A method of partially insulating an interior space of a pre-formed fluted core panel is disclosed herein. The fluted core panel includes a first facesheet, a second facesheet spaced apart from the first facesheet, and webs between the first facesheet and second facesheet. The interior space is defined between the first facesheet, the second facesheet, and adjacent webs. The method includes positioning a spacer in a first portion of the interior space, positioning a membrane between the spacer and a second portion of the interior space, and positioning insulation in the second portion of the interior space. Additionally, the method includes pressing the membrane against the spacer, curing the membrane, and removing the spacer from the first portion of the interior space.