Composite Wall Thermal Resistance Heat Loss Calculator

Calculator overview

Inputs and outputs

This summary comes from the calculator's published input and output contract.

Inputs

CWT Hot Side Film Coefficient Conditional: About this input

Positive user-entered hot-side heat-transfer coefficient used only with bulk-fluid boundary temperatures. No correlation or lookup is provided; the shipped value is a synthetic arithmetic demonstration.

Unit W/(m^2*K) Default 137.3 Range At least 0
Hot-side surface temperature: About this input

Hot-side surface temperature in surface mode or hot-side bulk-fluid temperature in film mode. The computed lower bound is absolute zero.

Unit deg C Default 100 Range At least -273.15 (conditional)
CWT Unit System: About this input

Select a coherent SI or U.S. customary entry path. Grid values and scalar values are not converted automatically when this selector changes.

Default SI metric Allowed SI metric, US customary
Plane-wall layers - thickness (mm), conductivity (W/(m*K)): About this input

Submit exactly six complete rows. Set Use layer? to Yes for each layer in series and No for unused rows. Every Yes row needs a 1-to-80-character non-whitespace label, positive thickness, and positive conductivity in the selected units. No rows may contain null or empty cells; shipped active thicknesses and conductivities are conspicuously synthetic arithmetic values, while unused rows use a nonblank label, zero thickness, and conductivity 1 as inert transport-safe sentinels. Numeric grid cells are transported as strings under contract 0.17 and are parsed explicitly.

Default 6 rows

Column Range or allowed values

Use layer? No, Yes

Layer label Not declared

Thickness At least 0

Thermal conductivity At least 0
Cold-side surface temperature: About this input

Cold-side surface temperature in surface mode or cold-side bulk-fluid temperature in film mode. It must not exceed the hot-side boundary temperature.

Unit deg C Default 20 Range At least -273.15 (conditional)
CWT Boundary Temperature Basis: About this input

Use known wall-surface temperatures for solid conduction only, or use bulk-fluid temperatures and add two positive user-entered film coefficients.

Default Known surface temperatures Allowed Known surface temperatures, Known bulk-fluid temperatures with entered film coefficients
CWT Heat Transfer Area: About this input

Positive area normal to heat flow, common to every active plane layer. Variable-area and radial geometries are outside scope.

Unit m^2 Default 10 Range At least 0
CWT Cold Side Film Coefficient Conditional: About this input

Positive user-entered cold-side heat-transfer coefficient used only with bulk-fluid boundary temperatures. No correlation or lookup is provided; the shipped value is a synthetic arithmetic demonstration.

Unit W/(m^2*K) Default 31.7 Range At least 0

Column	Range or allowed values
Use layer?	No, Yes
Layer label	Not declared
Thickness	At least 0
Thermal conductivity	At least 0

Outputs

CWT Total Areal Resistance: About this output

Total solid resistance plus two entered 1/h film resistances in bulk-fluid mode; equal to solid resistance in surface-temperature mode.

Unit m^2*K/W
CWT Overall Thermal Conductance: About this output

Reciprocal of total areal resistance for this idealized plane-wall stack and selected boundary basis.

Unit W/(m^2*K)
CWT Hot Surface Temperature Conditional: About this output

Hot bulk-fluid temperature minus the temperature drop across the entered hot-side film resistance. Hidden in surface-temperature mode because the surface value is already an input.

Unit deg C
Model Status: About this output

Returns actionable NOT VALID text for malformed fixed-grid or boundary inputs and derived overflow, CHECK for equal boundary temperatures, and OK otherwise. It does not certify an assembly or select a material.

No unit declared
CWT Total Solid Thickness: About this output

Sum of entered thicknesses for all Yes rows; unused rows contribute zero.

Unit mm
CWT Total Solid Areal Resistance: About this output

Sum of thickness divided by conductivity for every active plane-wall layer, excluding any surface-film resistance.

Unit m^2*K/W
CWT Heat Transfer Rate: About this output

Heat flux multiplied by the entered common plane-wall area. SI results are displayed in kilowatts; the internal SI result is in watts.

Unit kW
CWT Cold Surface Temperature Conditional: About this output

Cold bulk-fluid temperature plus the temperature rise across the entered cold-side film resistance. Hidden in surface-temperature mode because the surface value is already an input.

Unit deg C
CWT Boundary Basis Used: About this output

States whether heat transfer uses entered solid-surface temperatures alone or bulk-fluid temperatures plus two entered film resistances.

No unit declared
CWT Active Layer Count: About this output

Number of six fixed grid rows marked Yes. Noncontiguous and duplicate-labelled active rows count independently.

Unit layers
CWT Heat Flux: About this output

Nonnegative boundary temperature difference divided by total areal resistance.

Unit W/m^2
CWT Dominant Resistance Layer: About this output

First active grid label whose layer resistance equals the maximum active layer resistance.

No unit declared
CWT Dominant Layer Resistance Share: About this output

Largest active layer areal resistance divided by total solid areal resistance. The structural range is zero through one.

