R-Value Calculator
Insulation R-Value Calculator
Calculate the thermal resistance (R-value) of an insulation material based on its thickness and thermal conductivity (k-value).
R-Value Chart & Material Data
Chart showing R-Value vs. Thickness for the selected material/k-value.
| Material | Typical k-value (W/m·K) | Typical k-value (BTU·in/h·ft²·°F) | Approx. R-value per inch |
|---|---|---|---|
| Fiberglass Batt | 0.040 – 0.048 | 0.28 – 0.33 | 3.0 – 3.6 |
| Fiberglass Loose-fill | 0.049 – 0.060 | 0.34 – 0.42 | 2.4 – 2.9 |
| Cellulose Loose-fill | 0.038 – 0.045 | 0.26 – 0.31 | 3.2 – 3.8 |
| EPS Foam Board | 0.033 – 0.039 | 0.23 – 0.27 | 3.7 – 4.3 |
| XPS Foam Board | 0.027 – 0.032 | 0.19 – 0.22 | 4.5 – 5.3 |
| Polyiso Foam Board | 0.020 – 0.025 | 0.14 – 0.17 | 5.9 – 7.1 |
| Spray Foam (Closed Cell) | 0.020 – 0.025 | 0.14 – 0.17 | 5.9 – 7.1 |
| Spray Foam (Open Cell) | 0.036 – 0.043 | 0.25 – 0.30 | 3.3 – 4.0 |
| Rockwool/Mineral Wool | 0.038 – 0.045 | 0.26 – 0.31 | 3.2 – 3.8 |
Typical k-values and R-values for common insulation materials. Actual values can vary.
What is R-Value?
The R-value is a measure of thermal resistance used in the building and construction industry. It indicates how well a two-dimensional barrier, such as a layer of insulation, a window, or a complete wall or ceiling, resists the conductive flow of heat. The higher the R-value, the greater the insulating effectiveness and the better the material or assembly's ability to resist heat transfer. Understanding the R-value is crucial when using an R-Value Calculator.
The R-value depends on the type of insulation, its thickness, and its density. For some materials, it also depends on temperature, aging, and moisture content. When calculating heat transfer through a multi-layered assembly (like a wall with siding, sheathing, insulation, and drywall), the R-values of the individual layers are added together to get the total R-value.
Who Should Use an R-Value Calculator?
An R-Value Calculator is useful for:
- Homeowners: To understand the insulation levels in their homes and plan upgrades.
- Builders and Contractors: To select appropriate insulation materials and thicknesses to meet building codes and energy efficiency goals.
- Architects and Engineers: To design energy-efficient buildings and specify insulation requirements.
- DIY Enthusiasts: To choose the right insulation for home improvement projects.
Common Misconceptions
One common misconception is that doubling the thickness of insulation will double the R-value. This is generally true for a given material, but comparing different materials solely by thickness can be misleading; their thermal conductivity (k-value) also matters significantly, which is why an R-Value Calculator considers both.
R-Value Formula and Mathematical Explanation
The R-value is calculated by dividing the thickness of the material by its thermal conductivity (k-value or λ-value).
The formula is:
R = d / k
Where:
- R is the R-value (thermal resistance).
- d is the thickness of the material.
- k is the thermal conductivity of the material.
The units of R-value depend on the units used for thickness and k-value. In the US, R-values are commonly expressed in ft²·°F·h/BTU, which results from thickness in inches and k-value in BTU·in/h·ft²·°F. In the metric system (SI), R-values are expressed in m²·K/W, resulting from thickness in meters and k-value in W/m·K. Our R-Value Calculator can handle both systems.
Variables Table
| Variable | Meaning | SI Unit | US Unit | Typical Range (for insulation) |
|---|---|---|---|---|
| R | R-value (Thermal Resistance) | m²·K/W | ft²·°F·h/BTU | 1 – 60 (US) / 0.18 – 10.6 (SI) |
| d | Thickness | meters (m) | inches (in) | 0.01 – 0.5 m / 0.5 – 20 in |
| k | Thermal Conductivity | W/m·K | BTU·in/h·ft²·°F | 0.02 – 0.06 W/m·K / 0.14 – 0.42 BTU·in/h·ft²·°F |
Practical Examples (Real-World Use Cases)
Example 1: Fiberglass Batt Insulation
A homeowner wants to install fiberglass batt insulation in their attic, which has a typical k-value of 0.046 W/m·K (or about 0.32 BTU·in/h·ft²·°F). They plan to install batts that are 10 inches thick.
- Thickness (d) = 10 inches
- k-value = 0.32 BTU·in/h·ft²·°F
- R-value (US) = 10 / 0.32 = 31.25 ft²·°F·h/BTU (R-31)
Using the R-Value Calculator with 10 inches and a k-value of 0.32 BTU·in/h·ft²·°F gives an R-value of R-31.25.
