An elastomeric compositions having improved impermeability is disclosed. The cured elastomeric composition may comprise at least one C4-C7 monoolefin elastomer, a hydrocarbon polymer additive, and a clay. In some embodiments, the cured elastomeric composition has a permeation coefficient at 40° C. of 90 cc*mm/(m2-day) or less. In some embodiments, the elastomeric composition is substantially free of naphthenic oil.