The wall that loses heat and the centimetre that decides

Every insulation project reaches the same question: how much insulation is needed? And almost every answer is given the wrong way — by feel, by imitating the neighbour, or by accepting the thickness the quote proposes. Yet the wall's physics is a one-line formula: thermal resistances add up, and the U-value is the reciprocal of the sum.

In the reference — a 1970s wall at 1.2 W/m²K, ten centimetres of insulation with λ 35 — the U-value drops to 0.271: the wall's losses fall by 77.4%. But the zone's regulatory threshold is 0.26 — and those ten centimetres, which looked generous, leave the wall non-compliant by eleven thousandths.

This calculator gives the insulated wall's U-value, the index against the threshold you set, and the number that saves quotes: the minimum thickness of that insulant for compliance — the missing centimetre, discovered before the site opens.

The formula: resistances add up, the hyperbola decides

The resistance added by the insulant is thickness divided by conductivity; the existing wall's is the reciprocal of its U. The final U-value is the reciprocal of the sum — and this structure has a consequence that governs every decision: it is a hyperbola. The first centimetre cuts a lot, the tenth little, the twentieth almost nothing.

At the reference: the first two centimetres take the wall from 1.2 to 0.712 — a 40.7% cut — while the step from 18 to 20 centimetres gifts less than a point and a half percent. The last two centimetres are worth less than a thirtieth of the first two, at the same price per square metre.

λ is read in thousandths — the mW/(m·K) of the data sheets: aerogel around 15, polyurethane 22-28, XPS and EPS 30-38, mineral wools 32-40, wood fibre 38-50. At equal thickness, moving from λ 40 to λ 22 is worth almost as much as doubling the thickness.

The same wall, five thicknesses

The table holds the reference wall fixed — U 1.2, insulant with λ 35 — and moves only the thickness. The wall is the same in every row: what changes is how much you dress it.

The table tells the hyperbola better than any formula: at 5 centimetres two thirds of the work is already done; at 10, a good three quarters. The jump from 10 to 20 centimetres — double the material, double the reveals at the windows, sills and thresholds to redo — buys a scant ten percentage points.

But the same table explains why the regulatory thresholds sit where they sit: getting below 0.26 takes more than ten centimetres of this insulant — the legislator placed the bar exactly beyond the point where instinct would stop, because it knows the hyperbola better than whoever fills in the quotes.

How this calculator works

The current U-value is the wall as it is today: historic solid masonry 1.5-2.5, uninsulated 1960s-80s cavity walls around 1.1-1.4, already insulated walls below 0.5 — the energy audit provides it, and in its absence the construction era orients.

The thickness is the additional layer under evaluation; the conductivity λ is the one declared in the material's data sheet, in mW/(m·K). The regulatory threshold is the U limit of your climate zone and jurisdiction — in Europe typically between 0.24 and 0.34 for walls, with cold zones stricter.

The surface feeds the last output: the loss avoided in watts for every degree of difference between inside and outside — the number that connects this page to the system sizing.

How to read the results

The post-insulation U-value is the project number: the one that will end up in the technical report. The index against the threshold scales it: up to 0.5 the wall is well below the limit; up to 1 it is compliant; between 1 and 2 it is out but within centimetres; beyond 2 the chosen thickness is symbolic against the target.

The minimum thickness for compliance is the operational verdict: how many centimetres of that insulant are really needed — to compare with the catalogue formats, which climb in steps of two or three centimetres. The loss avoided per degree — 92.9 W/K at the reference — is the permanent dividend: at 20 degrees of difference, almost 1,900 watts the wall no longer asks of the system.

The missing centimetre

The reference case is the page's most instructive: ten centimetres — the thickness «everyone has always used» — leave the wall at 0.271 against a 0.26 threshold. Out by a whisker: but out is out, for the incentives that require compliance, for the technical report, for the final inspection.

The fix costs almost nothing if decided early: the 12-centimetre format of the same material, or the same thickness with λ 32 instead of 35 — the calculator shows both routes in two tries. The same correction decided late — with the cladding installed — costs a construction site.

That is why the minimum thickness must be computed before asking for quotes, not after: whoever arrives at the installer with the right number compares prices at equal results; whoever arrives without compares random thicknesses, and the thinnest always wins — until the inspection.

The site neighbours

This page's avoided loss goes straight into the heat load: the insulated wall reduces the watts the system must supply — which is why the right order of works is envelope first, system second: whoever sizes the boiler or heat pump on the uninsulated house buys power they will never use.