Example 2: XPS Foam Board
A builder is using XPS foam board with a k-value of 0.029 W/m·K for exterior sheathing. The board is 5 cm thick (0.05 meters).
- Thickness (d) = 0.05 meters
- k-value = 0.029 W/m·K
- R-value (SI) = 0.05 / 0.029 ≈ 1.72 m²·K/W
- Converting to US R-value (1 m²·K/W ≈ 5.678 ft²·°F·h/BTU): 1.72 * 5.678 ≈ R-9.8
The R-Value Calculator will confirm these values.
How to Use This R-Value Calculator
- Select Material Type: Choose a material from the dropdown. This will pre-fill the k-value. If your material isn't listed, select "– Enter Custom k-value –".
- Enter k-value (if custom): If you selected custom, enter the thermal conductivity (k-value) of your material and select its units (W/m·K or BTU·in/h·ft²·°F).
- Enter Thickness: Input the thickness of the insulation material.
- Select Thickness Unit: Choose the unit for the thickness you entered (inches, cm, or meters).
- Calculate: Click "Calculate R-Value" or observe the results updating as you type.
- Read Results: The calculator will display the R-value in both US (ft²·°F·h/BTU) and SI (m²·K/W) units, along with the thickness and k-value used in both unit systems.
The results help you compare different insulation options and understand their thermal performance. Higher R-values mean better insulation. When planning, consider the recommended R-values for your climate zone and building application (walls, attic, foundation). Our R-Value Calculator provides quick and accurate results.
Key Factors That Affect R-Value Results
- Material Type: Different materials have inherently different thermal conductivities (k-values). For instance, closed-cell spray foam has a much lower k-value (and thus higher R-value per inch) than fiberglass. Our R-Value Calculator allows you to select common materials or input custom k-values.
- Thickness: For a given material, the R-value is directly proportional to its thickness. Doubling the thickness generally doubles the R-value.
- Density: For some materials like fiberglass or cellulose, the density to which they are installed can affect the k-value and thus the R-value.
- Moisture Content: Moisture within insulation can significantly reduce its R-value because water is a much better conductor of heat than air or the insulation material itself.
- Temperature: The R-value of some materials can vary with temperature. Manufacturers usually provide R-values tested at a mean temperature of 75°F (24°C).
- Installation Quality: Gaps, voids, or compression in insulation can create thermal bridges and reduce the effective R-value of the assembly. Proper installation is crucial to achieve the rated R-value. An R-Value Calculator gives the material R-value, not the assembly R-value which is affected by installation.
- Aging: Some foam insulations, particularly those using blowing agents other than air, can experience a decrease in R-value over time as the blowing agent slowly diffuses out and is replaced by air.
Frequently Asked Questions (FAQ)
- What is a good R-value for an attic?
- It depends on your climate zone. In colder climates, R-49 to R-60 is often recommended, while warmer climates might require R-30 to R-49. Check local building codes and ENERGY STAR recommendations.
- Can I just add R-values together?
- Yes, for layers of insulation stacked directly on top of each other, you can add their R-values to get the total R-value. However, this doesn't account for thermal bridging through studs or joists.
- What is the difference between R-value and k-value?
- K-value (thermal conductivity) is an inherent property of a material indicating its ability to conduct heat per unit thickness. R-value (thermal resistance) is the material's resistance to heat flow for a specific thickness (R = thickness/k). The R-Value Calculator uses both.
- Does R-value change with temperature?
- For some materials, yes. The stated R-value is usually measured at a standard mean temperature. The effective R-value can decrease at very high or low temperatures for some insulations.
- What is RSI?
- RSI is the metric equivalent of R-value, expressed in m²·K/W. 1 m²·K/W is approximately equal to 5.678 ft²·°F·h/BTU.
- How does moisture affect R-value?
- Moisture significantly reduces the R-value of most insulation materials because water conducts heat much more readily than the insulation itself.
- Is a higher R-value always better?
- Yes, a higher R-value indicates better insulating performance and greater resistance to heat flow. However, there are diminishing returns, and the cost-effectiveness of adding more insulation needs to be considered.
- How do I find the k-value of a material not listed in the R-Value Calculator?
- The k-value (or thermal conductivity) is usually provided by the insulation manufacturer on the product's technical data sheet.
Related Tools and Internal Resources
- Insulation Guide – Learn more about different types of insulation and their applications.
- Energy Efficiency Tips – Discover ways to make your home more energy-efficient.
- Building Materials – Information on various building materials and their properties.
- Home Improvement Calculators – A collection of calculators for various home projects.
- Heat Loss Calculator – Estimate the heat loss from your home to determine insulation needs.
- Understanding k-Value – A deeper dive into thermal conductivity and how it relates to insulation.