Insulation Calculator
Calculate total R-value and U-value of any wall, ceiling, or floor insulation. Select materials and thicknesses from 50+ building materials for accurate thermal resistance.
About This Calculator
The Insulation R-Value Calculator helps homeowners, builders, architects, contractors, and energy auditors determine the total thermal resistance (R-value) of multi-layer building assemblies. Whether you are designing new construction, retrofitting existing walls, specifying attic insulation, or evaluating energy efficiency upgrades, this calculator provides accurate R-value and U-value calculations based on standard material properties from ASHRAE and international building codes.
Methodology: Our calculator uses the standard R-value accumulation formula: Total R-Value = Σ (Rₙ × tₙ), where Rₙ is the R-value per inch thickness of each material in °F·ft²·hr/BTU/in, and tₙ is the material thickness in inches. The U-value (thermal transmittance) is calculated as the reciprocal: U = 1 / Total R-Value, expressed in BTU/h·ft²·°F. For SI unit conversion, divide R-value by 5.678 to get RSI (m²·K/W), and multiply U-value by 5.678 to get U-value in W/m²·K.
Regional Notes
United States (US): The US uses R-value (°F·ft²·hr/BTU) as the standard measure of insulation. The Department of Energy recommends R-30 to R-60 for attics, R-13 to R-21 for walls, and R-25 to R-30 for floors, depending on climate zone. Building codes (IECC) specify minimum R-values for new construction.
United Kingdom (UK): UK building regulations (Approved Document L) specify maximum U-values rather than minimum R-values. Typical requirements: walls 0.18-0.28 W/m²·K, roofs 0.11-0.16 W/m²·K, floors 0.13-0.25 W/m²·K. Convert our calculator's output using RSI = R / 5.678 and U(W/m²·K) = 5.678 / R.
India (IN): The Bureau of Indian Standards and ECBC (Energy Conservation Building Code) recommend insulation based on climate zone. For composite climates (Delhi, Lucknow): R-20 to R-30 for roofs, R-11 to R-19 for walls. For cold climates (Shimla, Srinagar): R-30 to R-40 for roofs. Extruded polystyrene (XPS) and mineral wool are commonly available for residential insulation in India.
Frequently Asked Questions
What is R-value in insulation?
R-value measures the thermal resistance of insulation material — how well it resists heat flow. The unit is °F·ft²·hr/BTU per inch thickness. Higher R-values indicate better insulation performance. The total R-value of a building assembly is the sum of each material layer's R-value multiplied by its thickness in inches.
How do I calculate the total R-value of a wall?
To calculate total R-value, multiply each material's R-value per inch by its thickness in inches, then sum all layers. For example, a wall with 0.5-inch gypsum board (R-0.90/in), 3.5-inch fiberglass batt (R-3.40/in), and 0.75-inch cement board (R-0.05/in) has a total R-value of (0.5×0.90) + (3.5×3.40) + (0.75×0.05) = 12.35 °F·ft²·hr/BTU.
What is the U-value and how is it related to R-value?
U-value (thermal transmittance) is the reciprocal of R-value: U = 1 / R. It measures how well a building element conducts heat. Lower U-values are better for insulation. While R-value is used in the US, U-value (W/m²·K) is more common in the UK and Europe. To convert R-value to RSI (SI units), divide by 5.678.
What R-value is recommended for attic insulation?
Recommended attic insulation R-values vary by climate. In the US, the Department of Energy recommends R-30 to R-60 for attics depending on climate zone (R-30 for warm climates like Florida, R-49 to R-60 for cold climates like Minnesota). In the UK, building regulations require U-values of 0.16 W/m²·K or lower for roofs (equivalent to approximately R-36 or higher). In India, R-20 to R-30 is common for residential roof insulation.
What is the best insulation material for walls?
The best insulation material depends on your budget, space constraints, and climate. Spray polyurethane foam (PUR/PIR) offers the highest R-value per inch at R-6.5-7.2, making it ideal for tight spaces. Fiberglass batt (R-3.4/in) is the most cost-effective option. Rigid foam boards like EPS (R-4.0/in) and XPS (R-5.0/in) offer good mid-range performance. Cellulose (R-3.5/in) is an eco-friendly choice made from recycled paper.
Does thickness affect R-value?
Yes, R-value is directly proportional to thickness. Doubling the thickness of an insulation layer doubles its R-value, assuming the same material. However, adding too much insulation can lead to diminishing returns in terms of cost-effectiveness, and may cause moisture issues if vapor barriers are not properly installed. Building codes typically specify minimum R-values for different applications.
How does insulation save energy?
Insulation reduces heat transfer through walls, roofs, and floors, keeping buildings warmer in winter and cooler in summer. Properly insulated homes can reduce heating and cooling energy consumption by 20-50%. In the US, the EPA estimates homeowners can save an average of 15% on heating and cooling costs by adding proper attic insulation. In the UK, cavity wall insulation can save up to £250 per year on energy bills.
What is the difference between R-value per inch and total R-value?
R-value per inch is the material's inherent thermal resistance per unit thickness. Total R-value is the actual resistance of the installed layer, calculated by multiplying the R-value per inch by the thickness. For example, fiberglass batt has an R-value of 3.4 per inch, so a 6-inch batt has a total R-value of 20.4. Always use total R-value when comparing installed insulation performance.