Documented steady one-dimensional case

Planar-Wall Thermal Resistance & Heat-Transfer Calculator

Sum documented homogeneous layer and surface resistances, calculate U-value, and preserve the sign and direction of the steady heat-transfer rate.

R = d/λU = 1/RtotalSigned Q = UA(Ti−Te)

Assembly / project identifier

Boundary-temperature case, source and revision

Inside-side temperature Ti (°C)

Outside-side temperature Te (°C)

Documented net heat-flow area A (m²)

Area take-off / geometry source

Inside surface resistance Rsi (m²·K/W)

Rsi boundary-condition source

Outside surface resistance Rse (m²·K/W)

Rse boundary-condition source

Solid homogeneous layers, entered from either side

For each solid layer, cite the product/design conductivity source and revision. Air or gas layers and cavities require a different resistance method and are not accepted by this d/λ ledger.

Applicability gate: this calculator supports only a plane component represented as solid homogeneous material layers in series under a documented steady one-dimensional boundary case. It does not model air/gas layers or cavities, thermal bridges, studs/fasteners, air leakage, ground coupling, windows/doors, moisture, solar gain or transient thermal storage.

Documented planar-wall result

Layer resistance sum Σ(d/λ)

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Total resistance Rtotal

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Thermal transmittance U

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Signed temperature difference Ti−Te

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Signed heat flux q

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Signed heat-transfer rate Q

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Implemented arithmetic

Ri = di/λi
Rlayers = ΣRi
Rtotal = Rsi + Rlayers + Rse
U = 1/Rtotal
q = U(Ti−Te)
Q = A·q

Storis d is converted from millimetres to metres. Conductivity λ is entered in W/(m·K), resistance in m²·K/W, U in W/(m²·K), heat flux in W/m² and total rate in W. Positive Q means transfer from the entered inside side toward the entered outside side; negative Q means the reverse direction.

Standard status and scope

ISO 6946:2017, edition 3, is Published and was confirmed in 2022; ISO lists a corrected English version dated 2021-12. Its scope covers calculation of resistance and transmittance for building components using appropriate design conductivities or resistances and thermally homogeneous layers. The ISO page excludes doors/windows/glazing, curtain walling, ground heat transfer and components designed for air permeation, and states that bridged/inhomogeneous cases require additional treatment.

This page performs only the displayed series-resistance arithmetic. It does not claim an ISO 6946 compliance calculation because it does not implement the complete licensed procedure, air-layer rules, inhomogeneous-layer method or Annex F corrections. No ISO clause or table is asserted without checking the licensed text.

Why fixed defaults and presets were removed

The former calculator silently imposed Rsi=0.13 and Rse=0.04 m²·K/W, supplied unsourced conductivity presets and generic “typical U-value” ranges, and used |Ti−Te| so an outside-to-inside heat gain was still labelled heat loss. Surface resistance and design conductivity depend on the documented boundary and product/application case; this version therefore requires their sources and preserves the sign. An Energy Saving Trust report published by Ofgem documents missing λ-source evidence and a real calculator error caused by treating air layers with a fixed conductivity; this ledger therefore accepts only an explicitly selected solid homogeneous d/λ model.

Not an energy-code or condensation assessment: verify national annexes and code methods, design conductivity/resistance, geometry, surface orientation and environment, repeating/non-repeating bridges, openings, junctions, ventilation/infiltration, moisture/condensation, solar and long-wave radiation, transient conditions and uncertainty.