A vapor mitigation apparatus and method for use with the construction of a building. The apparatus includes a vertical member configured to be affixed to a foundation wall proximate a footing that is supporting the foundation wall. The vertical member has a first end located proximate to a footing and a second end. A horizontal foot is located at the first end of the securing piece. The horizontal foot is configured to rest against the footing. The apparatus may include a mechanical clip located at the second end of the vertical member and is configured to secure a gas impermeable membrane. In the alternative, the vertical member and the gas impermeable membrane are of a unitary structure. The method includes securing the apparatus to the foundation wall, sealing the gas impermeable membrane to a membrane that is covering the footings or granular base of the building and then creating a concrete slab by pouring concrete over the footing and apparatus. The apparatus may also include a membrane material and a rigid ring located at the approximate center of the membrane material. An inner portion of the membrane material is located inside the perimeter of the ring. The rigid ring is configured to be located around one of the at least one penetration such that the inner portion of the membrane material is configured to stretch around the one penetration resulting in the sealing of the apparatus to the penetration.