U-value

The U-value (thermal transmittance) measures the rate of heat transfer through a building element, expressed in watts per square meter per degree Kelvin (W/m2K). A lower U-value indicates better insulation performance — less heat escapes through the element. U-values are used to quantify the thermal performance of walls, roofs, floors, windows, and doors, and are central to building energy calculations and compliance with building codes.

Finnish building regulations specify maximum U-values for different building elements in new construction: exterior walls typically 0.17 W/m2K, attic structure 0.09 W/m2K, ground floor 0.16 W/m2K, and windows 1.0 W/m2K. These values ensure that new buildings achieve a high level of energy efficiency. Older buildings may have significantly higher U-values — a 1970s wall might have a U-value of 0.40 W/m2K, meaning it loses heat more than twice as fast as a modern wall.

The U-value of a composite element (such as a wall assembly) is calculated from the thermal resistance of each layer — including the interior surface film, each material layer, air cavities, and the exterior surface film. Adding insulation reduces the U-value (improves performance), while thermal bridges (structural elements that bypass the insulation, such as steel studs or concrete balcony slabs) increase the effective U-value and should be minimized in design.

When planning energy upgrades, U-value calculations help determine the most cost-effective improvements. For example, adding 100 millimeters of insulation to an uninsulated wall produces a dramatic U-value improvement, while adding the same amount to an already well-insulated wall produces a much smaller improvement. Windows are typically the weakest thermal element in the envelope, so upgrading windows often provides the best return on investment in older buildings.