Question

During winter, when people are in a well-insulated house, they usually feel comfortable if the air temperature is near \(22^{\circ} \mathrm{C}\left(72^{\circ} \mathrm{F}\right)\). If you have ever spent a night in a poorly insulated cabin in winter, however, you will recognize that paradoxically, when people are in poorly insulated buildings, they often feel chilly even when the air inside is heated to \(22^{\circ} \mathrm{C}\) or higher. One important reason for the difference in how warm people feel in the two sorts of buildings is that even if a well-insulated and poorly insulated building are identical in the air temperature inside, they differ in thermalradiation heat transfer. Specifically, a person standing in the two types of buildings experiences different heat exchange by thermal radiation in the two. Explain how thermal-radiation heat transfer accounts for the sense of chill in the poorly insulated building. (Hint: Think of the outer walls of the two types of buildings, and think specifically of the temperatures of the interior surfaces of those walls.)

Short answer

The feeling of being chilly in a poorly insulated building, despite having the air temperature at the same level as in a well-insulated building, is due to the lower level of thermal radiation. In the poorly insulated building, heat escapes quickly to the outside, lowering the temperature of the interior surfaces, and therefore the amount of thermal radiation emitted inside decreases, causing occupants to feel colder.

Step-by-step solution

Understanding Thermal Radiation

Recall that thermal radiation is a type of heat transfer that occurs due to the emission of infrared waves from a heated body. These waves can travel in all directions and not necessarily need a medium-like air to be transferred. In this context, the heat source is the heating system of the building and the persons inside the building are the receiving bodies.

Differentiating between Well-Insulated and Poorly Insulated Buildings

In a well-insulated house, insulation slows the transfer of heat from the interior to the exterior - during winter, it slows the loss of heat to the outside. The interior surfaces of the walls stay relatively warm. This warm surface temperature allows for increased thermal radiation towards the inhabitants, making them feel warm.

Understanding the Situation in a Poorly Insulated Building

However, in a poorly insulated building, heat is quickly transferred to the outside due to poor insulation, thus the inside surface temperature of the walls decreases. This reduction in temperature results in decreased thermal radiation to the inhabitants of the building from the walls, regardless of the air temperature being maintained at 22 degrees Celsius, making the inhabitants feel chilly.

Concluding the Phenomenon

Therefore, the perceived chilliness in a poorly insulated house can be attributed to lower levels of thermal radiation heat transfer based on the cooler interior wall surfaces, even when the air temperature inside the building is the same as in the well-insulated building.

Key concepts

Infrared Waves

In discussing heat transfer, it's essential to understand the role of infrared waves. Infrared radiation is part of the electromagnetic spectrum, lying just beyond the red portion of visible light. Objects emit infrared waves as a form of energy when they are warm or hot.

When it comes to the warmth we feel in a building, these infrared waves play a critical role. They allow heat to be radiated from warm objects, such as a heating system, to cooler surroundings, and ultimately to us, without the need for contact or an intervening medium.

This means that even in the absence of air currents, we can still receive heat directly from surfaces that emit infrared radiation. On a bright, sunny day, you may have experienced warmth from the sun despite the cold air; this is infrared radiation at work.

Building Insulation

Building insulation is a critical factor in maintaining a comfortable indoor climate. Insulation materials are designed to reduce the heat transfer between inside and outside of a building.

The effectiveness of insulation is often measured in terms of its 'R-value' - the higher the R-value, the greater the insulation performance. Good insulation acts as a barrier, keeping warmth in during the winter and out during the summer.

Types of Insulation

Insulation can come in various forms, such as fiberglass, foam, or cellulose. Each has its properties and applications, but they all work to slow down the process of heat transfer, helping maintain the desired inside temperature.

Heat Transfer Mechanisms

Heat can be transferred in three primary ways: conduction, convection, and radiation. Conduction occurs when heat moves through a solid material, like the wood framing of a house.

Convection involves the movement of heat through fluids - liquids or gases - which is how heated air circulates through rooms. Radiation, on the other hand, is the transfer of heat in the form of infrared waves, which can travel even through a vacuum.

In the context of building warmth, heat transfer mechanisms work in tandem to influence how heat is distributed and how it is retained or lost. Effective insulation minimizes conduction and convection losses, while still allowing beneficial infrared radiation to keep occupants comfortable.

Perceived Temperature in Buildings

Perceived temperature goes beyond the reading on a thermostat; it reflects how warm or cool a person feels within a particular environment. This is a subjective sensation influenced by numerous factors including air temperature, humidity, air movement, personal activity level, and thermal radiation.

In buildings, the temperature of the surrounding surfaces can significantly affect this perception. If the walls of a room are cooler, as is typically the case in a poorly insulated building, the infrared radiation from these surfaces will be less, leading inhabitants to feel colder, even if the air temperature is technically the same as in a better insulated, warmer-walled counterpart.