Early investigations of dielectric permittivity of rigid polyurethane foams at low frequencies were made on petrochemical-origin foams, mainly by means of parallel plate capacitors. In the present investigation biopolyol was synthesized from Latvia-grown rapeseeds’ oil by the transesterification method with triethanolamine, in an environmentally friendly process, without emission of harmful substances, at temperatures 175°C ± 5°C. Rigid, closed-cell rapeseed oil polyol polyurethane biofoams and petrochemical foams were made ensuring content of the renewable rapeseed oil polyol in ready foams 27 wt.%–29 wt.%. Dielectric permittivity of the polyurethane foams and the underlying monolithic petrochemical-origin polyurethane and biopolyurethane was measured with a non-destructive dielectric spectrometer equipped with a capacitive sensor of one-side access type at 16 discrete frequencies distributed geometrically over the band 10 Hz, …, 330 kHz. Permittivity value of the gaseous phase in the closed-cells was estimated to be εg ≈ 1.001 that corresponds to the values, characteristic for the most of gases. Dielectric permittivity of petrochemical polyurethane foams and the mentioned biofoams was compared with permittivity of polyurethane foams from industrial producers Sika JSC and General Plastics Manufacturing Co. Polyurethane foams of the developed formulation exhibit competitive, low dielectric permittivity, not exceeding that of the foams from industrial producers: petrochemical foams up to 550 kg/m3 and the mentioned biofoams, comprising the renewable rapeseed oil polyol, up to densities 230–250 kg/m3. Considering petrochemical-origin polyurethane foams as a heterogeneous media “Polymer—gaseous phase”, the applicability of the rule of mixture and Maxwell–Garnett equation to model mathematically the dependence of effective dielectric permittivity on the volume fraction of phases was showed. © 2020, Tech Science Press. All rights reserved.