The collector box is insulated wherever possible to reduce heat loss to the surrounding air. The insulation should be either a high temperature foam or low binder fiberglass. Residential-grade fiberglass contains binders that will outgas and condense on the collector's glazing. Ideally, the protective box that encloses the absorber system will be made of a durable material such as aluminum, especially if the collector has a high efficiency absorber. Wood components in flat plate collectors have spontaneously ignited during malfunction.
Wood is safer in low efficiency collectors.
Batch Collectors Batch collectors use a hot water storage tank as the solar energy absorber. A black bucket is a simple version of a batch collector. Change the bucket to a water tank, add an insulated box with glass on the south side and you have a typical batch solar hot water heater. Sometimes they are built with an insulated door that closes over the glass to prevent heat loss at night. The door usually has a reflective surface to increase heat gains by reflecting additional light into the box.