Formula used
- Conduction (per-area): R' = L / k
- Convection film (per-area): R'film = 1 / h
- Series total (per-area): R' = Σ(Lᵢ/kᵢ) + 1/hᵢ + 1/hₒ + R'contact
- Overall resistance: R = R' / A
- U-factor: U = 1 / R'
- Heat rate: Q = ΔT / R = ΔT · A / R'
How to use this calculator
- Select Single layer or Multiple layers.
- Enter area A and temperature difference ΔT.
- Provide thickness and conductivity for each layer.
- Optionally add inside and outside film coefficients.
- Press Calculate to view results above the form.
- Use the download buttons to export CSV or PDF.
Example data table
| Case | Area (m²) | Thickness (m) | k (W/m·K) | ΔT (K) | R' (m²·K/W) | R (K/W) | Q (W) |
|---|---|---|---|---|---|---|---|
| Insulation layer | 10 | 0.10 | 0.040 | 20 | 2.500000 | 0.250000 | 80.000000 |
| Dense board | 10 | 0.02 | 0.160 | 15 | 0.125000 | 0.012500 | 1200.000000 |
| Two-layer stack | 8 | 0.08 + 0.02 | 0.045 + 0.160 | 18 | 1.777778 + 0.125000 | 0.237847 | 75.679012 |
Thermal resistance guide
1) What R-value represents
Thermal resistance (R-value) describes how strongly a material or assembly resists heat flow. In this calculator, the primary output is the per‑area resistance R′ in m²·K/W, which is the most common engineering form for insulation layers. A higher R′ reduces heat transfer and improves energy performance for walls, roofs, ducts, and equipment enclosures.
2) Conduction through a layer
For a uniform layer, conduction resistance per area is R′ = L/k, where L is thickness and k is thermal conductivity. Typical insulating foams can have k near 0.020–0.035 W/(m·K), while mineral wool and fiberglass are often around 0.035–0.045 W/(m·K). Dense boards, masonry, and metals have much larger k values and therefore much smaller R′ at the same thickness.
3) Series assemblies and layer stacking
Real building envelopes are multi‑layer: gypsum board, insulation, sheathing, and cladding. In one‑dimensional steady flow, per‑area resistances add in series: R′total = Σ(Lᵢ/kᵢ). This is why adding even a thin high‑k layer usually has little impact, while adding a thicker low‑k insulation layer can dominate the overall resistance.
4) Surface films and realistic boundaries
Air on either side of a surface adds convective film resistance, modeled here as 1/h per side. Indoor film coefficients might be around 5–10 W/(m²·K) for calm air, and outdoor values can range from 15–40 W/(m²·K) depending on wind and surface conditions. These films are often significant for low‑R constructions.
5) Contact resistance and interfaces
Interfaces are rarely perfect. Small gaps, roughness, and imperfect bonding can introduce an additional per‑area resistance. Use the contact resistance option when you want to approximate imperfect layer contact or a thin trapped air layer. Even modest interface resistances can matter when the conduction stack is already highly insulating.
6) From R′ to overall R and U-factor
The calculator converts per‑area resistance to overall resistance using R = R′/A, where A is the heat‑transfer area. It also computes the U‑factor, U = 1/R′, in W/(m²·K). Designers often compare assemblies using U because it directly scales heat flux: q″ = U·ΔT.
7) Heat rate and energy intuition
With ΔT and area, the heat rate is Q = ΔT·A/R′. For example, an assembly with R′ = 2.5 m²·K/W, area 10 m², and ΔT = 20 K gives Q = 80 W, matching the example table. Doubling thickness doubles R′ and halves Q, assuming k remains constant.
8) Practical modeling notes
This tool assumes steady, one‑dimensional heat flow and uniform properties. It does not model thermal bridging from studs, fasteners, or corners, and it ignores radiation exchange unless you capture it indirectly through an effective h value. For detailed building analysis, treat this calculator as a fast, transparent first estimate.
FAQs
1) Is R′ the same as the common “R-value” on insulation labels?
R′ is the per‑area thermal resistance in m²·K/W. Product labels may use different unit systems. Convert carefully before comparing, and focus on consistent units for design decisions.
2) Why does the calculator also ask for area?
Area converts per‑area resistance R′ into overall resistance R = R′/A and allows heat rate calculation. Without area, you can still interpret R′ and U, but not total watts.
3) When should I include surface films?
Include films when your boundary conditions are air on both sides, such as walls or ducts. Films can noticeably affect low‑R assemblies. If you already embed boundary effects elsewhere, leave them off.
4) What if my material k changes with temperature?
Use an effective conductivity averaged over the expected temperature range. Many insulations vary modestly, but high‑temperature applications may need temperature‑dependent data and a more detailed model.
5) Can I model a layered wall with different thickness units?
Yes. Each layer supports its own thickness unit, and the calculator converts internally to meters. Just ensure the numerical values match the selected units to avoid scaling errors.
6) Why is U based on R′ and not overall R?
U is an area‑normalized measure: U = 1/R′ in W/(m²·K). It describes heat flux per area for a given ΔT, which is why it’s widely used for assemblies.
7) Does this include thermal bridges like studs or fasteners?
No. This is a one‑dimensional series model. To approximate bridging, you can compare parallel paths separately and area‑weight them, or use specialized building‑envelope tools.