EngineeringFormulaThermodynamics
U-Value Calculator
Calculate wall, roof, or floor U-value from material layers, surface resistances, optional air gaps, and heat-loss conditions.
A U-value, also called thermal transmittance, tells you how easily heat passes through one square metre of a building element for each degree of temperature difference. This calculator builds the assembly layer by layer: each material layer contributes R = thickness / conductivity, surface resistances are added, and the final U-value is the reciprocal of total area-normalized resistance.
Layer Stack
Thickness is entered in mm; conductivity is W/(m·K). Add each continuous layer in the assembly.
| Layer | Thickness (mm) | k W/(m·K) | R = L/k |
|---|---|---|---|
| 0.1325 m²·K/W | |||
| 2.857 m²·K/W | |||
| 0.1961 m²·K/W | |||
| 0.0619 m²·K/W |
Rsi · Inside air-film resistance in m²·K/W.
Rse · Outside air-film resistance in m²·K/W.
Rextra · Optional air gap or known added R in m²·K/W.
A · Optional assembly area for heat-loss estimate.
ΔT · Optional hot-cold side difference for heat loss.
Solution
Enter the material layers, then calculate to see Rtotal, U-value, and optional heat loss.
U = 1 / (Rsi + Σ(L/k) + Rextra + Rse)
Formula Sheet
- UU-Value / Thermal Transmittance
- RtotalTotal Thermal Resistance per Unit Area
- LLayer Thickness
- kThermal Conductivity
- RsiInternal Surface Resistance
- RseExternal Surface Resistance
- AArea
- ΔTTemperature Difference
Variables & Units
| Symbol | Variable | Description | Common Units |
|---|---|---|---|
| U | U-Value / Thermal Transmittance | Heat-transfer rate per unit area per degree temperature difference. Lower is better for insulation. | W/(m²·K), Btu/(h·ft²·°F) |
| Rtotal | Total Thermal Resistance per Unit Area | Sum of surface, layer, and extra resistances for the full assembly. | m²·K/W |
| L | Layer Thickness | Thickness of one material layer measured along the heat-flow direction. | mm, cm, m, in |
| k | Thermal Conductivity | Material property describing how readily the layer conducts heat. | W/(m·K), Btu/(h·ft·°F) |
| Rsi | Internal Surface Resistance | Inside surface-film resistance used in the assembly total. | m²·K/W |
| Rse | External Surface Resistance | Outside surface-film resistance used in the assembly total. | m²·K/W |
| A | Area | Optional surface area used to convert U-value into total heat loss. | m², ft² |
| ΔT | Temperature Difference | Optional indoor-outdoor or hot-cold side temperature difference. | K, °C, °F |
How to Use This Calculator
- 01Enter each material layer from inside to outside or outside to inside. The order does not change the simple series-resistance total, but keeping a real order makes the result easier to audit.
- 02Use thickness in millimetres and thermal conductivity in W/(m·K). Add only layers that are part of the continuous heat-flow path.
- 03Keep the default surface resistances for a quick vertical-wall estimate, or edit Rsi/Rse if your reference, code method, or heat-flow direction uses different values.
- 04Use Extra R for an unventilated air cavity or a known additional thermal resistance. Do not enter a material layer and the same R-value again.
- 05For framed walls, do not treat insulation as continuous through studs or rafters. Use a code-approved parallel-path or thermal-bridge method for compliance.
- 06Optionally enter area and temperature difference to estimate steady heat flux and total heat loss from the calculated U-value.
How the Formula Works
For each homogeneous layer, thermal resistance per unit area is Rlayer = L / k, where L is thickness in metres and k is thermal conductivity in W/(m·K). Insulation has low k, so the same thickness creates much larger resistance than brick, concrete, or glass.
The total resistance is Rtotal = Rsi + ΣRlayer + Rextra + Rse. Rsi and Rse represent the inside and outside surface films. The calculator defaults to 0.13 and 0.04 m²·K/W, commonly used as a quick vertical-wall starting point, but these are editable because roof, floor, and code-compliance workflows can use different values.
U-value is the inverse: U = 1 / Rtotal. Lower U-value means less heat transfer. If area and temperature difference are entered, heat flux is q = UΔT and total steady heat loss is Q = U A ΔT.
Worked Example 01
Cavity wall U-value
Known
- Brick: 102 mm at k = 0.77 W/(m·K)
- Mineral wool: 100 mm at k = 0.035 W/(m·K)
- Block: 100 mm at k = 0.51 W/(m·K)
- Plasterboard: 13 mm at k = 0.21 W/(m·K)
- Surface resistances: Rsi = 0.13, Rse = 0.04 m²·K/W
Formula
U = 1 / Rtotal
Substitution
Rtotal = 0.13 + 0.102/0.77 + 0.100/0.035 + 0.100/0.51 + 0.013/0.21 + 0.04 = 3.42 m²·K/W
Result
U = 1 / 3.42 = 0.292 W/(m²·K)
Most of the resistance comes from the mineral wool. The masonry layers add mass and structure, but much less insulation per millimetre.
