The U-value is the report card of the insulation. In a single-layer panel it is roughly the thermal conductivity divided by the thickness: if the conductivity of polyurethane foam is 0.022 W/mK and the thickness is 0.10 m, U ≈ 0.22 W/m²K; when the air film resistances at the surfaces are added it falls to 0.21. With the same foam a 60 mm panel gives 0.35, an 80 mm panel 0.26, a 120 mm panel 0.18, a 150 mm panel 0.14 and a 200 mm panel 0.11 W/m²K. In older sources and in trade language the same quantity appears as the "k-value"; its inverse, the thermal resistance, is denoted by R.
Its use is simple: heat gain = U × surface area × temperature difference. Example: a 0 °C room with a total of 100 m² of walls and ceiling, in 35 °C surroundings and with 100 mm panels, gains 0.21 × 100 × 35 = 735 W of heat. If the same room ran at −20 °C the difference would be 55 K and the 100 mm panel would pass 1,155 W; with a 150 mm panel this falls to 770 W. In cold rooms the target is to keep the heat flux through the panel below 8–10 W per square metre.
In design each surface is calculated separately, because the temperature differences differ: on a roof and an outer wall exposed to the sun the outer surface can reach 50–60 °C, on a partition wall facing a neighbouring cold room the difference falls to a few degrees, and at the floor the other side is ground at 10–15 °C. The U-value of doors is worse than that of the panel; glass, viewing windows and an uninsulated floor are the weakest surfaces. The U-value in the catalog is for the middle of the panel; joints, locks and corners make the true average 5–15% worse.
Points to watch: as foam ages its conductivity rises somewhat, so the declared "aged" design value is used in the calculation. Insulation that gets wet loses its value to a large extent; this is where the importance of the vapour barrier and of joint sealing comes from. The extra cost of a thicker panel is paid once, the gain from a low U-value is collected every day. When you change the panel thickness in the cooling load calculator you see directly how the conduction load changes; our unit converter gives the conversions between W, kcal/h and BTU/h.