A U-value measures how quickly heat passes through a building element, in watts per square metre per kelvin (W/m²K). The lower the number, the better the element resists heat loss. It is calculated as the reciprocal of the element’s total thermal resistance: U = 1/R. Every fabric heat-loss calculation starts here.
What does a U-value actually measure?
Take one square metre of wall. Hold a temperature difference of one degree between inside and outside. The U-value is the rate at which heat flows through that square metre, in watts. A wall with a U-value of 0.15 W/m²K loses 0.15 watts per square metre for every degree of difference between the room and the weather.
The figure rolls an entire build-up into a single number: blockwork, insulation, linings, cavities, and the thin films of still air that cling to the inner and outer surfaces. That is what makes it useful. The U-value is the common currency of fabric design, the same number for sizing a heating system as for comparing two wall build-ups.
Mind the direction. A U-value describes conductance, not resistance. High numbers mean leaky elements. Low numbers mean well-insulated ones. Designers occasionally trip over this when switching between U-values and R-values, which run the other way.
How do you calculate a U-value?
The maths is short and worth knowing by hand, even if a calculator does the work day to day.
- Find the thermal resistance of each layer: R equals thickness divided by lambda, where thickness is in metres and lambda (λ) is the material’s thermal conductivity in W/mK.
- Add the layer resistances together with the standard surface resistances from ISO 6946. For a wall, the internal surface resistance (Rsi) is 0.13 m²K/W and the external surface resistance (Rse) is 0.04 m²K/W.
- Take the reciprocal of the total: U = 1/R_T.
Two things follow directly from the formula. Doubling the thickness of an insulation layer doubles its resistance. Halving the lambda of the material you specify does exactly the same. Thickness and conductivity are the two levers, and they trade off against each other.
You can run the full calculation, layer by layer, in our free U-value calculator.
What is a good U-value?
It depends on what the number is for. A regulatory backstop is the worst element you are permitted to build. A performance target is what a low-energy building actually needs. The two are not the same, and the gap between them is where most fabric decisions get made.
| Benchmark | Element | U-value (W/m²K) |
|---|---|---|
| Ireland Part L backstop | Wall | 0.18 |
| Passivhaus rule of thumb, cool-temperate climate | Opaque elements | ≈0.15 |
| Passivhaus requirement | Windows, installed Uw | ≤0.80 |
The Passivhaus figures are not arbitrary. They are roughly what it takes, alongside an airtight envelope and heat recovery ventilation, to bring space-heating demand down to the standard’s limit of 15 kWh/m²a. Note that the window figure is an installed value. It accounts for the frame and the way the unit sits in the wall, not the glass alone. A good window installed badly is not a good window.
The headline lesson from the table: the distance between a compliant wall and a Passivhaus wall is smaller than many designers assume. The difference is rigour, not exotic materials.
Why does the lambda value you pick matter?
Because layer resistance is thickness divided by lambda, the conductivity figure you enter does as much work as the insulation thickness you draw. Pick an optimistic lambda and the calculated U-value flatters the wall. Pick the declared design value for the actual specified product and the number means something.
This is where many paper U-values part company with built reality. Substituting a product with a higher lambda at procurement stage, without revisiting the calculation, quietly degrades the element. So does ignoring the difference between a quilt’s nominal conductivity and its performance once compressed or imperfectly fitted.
Use real, declared values for real products. Our lambda value library collects typical conductivities for common insulation and structural materials so you can sense-check a specification quickly.
Do thermal bridges change the picture?
Yes, and increasingly so as the plane elements improve. A U-value describes the flat, repeating part of a wall, roof or floor. It says nothing about the junctions: the wall-to-floor connection, the window reveal, the balcony bracket, the fixings that pass through the insulation line. Heat takes the easiest path, and at every junction the easiest path bypasses the insulation.
In a poorly insulated building, junction losses disappear into the noise. In a well-insulated one they do not. As plane U-values fall towards Passivhaus levels, the bridges take a progressively larger share of the total heat loss, which is why the standard demands thermal-bridge-free detailing and explicit junction calculation rather than default allowances.
The practical takeaway: U-values and thermal bridges have to be assessed together. An envelope with excellent plane values and unresolved junctions will not deliver the heating demand the model promises. You can see how the pieces combine, element by element and junction by junction, in our fabric heat loss tool, and where junctions need proper numerical analysis, that is the work of thermal bridge analysis.
A short checklist for designers
When a U-value crosses your desk, ask four questions. Does the calculation include the ISO 6946 surface resistances? Are the lambda values declared design values for named products, not generic optimism? Is the window figure an installed Uw of 0.80 W/m²K or better, not a centre-pane value? And have the junctions around the element been detailed and calculated, not assumed away?
If the answer to all four is yes, the number on the page has a fair chance of turning up in the finished building.
Where Mosart fits
The four questions in the checklist above are most of the job, and they are the same ones our certifiers ask of the calculations that cross their desks. When a junction needs more than a rule of thumb, our thermal bridge analysis service puts a calculated number on it. Test your own build-up in the U-value calculator; it takes minutes.