Polyurethane sandwich structures are very useful in construction because of their excellent thermal insulating properties. Even though the chemicophysical processes occurring inside the production line crucially affect the quality of the final product, up to now, they are not monitored at all. Thus, the aim of this paper is to present a new fundamental approach for non-invasively monitoring these processes online by using air-coupled, low-frequency ultrasonic plate waves ("Lamb waves"). Hereby, a foaming cell incorporating the ultrasonic unit was constructed on a laboratory scale. Then, sandwich structures were fabricated with polyurethane consisting of various ratios of the educts di-isocyanate and polyol. Air, instead of liquid was used as the coupling medium to transport the ultrasonic waves that were subsequently monitored. For comparison, tensile strength and microscopic images were analyzed. Most importantly, various wave amplitudes and phases were recorded depending on the actual chemical properties of the polyurethane and its physical state. Consequently, this technique permits the direct and non-invasive tracking of the foaming process thereby avoiding expensive panel replacements.