Worked Example 02
Heat loss from a calculated wall U-value
Known
- Calculated U-value: 0.366 W/(m²·K)
- Wall area: 12 m²
- Temperature difference: 20 K
Formula
Q = U × A × ΔT
Substitution
Q = 0.366 × 12 × 20
Result
Q ≈ 87.9 W
This is only steady transmission through the entered assembly area. Air leakage, thermal bridges, and solar/internal gains are outside this simple check.
Applications
- 01Estimating U-value for walls, roofs, floors, slabs, or simple insulated panels
- 02Comparing insulation thicknesses and material conductivities during early design
- 03Converting a layer stack into a heat-loss input for HVAC or energy screening
- 04Checking why a high-conductivity layer contributes little insulation compared with foam or mineral wool
Common surface resistance starting points. Use the values required by your method or jurisdiction.
| Element / Heat Flow | Rsi | Rse |
|---|---|---|
| Vertical wall, horizontal heat flow | 0.13 m²·K/W | 0.04 m²·K/W |
| Roof or ceiling, upward heat flow | 0.10 m²·K/W | 0.04 m²·K/W |
| Floor, downward heat flow | 0.17 m²·K/W | 0.04 m²·K/W |
Typical thermal conductivities for rough screening only. Use manufacturer data for real design.
| Material | Approx. k | Notes |
|---|---|---|
| Mineral wool insulation | 0.035-0.045 W/(m·K) | Low conductivity; usually dominates assembly resistance |
| Expanded polystyrene (EPS) | 0.032-0.040 W/(m·K) | Varies by density and product grade |
| Plasterboard / gypsum board | 0.21-0.25 W/(m·K) | Thin finish layer, modest R contribution |
| Common brick | 0.6-0.8 W/(m·K) | High mass, relatively low resistance per thickness |
| Dense concrete | 1.4-2.0 W/(m·K) | Very conductive compared with insulation |
Assumptions
- 01Layers are treated as flat, homogeneous, continuous, and in series.
- 02Thermal conductivity is constant over the temperature range being estimated.
- 03Surface resistances are user-editable; defaults are only a quick vertical-wall starting point.
- 04Heat-loss output, when used, is steady-state transmission only.
Where This Model Stops
- 01Not a code-compliance U-value report and not a substitute for local building-regulation, ASHRAE, EN ISO 6946, or approved-energy-model calculations.
- 02Does not correct for thermal bridges, studs/rafters, fasteners, mortar joints, repeating framing, contact resistance, moisture, air leakage, or condensation risk.
- 03Does not decide vapor-control layers, dew-point risk, cavity ventilation, workmanship quality, or local energy-code compliance.
- 04Does not calculate window NFRC/SHGC performance or glazing edge effects; use manufacturer U-factor data for fenestration.
- 05Real assemblies can perform worse than a simple layer-by-layer result when insulation is interrupted or installed poorly.
References
- [1]Thermal Transmittance vs. Thermal Resistance
Engineering ToolBox
Reference for U = 1/R and layered thermal-resistance summation.
- [2]Energy in Buildings: Calculating U-values of Multiple Layers
Open University OpenLearn
Worked multi-layer wall example showing surface resistances, layer R-values, and U = 1/Rtotal.
- [3]In Situ Thermal Transmittance Measurements
Sustainability / MDPI
Open-access article summarizing EN ISO 6946's layer-resistance and thermal-transmittance relationship.
Frequently Asked Questions
Is U-value the same as R-value?
They are inverses when the units match. R-value measures resistance to heat flow, so higher is better. U-value measures heat transfer, so lower is better. This calculator reports metric U-value in W/(m²·K) and total metric resistance in m²·K/W.
Why are surface resistances included?
Real heat transfer includes thin inside and outside air films at the surfaces. U-value methods normally include these film resistances, usually shown as Rsi and Rse.
Can I use this for building-code compliance?
Use it for screening and learning only. Code-compliance calculations may require standard-specific corrections for thermal bridges, air cavities, fasteners, repeating members, ventilation, and approved material data.
How is this different from the Heat Loss Calculator?
This page calculates the assembly U-value from layers. The Heat Loss Calculator uses U-values plus areas and design temperatures to estimate room or zone heating load.
Can I enter manufacturer R-value instead of conductivity?
Yes, enter it as Extra R only if it represents the same area-normalized layer resistance. Do not also enter thickness and conductivity for that same layer, or the resistance will be counted twice.