A container insert for providing a thermally insulated enclosure. The container insert includes a bottom panel, side panels and end panels that are interconnected by a plastic backing sheet. The side panels and end panels can be pivoted upwardly to form side walls and end walls. The plastic backing sheet covers the seams between adjacent insulation panels to provide a moisture barrier. The plastic backing sheet can also urge adjacent panels together to reduce the gap between adjacent panels and improve thermal performance.