An intercomparison of cryogenic radiometers of HUT/VTT (Finland), PTB (Germany) and SP (Sweden) using silicon trap detectors was performed. Also results of monitoring of the responsivities of trap detectors over a 6 year period are presented. It was noticed that several non-idealities can undermine the agreement between results of calibrations. The non-uniformity of the responsivity of trap detectors made the results sensitive to alignment and beam size. The nonlinearity of some trap detectors was found to be large. The average relative differences in the results of the laboratories, when all known corrections are applied, are +1×10-4 for HUT-PTB, -3.7×10-4 for SP-PTB and−4.3×10-4 for SP-HUT, with standard uncertainties of 4.3×10-4, 3.0×10-4 and 4.4×10-4, respectively.