The experience of setting an indoor thermostat to 70°F (about 21°C) only to feel a persistent chill is common. This discrepancy arises because air temperature is only one component of thermal comfort. The human body continuously exchanges heat with its surroundings through physical mechanisms, and a variety of environmental factors influence these heat transfer processes. This results in a perceived temperature that is often much lower than the simple air temperature displayed on the wall.
The Impact of Air Movement
One immediate cause of feeling cold at 70°F is the movement of air, a process known as convection. The human body naturally maintains a thin, insulating boundary layer of warmed air directly adjacent to the skin. This blanket of air is heated by the body’s metabolic warmth and helps slow the rate of heat loss to the environment.
When air moves across the skin, this protective boundary layer is constantly stripped away and replaced with cooler, ambient air. This accelerated removal of heat is perceived as a draft or an indoor wind-chill effect, making the air feel colder than its measured temperature. Standard heating, ventilation, and air conditioning (HVAC) systems are sources of this air movement. Air flowing from vents, or movement across a large room, can trigger this cooling sensation.
Drafts from poorly sealed windows, doors, or electrical outlets can also create localized, high-velocity air streams. The effect is noticeable when sitting still, as the body’s metabolic heat production is low and localized heat loss increases. For thermal comfort, air speed is recommended to be below 0.15 meters per second, as exceeding this threshold rapidly increases the feeling of coolness.
Understanding Radiant Heat Transfer
A second factor influencing comfort is the transfer of heat through thermal radiation, which occurs via electromagnetic waves and accounts for a substantial portion of the body’s total heat loss. Unlike convection, this heat transfer does not require air movement; it is an exchange of heat between objects of different temperatures that are in line-of-sight. The body, with a typical surface temperature near 85°F (29°C), continuously radiates heat outward to surrounding surfaces.
The perceived temperature is influenced by the Mean Radiant Temperature (MRT), which is the average temperature of all solid surfaces surrounding a person, including walls, windows, floors, and the ceiling. If the air temperature is 70°F but surrounding surfaces are significantly colder—perhaps 55°F (13°C) due to poor insulation—the body continuously loses heat to them. This constant net loss of heat to a low MRT makes the person feel cold.
This phenomenon explains why sitting next to a large, uninsulated window in winter feels chilly, regardless of the thermostat setting, as the body radiates heat directly to the colder glass surface. A difference of just a few degrees between the air temperature and the Mean Radiant Temperature can significantly alter comfort, as radiation can account for nearly half of the body’s total heat dissipation. Achieving thermal comfort requires balancing the air temperature with the temperature of the surrounding building materials.
The Body’s Thermoneutral Zone
The physiological response of the human body provides another explanation for why 70°F feels cold, particularly for an inactive person. The body strives to maintain a stable core temperature, doing so most efficiently within the thermoneutral zone (TNZ). Within this specific range of ambient conditions, the body regulates its temperature simply by adjusting peripheral blood flow, without expending additional energy.
For a lightly clothed person at rest, the lower boundary of the thermoneutral zone is often cited between 60°F and 70°F (15°C to 21°C). When the ambient temperature drops near or below this lower critical temperature, the body must actively conserve heat. The primary mechanism for this is peripheral vasoconstriction, the narrowing of blood vessels near the skin’s surface, particularly in the hands and feet.
This reduction in blood flow to the extremities prioritizes the warmth of the core organs. While effective, this action lowers the skin temperature of the hands and feet, which are highly sensitive to cold, leading to the sensation of feeling chilly. If the temperature drops further, the body must increase its metabolic rate above its resting level, which is a significant energy expenditure and is perceived as discomfort.
How Low Humidity Increases Cooling
The amount of moisture in the air, or relative humidity, also plays a role in thermal sensation. Central heating systems, especially forced-air furnaces, draw in cold outdoor air that holds little absolute moisture and then heat it. As the air is warmed, its capacity to hold water vapor increases, causing the relative humidity inside the home to plummet, sometimes falling below 30%.
This dry air accelerates the process of evaporative cooling from the body’s surfaces. Moisture naturally evaporates from the skin and from the mucous membranes of the respiratory tract with every breath. Since the dry air has a greater capacity to absorb this moisture, the rate of evaporation increases, drawing heat away from the body more quickly. This accelerated latent heat loss contributes to a feeling of coldness.
This rapid evaporation can lead to dry skin and irritated sinuses, which are signs of a dry indoor environment in winter. The sensation of coolness from the dry air, combined with the discomfort of physical dryness, prompts many people to increase the thermostat setting. Maintaining an optimal indoor humidity level, typically between 40% and 60%, helps slow this evaporative heat loss, making the same air temperature feel warmer and more comfortable.