Unit fraction

What it is

The Composite Wall Thermal Resistance and Heat Loss Calculator computes the steady one-dimensional heat flux and heat rate through a flat wall built from up to six layers in series. Each layer carries a thickness and a thermal conductivity entered in a fixed six-row grid, with a Yes or No selector saying whether that row is part of the stack, and the wall carries one common area normal to the heat flow.

It reports the total areal resistance with and without the surface films, the overall conductance, the heat flux, the heat rate, the total active thickness, the active layer count, which layer contributes the most resistance and what share of the total it holds, and plots the temperature profile through the stack.

Boundary temperatures are entered on one of two bases. With known surface temperatures the calculation covers the solid stack alone. With known bulk-fluid temperatures you add a film coefficient on each side, the model puts two 1/h resistances in series with the wall, and it returns the wall-surface temperatures it derived. In surface-temperature mode the film fields are hidden and have no effect on any output.

Entry is in SI metric or US customary. The selector relabels the fields and switches the temperature floor. It converts nothing you have already entered, in the grid or anywhere else.

The model is plane and one-dimensional. Contact resistance, thermal bridging through studs, ties and fasteners, edge effects, and any radial or variable-area geometry are outside its scope. The result describes an idealised uninterrupted section, not an assembly as built.

Methodology

Purpose and model boundary

This model calculates steady, one-dimensional heat flow through a constant-area plane wall containing up to six active layers in series. It can begin from known wall-surface temperatures, or from bulk-fluid temperatures with user-entered hot- and cold-side film coefficients. It does not select materials, estimate film coefficients, or perform transient, moisture, fire, or code analysis.

Inputs and units

The unit-system selector controls the displayed temperature, thickness, area, conductivity, film-coefficient, resistance, flux, and heat-rate units. The wall table contains exactly six rows; active rows require a label, positive thickness, and positive thermal conductivity. Inactive rows remain part of the fixed submitted values but are excluded from the thermal path. Bulk-fluid mode also uses both positive film coefficients. Surface-temperature mode excludes the film resistances.

Governing relationships

For active layer i, the areal conduction resistance is:

R_i = L_i / k_i

The solid resistance and thickness are:

R_solid = Σ R_i

L_total = Σ L_i

When bulk-fluid temperatures are selected, the two film resistances are:

R_hot-film = 1 / h_hot

R_cold-film = 1 / h_cold

The selected total resistance, conductance, heat flux, and heat rate are:

R_total = R_solid in surface-temperature mode

R_total = 1/h_hot + R_solid + 1/h_cold in bulk-fluid mode

U = 1 / R_total

q'' = (T_hot - T_cold) / R_total

Q = A × q''

Interface temperatures are found by subtracting the heat-flux times the cumulative resistance from the hot-side temperature. The chart plots that workbook-calculated interface profile against cumulative active thickness.

Calculation sequence

Validate the selected unit and boundary modes, temperature order, area, active layer rows, and any active film coefficients.
Convert values to the workbook's common calculation basis.
Sum active L/k terms and, when selected, the two 1/h terms.
Calculate U, heat flux, total heat rate, surface temperatures, and each layer-interface temperature.
Identify the active layer with the largest resistance and its share of the solid resistance.
Evaluate the workbook status in the precedence shown below.

Outputs and interpretation

Heat flux and total heat-transfer rate are the headline results. Supporting outputs expose total and solid-only areal resistance, overall conductance, active-layer count and total thickness, hot/cold surface temperatures, the selected boundary basis, and the dominant resistance layer. The dominant-layer result is a resistance comparison, not a material recommendation.

Validation and status logic

Condition, evaluated in order	Returned status
A selection, boundary temperature, area, active film coefficient, or six-row layer input is invalid	`NOT VALID: correct visible boundary, area, film, or six-row layer inputs`
A derived numeric result exceeds the workbook's supported calculation range	`NOT VALID: derived result exceeds the supported calculation range`
The hot and cold boundary temperatures are equal	`CHECK: equal boundary temperatures produce zero heat transfer`
None of the preceding conditions applies	`OK`

The status formula above was verified against the workbook's named Model_Status calculation. Invalid states publish protected zero-valued numeric outputs rather than Excel error tokens; those zeros must not be interpreted as physical results.

Assumptions and limitations

Heat transfer is steady and normal to a plane wall with the same area through every layer.
Each layer is homogeneous, has constant conductivity, has no internal heat generation, and is in perfect contact with adjacent layers.
Cylindrical, spherical, radial, finned, tapered, multidimensional, and thermal-bridge geometries are outside scope.
Radiation, moisture transport, contact resistance, gaps, edge loss, phase change, temperature-dependent properties, and transient storage are not modeled.
The workbook contains no material-property, insulation, film-coefficient, or assembly U-value library. All properties are user-entered.

