Prefer spray-on type insulation. Polyurethane foam works best in combination with ceramic insulation paint. The paint is obvious for the exterior of the shipping container, while the foam is for the interior. The paint contains a special gas that contains inside bubbles, which gives it a high thermal efficiency. The spray-on foam is available in both open-cell and closed-cell. The difference is in the price tag, density, strength, R-factor and in waterproofing.