Foam has the potential to effectively carry and distribute either aqueous or gaseous amendments to the deep vadose zone for contaminant remediation. However, the transport of foam in porous media is complicated because flow characteristics such as effective viscosity are affected not only by foam properties but also by the sediment properties and flow conditions. We determined the average effective foam viscosity via a series of laboratory experiments and found that the effective foam viscosity was significantly higher than the viscosity of gas or water and increased with the liquid fraction in foam, the injection rate, and sediment permeability. These impacts are successfully described with a mathematical expression, which is further demonstrated with data from the literature. These results suggest that, when foam is used in deep vadose zone remediation, foam flow is almost not affected by gravity and is capable of distributing amendments uniformly in heterogeneous sediments.