A thermal detector with a monolithic structure comprises a layer of material sensitive to infrared radiation and an insulating layer constituted by a thermostable polymer that can be deposited as a thin layer and has a microporous structure. This insulating layer enables the thermal decoupling of the sensitive layer from the substrate comprising reading circuits with which the detector is provided. The performance characteristics of currently used monolithic infrared detectors can thus be substantially improved through the notable reduction of the thermal losses in the sensitive layer. This is achieved through the greatly reduced thermal conductivity of the layer of dielectric polymer. Application to infrared imaging devices.