U-value: definition and what it means for a floor
+ Definition
The amount of heat passing through a whole building element, all layers included, in W/m².K: the lower it is, the better the element insulates.
The amount of heat passing through a whole building element, all layers included, in W/m².K: the lower it is, the better the element insulates.
The U-value measures the amount of heat that passes through an entire wall, per square meter and per degree of temperature difference between its two sides. It is expressed in W/m²K. The lower it is, the less heat the wall allows to escape.
For a floor, the wall includes everything: the slab, insulation, screed, flooring, and even the ground beneath a slab laid on grade. It is the metric that describes the floor as it is constructed.
These are three levels of measurement. Lambda describes a material; see our fact sheet on thermal conductivity. The R-value describes an installed layer; see the fact sheet on thermal resistance. The U-value describes the complete floor.
U is the inverse of the sum of the resistances of all layers. Adding an insulating layer increases this sum, thereby lowering the U-value. A screed, however, barely changes the U-value: its resistance is very low compared to that of insulation.
The value commonly required for a new floor or for floor insulation in renovations is a maximum of 0.24 W/m²K. This figure appears in the EPB brochure for the Flemish Region and in Brussels PEB forms, and the Walloon Housing Bonus makes it a condition for floor insulation.
The exact calculation, which incorporates the ground and junctions, is performed by the project's PEB consultant. For a private individual, keeping the order of magnitude in mind is sufficient: an uninsulated slab is far from this value, and it is the insulation that does almost all the work.
On-site, the U-value depends on the thickness of insulation that the available height allows, as well as the continuity of the layer. This is what we verify before choosing between sprayed PUR, panels, or insulating mortar. See our floor insulation solutions.
Feel free to contact us directly: our team is always available to assist you and provide a 100% free quote.
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