The thermal conductivities and the thermoelectric powers of two-dimensional hole gases in fully strained Si/Si0.8Ge0.2 heterostructures have been measured for the first lime in the temperature range 3 K to 25 K, and for carrier sheet densities in the range 0.2 to 1 x 10(12) cm(-2). The thermopower is dominated by the phonon drag contribution and a fit to theory yields a value of 4.5 eV for the acoustic phonon deformation potential, with no other adjustable parameters.