Thermal R-Value Calculator
Use this insulation calculator to estimate R-value, U-value, and optional heat loss from thickness and thermal conductivity (k-value).
R ↔ U Quick Converter
Enter either R-value or U-value. If both are filled, R-value is used as the primary input.
What is an R-value?
In building science, the R-value measures thermal resistance. In simple terms, it tells you how strongly a material resists heat flow. A higher R-value means better insulation performance for the same conditions.
If you are comparing wall insulation, attic insulation, floor systems, or rigid foam boards, R-value is one of the most useful first-pass metrics. But it should always be considered alongside air sealing, moisture control, and thermal bridging.
R-value vs U-value
R-value and U-value are inverses of each other:
U = 1 / R and R = 1 / U
- R-value: higher is better (more resistance).
- U-value: lower is better (less heat transfer).
Most modern performance specifications for complete assemblies (walls, roofs, windows) are often written in U-value, while insulation products are frequently marketed using R-value.
How this R-value calculator works
Core equation
For a homogeneous layer of material:
R = L / k
- L = thickness in meters (m)
- k = thermal conductivity in W/m·K
- R = thermal resistance in m²·K/W
The calculator also provides an estimated imperial conversion: R (US) ≈ R (SI) × 5.678.
Optional heat-loss estimate
If you enter area and temperature difference, the calculator estimates steady-state heat flow:
Q = U × A × ΔT
- Q in watts (W)
- A area in square meters (m²)
- ΔT temperature difference in °C (same interval as K)
This gives a practical way to compare design options. For example, lowering U-value from 0.35 to 0.20 can significantly reduce heat loss over an entire heating season.
Typical k-values for common building materials
Thermal conductivity depends on density, moisture, manufacturing method, and temperature. Always verify with manufacturer data sheets when making final decisions.
| Material | Typical k (W/m·K) | Insulation quality |
|---|---|---|
| Polyisocyanurate (polyiso) | 0.022–0.028 | Very high |
| XPS foam board | 0.028–0.034 | High |
| EPS foam board | 0.032–0.040 | Moderate to high |
| Mineral wool | 0.034–0.040 | Moderate to high |
| Fiberglass batt | 0.035–0.045 | Moderate |
| Wood (softwood) | 0.10–0.14 | Low as insulation |
| Brick | 0.6–1.0 | Poor |
| Concrete | 1.0–2.0 | Very poor |
How to use the calculator effectively
- Choose a material preset or enter k-value manually.
- Enter thickness and select units (mm, cm, m, or inches).
- Optionally include interior/exterior air films for an assembly-level approximation.
- Optionally add area and ΔT to estimate heat transfer rate in watts.
- Use the R ↔ U converter to compare product data sheets and code targets.
Common mistakes when comparing insulation
- Ignoring thermal bridges: studs, rafters, and fasteners reduce real-world performance.
- Comparing nominal values only: installed quality and compression can change effective R-value.
- Skipping air sealing: uncontrolled air leakage can dominate heat losses even with high R-value insulation.
- Forgetting moisture effects: wet insulation usually performs worse.
- Mixing unit systems: SI and US R-values are not numerically equal; convert correctly.
Quick example
Suppose you have 140 mm of mineral wool with k = 0.036 W/m·K.
- Thickness in meters: 0.14 m
- R = 0.14 / 0.036 = 3.89 m²·K/W
- U = 1 / 3.89 = 0.257 W/m²·K
For a 50 m² surface and ΔT = 20°C, estimated heat loss is: Q = 0.257 × 50 × 20 = 257 W.
Final notes
This r value calculator is designed for quick planning, retrofit comparisons, and education. For permitting, compliance, or detailed energy modeling, use full assembly calculations and local code requirements. If you are evaluating envelope upgrades, combine better insulation with airtightness and proper ventilation strategy for the best total result.