🧱 Thermal Transmittance (U-Value) Calculator

Calculate the overall heat transfer coefficient (U-Value) for a multi-layered wall or surface.

Inputs

Inside Conv. Coefficient (h_i)

Outside Conv. Coefficient (h_o)

Layer 1

Layer 1 Cond. (k₁)

Layer 1 Thickness (d₁)

Layer 2

Layer 2 Cond. (k₂)

Layer 2 Thickness (d₂)

Results

Thermal Transmittance (U-Value)

0.000BTU/hr·ft²·°F

Total Thermal Resistance (R_t)

0.000(h·ft²·°F)/Btu

Wall Thermal Profile

Calculator Description

The U-value (thermal transmittance) is the heat transferred per unit area per unit temperature difference through an envelope such as a wall, roof or floor, or a heat-exchanger wall. A smaller value means better insulation. It is defined as the reciprocal of the total thermal resistance R_t.

What this calculator finds

Enter each layer's thickness d and thermal conductivity k, plus the interior/exterior convective coefficients h_i and h_o, to compute the multi-layer wall's total resistance R_t and U-value.

Why it matters

Building energy efficiency, insulation design and code (U-value) compliance
Heating/cooling load and energy-cost estimation
Heat-exchanger wall heat-transfer design

Formula

U-Value & Thermal Resistance

Each layer's conductive resistance (d/k) adds in series, and the two convective resistances (1/h) are added at the surfaces. The reciprocal of the total resistance is the U-value.

R_t = \dfrac{1}{h_i} + \dfrac{d_1}{k_1} + \dfrac{d_2}{k_2} + \cdots + \dfrac{1}{h_o}

U = \dfrac{1}{R_t}

R_t — Total thermal resistance [m²·K/W or h·ft²·°F/Btu]
U — Thermal transmittance / U-value [W/(m²·K) or Btu/(h·ft²·°F)]
h — Convective heat transfer coefficient [W/(m²·K)]
k — Thermal conductivity of material [W/(m·K)]
d — Material thickness [m]

How the formula works

Smaller k (insulation) and larger d raise that layer's resistance, lowering U.
A larger h (forced convection) lowers the surface resistance 1/h, reducing its influence on U.
Since U = 1/R_t, increasing resistance only approaches U = 0, never reaching it.

Worked example

For a wall with 100 mm concrete (k=1.7), 50 mm insulation (k=0.04), h_i=8, h_o=20 W/(m²·K): R_t = 1/8 + 0.1/1.7 + 0.05/0.04 + 1/20 ≈ 0.125 + 0.059 + 1.25 + 0.05 = 1.484 m²·K/W, so U ≈ 0.67 W/(m²·K).

Useful Tips

Practical tips

To boost insulation most, thicken the layer with the smallest k (best payoff).
Real construction has thermal bridges and air gaps — expect slightly higher U than design.
Treat a still air gap by its combined radiative/convective equivalent resistance.

Limitations & cautions

This model assumes 1-D steady conduction; moisture, wind and radiation variations need correction.
h depends strongly on surface condition and fluid velocity — use reliable values.