A fume blocking drain cap could include a drain cap body having a top side and an underside defining at least one aperture extending therethrough and a dissolvable membrane attached to the underside of the drain cap body. The dissolvable membrane is arranged over the at least one aperture sealing the aperture from the underside of the drain cap body. A retainer ring may be provided that abuts the bottom surface of the dissolvable membrane to retain the dissolvable membrane against the underside of the drain cap body. The cap could also include a collar having an outside surface and inside surface. The collar is attached to and depends from the underside of the drain cap body and surrounds the dissolvable membrane.