Restrictions and non-computing states

This calculator requires exactly six complete layer rows. Active rows need nonblank 1-to-80-character labels and positive thickness/conductivity. At least one layer must be active. The hot-side temperature cannot be below the cold-side temperature, area must be positive, and film coefficients must be positive whenever bulk-fluid mode makes them active. Values outside the published limits are rejected before the calculation runs.

Errors and warnings

NOT VALID means the visible state cannot form a supported wall calculation. CHECK preserves a valid zero-driving-force result but asks the reader to confirm that the equal temperatures are intentional. A connection, publishing, or calculation-service error is not a thermal result and must not be read as zero heat loss.

References

The equations, status precedence, visibility rules, and output roles were checked against the delivered workbook, its published input rules, tests, reviewer packet, and formulas bound to the named outputs. The workbook identifies these technical sources:

DOE-HDBK-1012/2-92, Thermodynamics, Heat Transfer, and Fluid Flow, Volume 2 — Fourier conduction, series thermal resistance, and heat-transfer relationships.
NASA, Introduction to Numerical Methods in Heat Transfer — Independent confirmation of steady one-dimensional Fourier conduction.
NIST Guide to the SI, Appendix B — Exact unit conversions used by the SI/US modes.

Frequently asked questions

Why do my film coefficients make no difference?

Because you are almost certainly on the known-surface-temperatures basis, where both fields are hidden and completely inert. On the shipped default, setting both to zero and setting both to ten to the three hundredth power give the same 0.65 total areal resistance, the same 123.1 watts per square metre and the same OK status. Confirm it by comparing the total areal resistance against the total solid areal resistance: equal means no film was added. Switch to bulk-fluid temperatures and the two 1/h resistances join in.

Will my real wall perform as well as this says?

Probably not, and the gap is often large. This is strictly one-dimensional, with no contact resistance between layers, no thermal bridging through studs, ties or framing, and no edge effects. Real assemblies lose a substantial fraction of their calculated performance to bridging alone. Treat the result as an idealised uninterrupted section, not the assembly as built.

Which layer should I make thicker?

The one with the lowest conductivity, and the dominant-layer output names it. Layer resistance is thickness over conductivity, so a millimetre buys resistance in inverse proportion to the conductivity it joins. On the shipped default, layer B is 23 of the 71 millimetres but carries 0.500 of the 0.650 total, the 76.9 percent reported, because its conductivity of 0.046 is eight times lower than layer A's. A millimetre added to B is worth about eight added to A.

I switched to US customary and the answer changed. Is that a bug?

No. The selector relabels the fields and switches the temperature floor from -273.15 to -459.67, but converts nothing already entered. Your millimetres are now read as inches and your watts per metre kelvin as Btu per hour foot degree Fahrenheit. The same wall entered correctly in both systems gives total areal resistances of 7.7931 and 1.3724, one resistance in two units. Re-enter every value after switching.

Why did it refuse a hot side colder than my cold side?

Because it declines to report a signed negative flux that could be misread. It returns NOT VALID and expects you to swap the two boundary labels. Equal boundaries are treated more gently: those return CHECK, saying equal boundary temperatures produce zero heat transfer, with resistance, thickness and layer detail still reported and the flux and heat rate at zero.

What are the rules for the layer grid?

All six rows must be submitted with all four cells whether you use them or not. A missing cell, a null cell, an empty label, a seventh row or a single row is refused up front. Every Yes row needs a label of 1 to 80 non-whitespace characters, a positive thickness and a positive conductivity; a zero on either returns NOT VALID. No rows still need a nonblank label and numeric cells, by convention zero thickness with conductivity 1. Active rows need not be contiguous, and duplicate labels are separate layers.

Why did every number come back as zero?

Because the model rejected the state. A zero area, a zero film coefficient while bulk-fluid mode is live, a zero thickness or conductivity on an active row, no active rows, or a derived resistance that overflows or underflows all return NOT VALID, which zeroes the numeric block and sets the two text outputs to NOT AVAILABLE. Read the status line first. Values below a declared floor never get that far, being refused before the model runs.

Why does the profile chart end in a flat line?

Because the chart carries seven points, enough for six layers plus both faces, and with fewer active layers the surplus points repeat the final thickness and cold-surface temperature. The tail is a plotting artefact, not a plateau inside the wall.

Can I trust the dominant-layer label?

When one layer clearly leads, yes. When two are close, read the share instead. The model names the first active layer whose resistance equals the maximum, tested on exact equality, so two layers intended to be identical can be separated by the last bit of binary arithmetic and the label follows that bit. Two rows sharing a label are also two layers, so the name may identify a material, not a place.

Found a problem, or have an idea?

Tell us if a result looks wrong, a label is unclear, or something is missing. We read every message.

This page is provided by LogicCommons for informational purposes only. Results are analysis outputs computed from the inputs you supply and are not engineering advice, a design, or a substitute for review by a licensed professional under the codes adopted where the work is built. Verify all inputs and results independently.

LogicCommons is in beta. If a result, label, or reference looks wrong, tell us here; we read every message.