U-value and lambda in mineral-wool panels: assessing a wall’s thermal performance

Lambda, written λ, describes heat conduction through a material, in W/(m·K). At equal thickness, a lower value means greater thermal resistance. Do not take one lambda value from the internet and apply it to every mineral-wool panel.

A particular core may combine wool types with lambda values of 0.038 and 0.040 W/(m·K). These belong to specified materials, not to all panels. The arrangement of wool within the finished product also matters. Panel U-values are therefore more useful for comparison than simply choosing the lowest advertised lambda.

U-value is expressed in W/(m²K). A lower figure means less heat flow per square metre at the same temperature difference. Documentation may also use Uc: check its definition and scope. It is not the core’s lambda value.

For a simple layer, thermal resistance is R = d/λ, with thickness d in metres. Thus 0.10 m of material at lambda 0.040 gives R = 2.5 m²K/W. This is a layer calculation, not a declared U-value for a panel; it omits factors considered in the manufacturer’s product assessment.

Keep units consistent: 100 mm equals 0.10 m, not 100 m. Do not add the U-values of layers together; the relevant thermal resistances are combined before determining the result. Use manufacturer data for a finished panel. If a simplified calculation differs, compare the assumptions rather than replacing the declared value.

The following examples compare two mineral-wool panel families. All figures are U-values in W/(m²K), not a common specification for all wall panels.

Different results need not mean a table is wrong: the products differ in construction and assessment. Do not transfer these values to every panel with a similar core. Specify the exact variant and thickness in the enquiry, and obtain renewed confirmation if the product is substituted.

The panel field is only part of the envelope. Bases, corners, roof junctions and door surrounds can create local heat-loss paths. Fasteners and how the cores meet also matter. Air leakage can add heat loss and moisture problems.

Connections need the correct compression and seals suitable for the building. Increasing panel thickness does not replace these. A well-built wall of correctly selected panels may perform better than a thicker envelope with faulty details. List connections that will become difficult to inspect once flashings are fitted.

First establish the required performance of the complete wall, based on its use and the design assumptions. Then select a panel allowing for the effects of details. An article or calculator should not determine building compliance from one product table.

An illustrative design target of 0.20 W/(m²K) does not mean that every panel with catalogue U = 0.20 achieves that value in a completed wall. Determine which corrections and thermal bridges apply. Summer overheating, moisture and ventilation also need separate assessment: a good U-value does not solve every envelope issue.

Request the current technical sheet, not just a screenshot of a table. Check product name, edition and units, and whether the value concerns a wall panel, roof panel or insulation alone. One manufacturer can offer several similarly named product families.

If only lambda is quoted, also request the declared U-value for the ordered thickness. If a whole-wall U-value is stated, ask which details and calculations support it. Three decimal places do not make a result more reliable when important connections are omitted.

It helps in a comparable build-up, but thickness, panel construction and wall details also affect the result.

That is a simplification for a homogeneous layer. It does not replace finished-panel data or a calculation of the complete wall.