Thermal Resistance Calculator
A wall's resistance to heat, layer by layer — R values, the U number and the watts that slip through per square metre.
Thermal Resistance Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Conduction only — no surface films or bridges
- Dry materials at steady state
In short: Two hundred millimetres of brick, fifty of insulation and a skin of plaster stack to 1.56 m²K/W of resistance — a U value near 0.64. Across twenty degrees of indoor–outdoor difference that wall loses about 12.8 watts per square metre, 128 watts over a ten-metre panel. The insulation layer is three-quarters of the resistance in a fifth of the thickness: heat negotiates with the material's conductivity, not its bulk.
Formula
R = L ÷ k — Rtotal = ΣR — U = 1 ÷ Rtotal
Each layer resists in proportion to its thickness and in inverse proportion to its conductivity; stacked layers add like series resistors, and the U value is the reciprocal of the sum. Multiply U by the area and the temperature difference and the steady-state watts fall out. Surface films — the still air clinging to each face — add roughly 0.17 m²K/W combined on top, which this screen leaves off so the materials speak for themselves.
Worked Example
- Divide each layer's thickness by its conductivity.
- Sum the R values; invert for U.
- Multiply U by area and ΔT for the watts.
Defaults: brick, wool and plaster over ten square metres at twenty degrees — U 0.639, 12.8 W/m², 128 watts of steady leak. Delete the insulation layer and the leak quadruples; the bricks never moved.
Strengths & Limits Of This Model
Where this engine is strong
- Layer card shows which layer owns the wall
- Series addition matches how builds are actually quoted
Where it stops
- Ignores thermal bridging and air leakage entirely
Practical Use Cases
U-value checks
does the build-up meet the spec
Retrofit maths
what the insulation layer buys
Heating loads
steady-state watts per envelope
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Thermal Resistance Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why do stacked layers simply add?
Steady heat is a flow through a chain: whatever leaves one layer enters the next, so the drops add at constant flux — the exact analogue of series resistors. That is why one good insulation layer dominates a wall of heavy materials.
What is the difference between R and U?
R resists, U transmits — U is one over the total R. A bigger R is better for the wall; a smaller U says the same thing in a form heat-loss sums prefer.
Are the surface air films included?
No. Inside and outside films add roughly 0.12 and 0.06 m²K/W respectively, so a real wall reads slightly better than this stack. Including them is a one-line addition to the total.
Why did my insulation barely change the U?
Then something else dominates — often a heavy structural leaf or an unmapped thermal bridge. Check the layer card: the biggest R owns the wall, and improving anything else buys little.
Conductivity versus conductance?
k is the material property, watts per metre per degree; a construction's U folds geometry in. Two products can quote the same k and build to very different U once thickness enters.
Does this apply to windows?
The arithmetic chain does, but glazing U is quoted complete with films and frames. Use the quoted U directly in the leak line rather than building the glass from layers.
Where does moisture fit?
Wet insulation conducts several times better than dry — a damp layer's k can quietly double. The screen computes the dry build-up; vapour control is a design question that protects the numbers.
Steady state — what about a cold night?
This is the watts per hour the wall moves once temperatures settle. Thermal mass delays and smooths the flow in real weather, which is a dynamic story this steady-state screen deliberately skips.