The interaction of powerful laser and X-ray pulses with planar low average density (0.5–10 mg/cm3) porous agar-agar targets was experimentally studied. At a laser power density of ∼5 × 1013 W/cm2 (λ = 1.054 μm) the laser light absorption and following energy transfer processes, as well as dynamics of produced plasma were investigated in detail with a variety of optical and X-ray diagnostic methods. Volume absorption is shown to occur in experiments with laser-irradiated agar targets. An extended laser energy deposition region filled with hot (0.8–1 keV) plasma is formed inside a porous target. The laser light absorption efficiency is as high as ∼80%. The emission of 2ω0 and 3ω0/2 harmonics from laser-produced plasma is observed over the time of the laser pulse even with agar targets of 0.5 mg/cm3 average density. Characteristics of energy transfer in low-density porous media are measured in experiments on illumination of agar targets by laser pulses or X rays emitted by a thin Cu converter. The hydrodynamic mechanism is responsible for the energy transfer in laser-illuminated porous targets and the radiative energy transfer seems to be dominant in the case of X-ray irradiation. The experimental data are in reasonable agreement with predictions of a developed theoretical model describing the hot plasma layer formation and the two-stage homogenization process within the illuminated porous targets.