Polybutylene terephthalate can be used as a gas diffusion barrier for closed-cell rigid polyurethane foams. A thermal insulation element containing a closed-cell, rigid polyurethane foam, which is at least partially covered by a layer system containing at least one layer formed by a polybutylene terephthalate composition, is useful. Articles and devices may contain a corresponding thermal insulation structure, such as refrigerators, insulation panels, pipe insulations, water heaters, and thermally insulated transport boxes.