanurate (PIR / Polyiso) rigid foam boards provide the highest thermal resistance per inch of thickness, rating approximately R-6.5 to R-6.8 per inch. Extruded polystyrene (XPS) delivers R-5.0/inch, while traditional fiberglass and cellulose batts provide R-3.2 to R-3.8 per inch.
No. The relationship follows the law of diminishing returns. Going from zero insulation (R-2) to R-12 cuts heat loss through that surface by 83%. Adding another R-10 (reaching R-22) only saves an additional 9% of heat. While high R-values are essential in cold climates, air sealing is equally important once basic insulation is achieved.
Nominal R-value is the rating printed on the insulation batt (e.g. R-21 fiberglass). Effective assembly R-value accounts for thermal bridging through wooden 2x4 or 2x6 studs (which only provide ~R-1.2 per inch). In a standard stud wall where wood framing represents 20%–25% of the wall area, an R-21 cavity batt delivers an effective whole-wall performance of only ~R-15.
Continuous insulation (ci) consists of rigid foam boards installed over the exterior sheathing across all structural members without thermal bridges (other than fasteners). Adding just R-5 continuous exterior foam eliminates wood stud thermal bridging, boosting effective wall performance by over 30%.