A regular blanket keeps you warm by slowing the loss of heat from your body. Its fibers and the air they hold form an insulating layer between you and the cooler room. Your body supplies the warmth; the blanket helps it escape more slowly.
That explains a familiar change: a blanket can feel cool when you first pull it over you, then become comfortable a little later. It warms as it receives heat from your body, while reducing how quickly that heat reaches the surroundings.
The air in a blanket does useful work
Heat naturally moves from warmer regions toward cooler ones. In a cool room, that means heat leaves your warmer body. A blanket adds resistance along part of that path.
Air is a poor conductor of heat when it’s relatively still. A blanket’s structure holds small pockets of air among its fibers. Heat passes through those pockets less readily than through a good conductor such as metal.
Wool and down both incorporate small pockets of air that help them insulate. The useful part of a fluffy covering is therefore more than the amount of solid material it contains: its airy structure matters too.
Keeping warm air nearby matters too
Warm air can carry energy away as it moves. Cooler air can then take its place and warm up against your skin. This transfer through the movement of a fluid, including air, is called convection.
A blanket limits the exchange between the air close to your body and the room. An open edge that lets a draft pass underneath can let some of that warmed air escape. Layers can help retain air as well: the National Weather Service’s clothing guidance explains that air trapped between layers provides insulation.
Heat can also travel as thermal radiation, which warm objects emit even without direct contact. An ordinary blanket changes the heat-transfer conditions around you; it doesn’t stop every form of heat loss.
Does a thicker blanket always feel warmer?
A thicker insulating layer can slow heat conduction when other conditions are similar. But thickness alone doesn’t tell the whole story. The material, how much air its structure holds, and how readily room air moves through or around it also affect the result.
A heavy, tightly packed fabric and a light, fluffy covering can therefore behave differently. Weight alone isn’t a reliable measure of insulation. You need to consider the structure of the covering as well as how much material is there.
Where the warmth comes from
The explanation here applies to an ordinary, unpowered blanket in cooler surroundings. An electric blanket adds heat from a separate energy source. A regular blanket mainly changes the rate at which warmth already supplied by your body is lost.
Related reading
The same idea—how quickly heat moves—helps explain why metal feels colder than wood, even when the two objects are at the same temperature.
Sources and illustration
This explanation applies the linked sources’ heat-transfer principles to an everyday blanket. The original Whybranch diagram is simplified: its arrows indicate heat flow, not measured amounts or a comparison of particular products.