Radon is generated in the radium-bearing grains of soils and building materials by radioactive decay of radium and diffuses out of the ground through pore spaces. The decay products of radon with short-half lives in the breathing air induce considerable health hazards to the human. An experimental model was suggested to extract the effective diffusion coefficient (effective diffusion length) of radon in porous materials. The treatment includes the short-time accumulation method by the transient-diffusion measurement of the radon exhalation rate. It is shown that the one-dimensional diffusion theory applies to samples packed in the airtight-sealed radon chamber. From full analyses of the growth curves of concentration of radon, the effective diffusion length, relaxation time, and the factor of underestimation of free exhalation rate were determined. The variability of the relaxation time is determined by effective diffusion length and sample thickness. It is found that the exhalation rates deduced from measurements differ from free exhalation rates by a factor of 1.27 at most. The factor decreases with decreasing effective diffusion length, i.e. smaller grain size and lower porosity, which provides the effectiveness of the radon cover to be about 2. The present results can be employed in one-time measurement methods for radon monitoring.