The type of cavity insulation has an impact on performance. Equally important is whether a layer of rigid foam insulation is placed over the sheathing. Although it complicates construction, this extra layer of insulation reduces thermal bridging and boosts the home’s overall energy efficiency significantly. The thickness of the foam layer should be carefully calculated to prevent condensation inside walls. Details vary by climate, but the foam must be thick enough to prevent the accumulation of moisture on the back of the sheathing in cold weather. Because the rigid foam will prevent the wall assembly from drying to the exterior, a vapor retarder, such as polyethylene sheeting, should not be used on the interior.