Insulation R-Value Calculator
Convert between R-value and U-value, estimate required insulation thickness, and combine layers to get total R-value for walls, roofs, and floors.
R-Value ⇄ U-Value Converter
Choose your unit system, enter either R or U, and the calculator will compute the other. Works for both imperial and metric R-values.
Unit: ft²·°F·h/BTU
Unit: BTU/(ft²·°F·h)
Tip: Enter either R or U and leave the other blank. The calculator will fill in the missing value.
What is insulation R-value?
R-value is a measure of thermal resistance – how well a material resists heat flow. The higher the R-value, the better the insulation performance.
In building applications you will usually see:
- Imperial R-value: ft²·°F·h/BTU (used in the U.S. and Canada)
- Metric R-value: m²·K/W (used in most other countries and in standards)
Core relationships
For a homogeneous layer of thickness \(L\) and thermal conductivity \(k\):
\[ R = \frac{L}{k} \]
U-value is the inverse of R-value (for the whole assembly):
\[ U = \frac{1}{R_{\text{total}}} \]
R-value vs U-value
U-value (overall heat transfer coefficient) tells you how easily heat flows through an assembly. It is measured in:
- BTU/(ft²·°F·h) in imperial units
- W/(m²·K) in metric units
They are exact inverses in consistent units:
\[ U = \frac{1}{R} \quad\text{and}\quad R = \frac{1}{U} \]
So a wall with R-20 (m²·K/W) has U = 0.05 W/(m²·K). A lower U-value means less heat loss.
How to combine R-values of multiple layers
For layers stacked in series (e.g., interior air film, drywall, insulation, sheathing, exterior air film), the total R-value is the sum of the individual R-values:
\[ R_{\text{total}} = R_1 + R_2 + R_3 + \dots + R_n \]
Then:
\[ U_{\text{total}} = \frac{1}{R_{\text{total}}} \]
The layered assembly tool above automates this for you.
Example: simple stud wall
- Interior air film: R = 0.68 (imperial)
- 1/2" drywall: R ≈ 0.45
- Fiberglass batt: R-13
- OSB sheathing: R ≈ 0.5
- Exterior air film: R = 0.17
Total R ≈ 0.68 + 0.45 + 13 + 0.5 + 0.17 = 14.8. Equivalent U ≈ 1 / 14.8 ≈ 0.068 BTU/(ft²·°F·h).
Estimating insulation thickness from R-value
If you know the material’s thermal conductivity \(k\) and the target R-value, you can estimate the required thickness:
\[ R = \frac{L}{k} \quad\Rightarrow\quad L = R \cdot k \]
where:
- \(L\) = thickness (m or in)
- \(k\) = thermal conductivity (W/m·K or BTU·in/ft²·°F·h)
- \(R\) = thermal resistance (m²·K/W or ft²·°F·h/BTU)
Typical k-values for common insulation materials
| Material | k (W/m·K) | Approx. R per inch (imperial) |
|---|---|---|
| Fiberglass batt | 0.040–0.045 | R-3.0 to R-3.5 |
| Cellulose (loose-fill) | 0.040–0.045 | R-3.2 to R-3.7 |
| Mineral wool | 0.036–0.040 | R-3.7 to R-4.2 |
| Expanded polystyrene (EPS) | 0.032–0.038 | R-3.6 to R-4.2 |
| Extruded polystyrene (XPS) | 0.028–0.032 | R-4.5 to R-5.0 |
| Closed-cell spray foam | 0.020–0.025 | R-6.0 to R-7.0 |
Values are typical ranges; always check manufacturer data sheets for design.
Frequently asked questions about R-value
Is higher R-value always better?
Higher R-value reduces heat loss, but there are diminishing returns and practical limits (cost, space, moisture control). Building codes specify minimum R-values by climate zone; going somewhat above code is often cost-effective, but extreme R-values may not pay back.
Why do my R-values not match code tables exactly?
Code tables usually refer to assembly R-values (including framing, air films, and finishes), not just the insulation layer. They may also include safety factors and assumptions about installation quality. Use this calculator for quick design checks, but rely on local codes and professional guidance for compliance.
What about thermal bridges?
Framing members, fasteners, and structural elements can bypass insulation and reduce effective R-value. The simple layer-sum method assumes uniform layers without thermal bridges. For more accurate results, you need detailed modeling or manufacturer/standard tables that account for framing fractions.
How do R-values relate to energy savings?
Heat loss through an assembly is approximately:
\[ Q = U \cdot A \cdot \Delta T \]
where \(Q\) is heat flow (W or BTU/h), \(A\) is area, and \(\Delta T\) is temperature difference. Reducing U (increasing R) reduces \(Q\), which lowers heating and cooling loads.
Insulation R-Value FAQ
What is insulation R-value?
R-value measures how well a material resists heat flow. Higher R-values mean better insulation. It depends on material type, thickness, density, and installation quality.
How do I convert between R-value and U-value?
In consistent units, U = 1 / R and R = 1 / U. Use the first tab of this calculator: enter either R or U and it will compute the other automatically.
Can I add R-values of different layers?
Yes. For layers in series, total R-value is the sum of each layer’s R-value. The “Layered Assembly” tab lets you enter each layer and shows the total R and equivalent U.
Does compressed insulation keep the same R-value?
No. Compressing fibrous insulation (like fiberglass batts) usually reduces its R-value per inch. Always follow manufacturer instructions for cavity depth and installation.