The other neighbour is the heat pump savings: insulation lowers the required flow temperature, and the heat pump yields more precisely when the flow temperature drops — the two works multiply instead of adding. The envelope-then-system sequence is not site folklore: it is arithmetic.

The complete path: current U and minimum thickness (this page), heat load of the insulated house, system sized on that load — three calculations in a row, and the project stops being a sum of disconnected quotes.

Where the numbers come from

The current U comes from the energy audit or, in its absence, from the era-of-construction tables every jurisdiction publishes: the typical error is optimistic — real walls lose more than the tables, through moisture and discontinuities. The λ is in the data sheet: take the declared design value (λ_D), not the laboratory one.

The threshold depends on climate zone and jurisdiction, and changes if the works access incentives — which often require stricter values than the legal minimum. It pays to check both bars before choosing the thickness, because the difference between them is typically exactly that couple of centimetres.

The practical advice: run the count for every different wall — the uninsulated north and the already-refurbished south do not settle for the same thickness, and the uniform cladding is a site convenience, not a technical optimum.

Limits of the estimate

The model adds resistances in series for a flat wall: it does not see thermal bridges — corners, balconies, pillars, reveals — which in insulated walls become the heat's main escape route and can account for 10-30% of real losses. The calculated U is the floor, not the ceiling, of the true losses.

Surface resistances also stay out (they slightly improve the result), and above all the hygrometric check: every build-up must be verified for interstitial condensation, because a warmer but wet wall is a terrible deal. And the minimum thickness computed here is thermal: incentives may add further requirements.

A parametric estimate for information purposes: it supports the choice of thicknesses and materials and the dialogue with quotes; it replaces neither a designer's technical report nor the build-up's hygrometric verification.

The material scale

The first table holds wall and thickness fixed — U 1.2 and ten centimetres — and changes only the material, that is, the λ: the same centimetres, five different physics.

With the threshold at 0.26 the table becomes a decision map: at ten centimetres, PIR and XPS pass; EPS, rock wool and wood fibre do not. The same thickness crosses compliance just by changing the data sheet — which is why the quote that declares the centimetres but not the λ is not a quote: it is a random number.

The inverse reading is the real negotiation tool: when the thickness is constrained — narrow reveals, balconies, boundary distances — the λ is the only lever left, and dropping from 40 to 22 is worth almost as much as doubling the centimetres that do not fit.

The starting-wall scale

The second table holds the build-up fixed — ten centimetres of λ 35 — and changes the wall it is installed on: from the already decent wall to historic masonry.

The table contains a phenomenon that surprises whoever fills it in: the walls converge. The 2.5 historic masonry and the decent 0.6 wall — a factor of four at the start — end at 0.307 and 0.221: once dressed, almost the same wall. With enough insulation, the starting wall stops mattering: the insulant commands.

The practical corollary: the worst wall returns the most — an 87.7% reduction against 63.2% — at equal spend. On budget-limited sites, the right order of façades is from the worst wall to the best: every square metre of cladding yields where the start is lowest.

Material or thickness: the substitution you do not expect

Thickness and λ enter the formula only as a ratio: ten centimetres of λ 35 and eight centimetres of λ 28 are the same resistance — the same wall, for all thermal purposes. The conversion is immediate: equivalent thickness = thickness × (new λ / old λ).

It is the key to geometric constraints: where the cladding steals light from the windows, protrudes over the boundary or does not fit in the reveal, the high-performance material buys centimetres of space instead of resistance. It costs more per square metre — and often it is worth every cent, because space is not for sale.

The reverse also holds: where space abounds, the cheap material at increased thickness produces the same U at lower cost — and the choice stops being ideological («the best material») and becomes geometric: how much space there is, what a cubic metre of resistance costs in each material.

Thermal bridges: where the calculation ends and the site begins

This page's U describes the running wall — the flat field, far from the edges. But heat is lazy and looks for the easiest way: the more the running wall improves, the more the weak points become the main item. Corners, window reveals, balcony junctions, exposed pillars: in well-insulated walls the thermal bridges account for 10-30% of real losses.

The paradox is hygrometric as well as energetic: the thermal bridge in the insulated wall is colder relative to before — and the cold spot in a more airtight house is where condensation and mould choose to live. Half-done insulation can worsen the visible symptoms while improving the bill.

The site rule: the cladding wraps into the reveals (even just 2-3 thin centimetres), the balconies get treated — thermal break or wrapping — and the designer verifies the junctions. The best U in the neighbourhood does not save the project that forgot the edges.

