PUF is created by mixing two liquid chemicals that react, expand and set into a rigid closed-cell foam. In a cabinet, it is injected between the outer skin and inner liner, where it fills every gap, bonds to both surfaces and turns the shell into a stiff sandwich panel. Cold-room panels are made the same way in a factory press.
Good PUF has a thermal conductivity typically around 0.022 to 0.024 W/m·K, much lower than older materials such as EPS or glass wool. That means thinner walls for the same insulation, so more storage space for the same outside size. Modern foams use blowing agents with no ozone impact, often cyclopentane or other low-GWP options.
Quality depends on density, uniform filling and curing. Voids from poor injection create warm spots, condensation and wasted energy. Once foam soaks up moisture, through a damaged liner or a cracked panel joint, it insulates less well. A buyer cannot see inside the walls, so the evidence is consistent outer surface temperature, no sweating on the outside and stable energy use.