A blanket does not generate heat on its own; instead, it works as a highly effective thermal barrier. The feeling of warmth comes from the blanket’s ability to slow down the natural process of heat escaping from your body into the cooler environment. By preventing this heat loss, the body’s continuous energy production creates a warm microclimate underneath the fabric.
The Body as the Heat Generator
The human body functions as a constant heat source due to metabolic processes occurring within cells. This internal energy production, known as the metabolic rate, continuously generates heat, which is transferred to the skin’s surface by blood circulation. In a resting state, the body generates approximately 100 watts of thermal power to maintain a core temperature near 98.6°F (37°C).
When the body is uncovered, this heat escapes into the environment through three main mechanisms. Conduction is the transfer of heat through direct contact with a cooler surface. Convection is the heat loss that occurs as warm air near the skin rises and is replaced by cooler air, carrying heat away. Radiation involves the body emitting infrared electromagnetic waves directly to cooler surrounding surfaces, accounting for about half of the total heat loss.
Blocking Heat Loss Through Insulation
The primary function of a blanket is to create a localized thermal barrier that interferes with all three methods of heat transfer. A blanket’s effectiveness relies on trapping a layer of air close to the body, since still air is a poor conductor of heat. The fibers and weaving create countless tiny pockets that capture this air, preventing it from moving freely.
This trapped, stationary air drastically reduces heat loss by convection, as the warm air layer surrounding the skin cannot circulate and be replaced by cooler air. The blanket’s construction also limits conduction, because the solid fibers have a low thermal conductivity. The thickness or “loft” of the blanket is directly related to its insulating power, since a greater loft means more trapped air pockets.
The fabric also reduces heat loss through radiation by acting as a partial shield against the infrared waves emitted by the body. By slowing down these avenues of escape, the blanket allows the heat continuously produced by the body to accumulate between the skin and the fabric. This accumulation creates a warmer layer that reduces the temperature difference between the body and its surroundings.
Why Material Matters
The warmth provided by a blanket depends on the material’s ability to create and maintain this insulating layer of trapped air. Natural materials like wool have a crimped fiber structure that creates numerous small air pockets, supporting a high degree of loft. Down feathers offer superior insulation because their three-dimensional clusters expand to trap a large volume of air for a minimal weight.
Synthetic options, such as polyester fleece, achieve a similar effect through a dense pile construction engineered to maximize air capture. The thermal performance of any material is also influenced by its moisture management properties. Wool resists absorbing water, which helps maintain its insulating loft even in damp conditions.
If a blanket material absorbs moisture, water molecules replace the trapped air in the fiber structure. Since water is a much better conductor of heat than air, this compromises the blanket’s insulating ability and results in a colder feeling. Therefore, the best materials are those that are lightweight, low density, and possess a fiber structure that effectively locks still air in place.