Envelope first, system second: the order that multiplies

This page's avoided loss — 92.9 W/K at the reference — is not only bill savings: it is power the system no longer needs to have. The insulated house asks for a smaller boiler or heat pump, which costs less to buy and works better at steady state.

For heat pumps the effect is double: the better envelope allows lower flow temperatures, and the machine's efficiency rises precisely when the flow temperature drops. The two works do not add up: they multiply — which is the arithmetic reason why the wrong order (new system on a leaky house) is the refurbishment's most expensive mistake.

The project sequence: this page for the U and the minimum thickness, the heat load on the insulated house, the system sized on that load. Whoever skips the first step buys kilowatts they will pay for twice: at purchase and at every cycling of the oversized machine.

Mistakes that distort the U-value

The first is the laboratory λ instead of the declared λ: the data sheet reports both, and the difference — moisture and ageing — is worth 5-15%. In counts that end up in a report, the λ_D must be used, always.

The second is the optimistic starting U: the era-of-construction tables describe ideal walls, and real ones — damp, discontinuous, with collapsed cavities — lose more. In doubt, run the count with the worst plausible U: if the thickness suffices there, it suffices everywhere.

The third is the double unit jump on λ: 0.035 W/(m·K) and 35 mW/(m·K) are the same number, but mixing them produces results wrong by a thousandfold — this page asks for mW precisely to remove the decimal point from the most fragile spot. The fourth is declaring the thickness without the material: ten centimetres are not a performance, they are a measurement — the performance is the s/λ ratio, and the serious quote declares both.

Frequently asked questions

What is the thermal transmittance U?

It is the power crossing one square metre of wall for every degree of difference between inside and outside, in W/m²K: the lower, the better it insulates. Historic solid masonry sits at 1.5-2.5, a 1970s cavity wall around 1.1-1.4, a well-insulated wall below 0.3.

How is the U-value after insulation computed?

Thermal resistances add up: the existing wall's (1/current U) plus the insulant's (thickness divided by conductivity). The final U is the reciprocal of the sum — at the reference, 1/(1/1.2 + 0.10/0.035) = 0.271 W/m²K.

Why is the first centimetre worth more than the last?

Because the U-value is the reciprocal of a sum: it is a hyperbola. At the reference the first 2 cm cut 40.7% of the losses, the step from 18 to 20 cm less than 1.5%. The last centimetres cost the same as the first and return a thirtieth.

What is the conductivity λ and where do I find it?

It is how much the material conducts heat, in mW/(m·K) on the data sheets: aerogel ~15, PU/PIR 22-28, XPS 30-36, EPS 31-38, mineral wools 32-40, wood fibre 38-50. Use the declared design value (λ_D), not the laboratory one.

Which regulatory threshold should I enter?

The U limit for walls in your climate zone and jurisdiction — in Europe typically between 0.24 and 0.34, stricter in cold zones. Beware: incentives often require values tighter than the legal minimum, and it pays to check both bars.

What does the minimum thickness for compliance mean?

It is how many centimetres of that insulant it takes to bring the wall exactly to the threshold: at the reference 10.5 cm — more than the assumed 10. Compare it with the catalogue formats, which climb in 2-3 cm steps: the right format is the first one above the minimum.

Can the wall be compliant even without insulation?

Yes, if its current U is already below the threshold: in that case the minimum thickness shown is zero. It happens with walls insulated in the past — and there the right question is not compliance but the marginal return of the additional centimetres.

What is the loss avoided per degree?

The difference between the U before and after, multiplied by the square metres: at the reference (1.2−0.271)×100 = 92.9 W/K. At 20 degrees of difference, almost 1,900 watts the system no longer has to supply — the number to carry into the heat load calculation.

Why does the calculator show no currency?

Because the physics is identical in every country: U, λ and thresholds are international quantities, and the index against the threshold is a pure number. Costs come later, in comparing quotes at equal thermal results.

Does the calculation work for roofs and floors too?

The formula is the same — resistances in series — but the regulatory thresholds change (roofs have stricter limits than walls) and so do the surface resistances. For orientation it is fine; the technical report requires each structure's specific coefficients.

What are thermal bridges and why do they matter?

They are the points where the insulation is interrupted — corners, balconies, pillars, reveals — and where heat escapes anyway. In insulated walls they become the main escape route: they can account for 10-30% of real losses, and the calculated U does not see them.

Is this calculation enough for the building permit?

No: it is the orientation tool for choosing thickness and material before the quotes. The technical report requires the full build-up calculation with surface resistances, thermal bridges and the hygrometric condensation check — the designer's job.

Updated July 2026