Heat transfer coefficient (U-value)
Also known as: U-value, k-value, thermal transmittance
Short definitionThe number that tells how easily a building element lets heat through: watts per square metre per kelvin (W/m²K). A low value means good insulation; panels, doors and glazing are compared by it.
When a customer asks “how thick a panel do I need”, the answer comes from the U-value. A polyurethane-filled sandwich panel passes 0.21 W/m²K at 100 mm; at 150 mm this falls to 0.14 and at 200 mm to 0.11. Multiply the number by the surface area and the temperature difference between inside and outside, and you have the watts entering through that surface. Older installers call the same thing the k-value.
If a −18 °C store stands in a 32 °C ambient, the difference is 50 K; 10.5 W passes through each square metre of 100 mm panel, 7 W through 150 mm. Since the aim is to stay under the 8–10 W/m² limit, thick panels are chosen for freezer rooms. Do not calculate the roof, the partition wall and the floor with the same difference; a roof in the sun is much hotter. Doors and viewing windows are weaker than the panel, and joints spoil the brochure value a little. Foam that has taken in water does not insulate.