A foam insulated vessel, such as a water heater. The water heater includes a tank to contain water to be heated and a jacket is spaced outwardly of the tank to provide an annular cavity therebetween. An annular flexible strip is secured to the surface of one of said members, and is in sealing relation with the other of said members, thus forming a closure or dam at the lower end of the cavity. In a preferred form, the strip includes a flange secured to the tank and a body portion extends outwardly from the flange and terminates in a downwardly extending lip, which is located at an acute angle with respect to the body portion. The lip is in sealing engagement with the jacket. When a liquid foamable resin is introduced into the upper end of the cavity, the pressure acts to force the body portion of the seal downwardly, increasing the sealing effect against the jacket to retain the liquid resin within the cavity.