RE: Prevents conductive heat loss to cold ground.
External Vapor Barrier
RE: Protects the insulation from water (rain, snow, wet ground, etc.) and reduces convective heat loss from wind.
RE: Trap heat. A minimum of two layers is typically required; however, more may be necessary depending on the type of insulation and the conditions.
RE: Add heat to the patient. Hot packs are typically sandwiched between insulation layers to prevent thermal damage to the patient. They may also be wrapped with multiple socks or sweaters if multiple insulation layers are not used. In the severely hypothermic patient, hot pack are placed only on the patient's trunk to prevent systemic vasodilation and a catastrophic drop in blood pressure. Once a patient's core is warm and hydrated, consider placing a hot pack at the patient's feet to increase comfort.
Internal Vapor Barrier
RE: Minimize heat loss due to infrared radiation and reduce vapor loss through the packaging. It also acts to protect insulation layers from urine or feces; consider diapering the patient prior to placing in the hypothermia package.