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Canopy effect refers to the phenomenon where moisture accumulates underneath an impervious cover. Field observation reveals that canopy effect can take place in relatively dry soils where the groundwater table is deep and can lead to full saturation of the soil immediately underneath the impervious cover. On the other hand, numerical analysis based on existing theories of heat and mass transfer in unsaturated soils can only reproduce a minor amount of moisture accumulation due to an impervious cover, particularly when the groundwater table is relatively deep. In attempt to explain the observed canopy effect in field, this paper proposes a new mechanism of moisture accumulation in unsaturated freezing soils: vapour transfer in such a soil is accelerated by the process of vapour-ice desublimation. A new approach for modelling moisture and heat movements is proposed, in which the phase change of evaporation, condensation and de-sublimation of vapor flow are taken into account. The computed results show that the proposed model can indeed reproduce the unusual moisture accumulation observed in relatively dry soils. The results also demonstrate that soil freezing fed by vapour transfer can result in a water content close to full saturation. Since vapour transfer is seldom considered in geotechnical design, the canopy effect deserves more attention during construction and earth works in cold and arid regions.