How do U-values and thermal conductivity relate to heat loss through a mineral wool panel?

Thickness divided by conductivity gives the thermal resistance of a homogeneous layer. This explains the trend, but a finished panel has joints and a particular geometry. If a declared U-value is available for the complete element, use it in the order rather than deriving one solely from the core.

The U-value, in W/(m²K), relates heat flow to area and temperature difference. Check whether the data sheet gives the value for the panel field or a corrected value for the product. Compare figures determined by the same method for the actual variants on order.

In one wall panel range, 120 mm gives 0.31 W/(m²K) and 180 mm gives 0.21 W/(m²K). Both have the same core design but different thicknesses. Other systems can differ because of the joint, geometry and the way joints are accounted for.

Take 324 m² of panel area and a temperature difference of 26 K. Heat loss rate through that area is estimated as Q = U × A × temperature difference. At U = 0.31 this gives 2,611.44 W, or about 2.61 kW. At 0.21 it gives 1,769.04 W, or about 1.77 kW.

The difference is about 0.84 kW under these conditions. This is the effect of increasing the thickness of this wall area, not a corresponding reduction in the whole building’s heating capacity. The calculation assumes steady temperatures on both sides and excludes doors, ventilation, roof and other elements.

Use the same area for both variants. If one calculation includes windows and the other omits them, geometry affects the result too. Here the 324 m² refers solely to the panel area under consideration. Recording this makes the calculation reproducible.

Real walls have edges, corners, plinths, reveals and fixings. The designer assesses their effect on heat loss and surface temperatures. Avoid counting effects already included in the declared U-value a second time. Use a consistent method for the whole envelope.

A large, poorly insulated vehicle door can change the result for the façade. Opening it often increases air exchange too. Compare each element’s share before paying extra for thicker panels. Improving a small area cannot resolve a dominant loss elsewhere.

Kilowatts describe power at a particular moment; annual energy use is measured in kilowatt-hours. Estimating it requires varying outdoor temperatures, heating hours, heat gains and system efficiency. Multiplying a cold-day heat loss by every hour of the year gives a misleading answer.

Treat payback estimates just as carefully. Energy prices and use patterns change. A warehouse heated only during working hours differs from one maintained at temperature around the clock. State the operating scenario instead of promising a single payback date.

Record panel area, temperatures, operating hours and each variant’s U-value. Include changes to plinths, windows and flashings. Thicker panels may need longer fasteners or revised details; these belong in the cost comparison.

Start with any thermal performance specified by the design. Then check fire, mechanical and environmental properties. A low U-value is useful only if the panel can be installed correctly across the complete wall and suits the way the building operates.