Thermodynamics of Home Energy Efficiency

Plummeting temperatures and the appearance of snow are common signs that winter is approaching. For many homeowners, this means cranking up the thermostat and preparing for the impending cold weather. The change in outside temperatures greatly influences the comfort of our homes because energy is used to maintain desirable temperatures. In order for occupants of residential buildings to stay comfortable, they must maintain a certain level of thermal control. The idea of the remote control goes far beyond a thermostat that maintains a consistent temperature inside a home. Thermal control means containing thermal energy and reducing the amount of energy that is lost. By containing the heat that we produce, we can minimize the fluctuation of indoor temperatures and maximize energy efficiency.

Thermodynamics can explain why changes in outdoor temperatures affect residential structures. Radiation, convection, and conduction are solely responsible for energy transfer, but in this aspect, we are examining these processes in relation to the transfer of thermal energy. The three processes of energy transfer are the main reasons why certain materials are more productive at limiting thermal energy transfer than others. An important aspect of thermal control that will be further discussed is insulation. Generally, insulation is used to limit the amount of heat that is being transferred between two objects or areas. The following paragraphs will illustrate the ways that energy and heat are transferred and will also discuss strategies on how to maximize energy efficiency for outdated homes.

In this day and age, energy efficiency seems to be a primary goal for residential buildings. In order to make a residential building more energy-efficient, we must first examine how energy is lost. All thermal changes for residential homes are determined by thermodynamics. Britannica.org states that thermodynamics is the “science of the relationship between heat, work, temperature, and energy. In broad terms, thermodynamics deals with the transfer of energy from one place to another and from one form to another.” Thermodynamics can explain why heat is being transferred from your home to the outside world. The reason why our homes radiate heat in the winter is that every thermodynamic system is pursuing a state of thermodynamic equilibrium. It is the nature of temperature to tend toward a consistent level. This means that technically, your house isn’t getting colder; the air outside is getting warmer. Every time thermal energy is released from residential buildings, the surrounding area experiences an increase in temperature. Because it would be nearly impossible to heat the outside air up to a comfortable level, we notice a more drastic temperature change indoors.

Now that we know why our houses get colder in the winter, it is important to understand how this happens. There are three main ways that this type of energy is moved around. An article from Building Science Corporation states that “Conduction is the flow of heat through a material by direct molecular contact. Convection is the transfer of heat by the movement or flow of molecules (liquid or gas) with a change in their heat content. Radiation is the transfer of heat by electromagnetic waves through a gas or vacuum.” A campfire is a good example of the relationship between these three types of energy transfer. You will notice the effect of conduction right away if you made direct contact with the hot coals. These two objects would gravitate toward thermal equilibrium because of the difference in thermal energy between a human hand and burning hot coal. Another way that a thermal equilibrium can be established is through convection. Convection transfers heat through the flow of particles. The process of convection can be noticed when a gust of wind blows heat from the campfire. In the campfire example, the most common way of transferring thermal energy is by radiation. The campfire radiates a certain degree of heat in pursuit of thermal equilibrium. Many people are familiar with radiative heat transfer because it is what roasts our marshmallows and cooks our food.

These methods are important factors for home energy efficiency as well. During winter months, convection, conduction, and radiation are responsible for the change in household temperatures. All three of these heat transfer methods contribute to the loss of heat in residential buildings, but conduction and convection are the most common forms of heat loss. Heat loss in homes due to conduction occurs when any object comes in contact with the building directly. For example, basements and flooring can contribute to the loss of thermal energy throughout a structure. The cold ground pulls the heat that is produced from inside the house through direct contact. A similar process can be observed through the walls, roof, and windows of a residential building, but in this case, heat loss is due to convection. As the cold air moves past the house it draws heat from exposed areas, lowering the inside temperature.

Convection and conduction often work together during the transfer of thermal energy. An example of this is when heat from inside the home is trapped in the walls and then pulled towards the outer parts of the house by conduction, where it is then removed through convection. Radiation plays a significant role in the loss of heat in residential buildings by releasing thermal energy through poorly insulated walls and windows. Heat loss in the form of radiation can be further explained through the third law of thermodynamics which states that “The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K).” This means that any object that is not at absolute zero will have thermal energy to radiate. With an increase of thermal energy in an object, there will also be an increase in the rate at which it expels that energy. Radiation of thermal energy is the result of objects attempting to reach absolute zero, which is a stable and constant temperature where the molecules have no movement at all.

Solids, liquids, and gasses behave differently when they radiate heat. Liquids and gases transfer thermal energy through radiation more easily because molecules in those substances move faster than the molecules of a solid object. By nature, molecules of gas move the fastest and the molecules of a solid object move the slowest. This is not true for solids because conduction is the most efficient way for thermal energy transfer between two solid objects. This is the reason that metal wires are used to transfer energy. The direct contact that is made between certain solid objects allows for a quick transfer of energy between the molecules. Convection is not possible for solids because it relies on the movement of free-flowing particles to transfer energy. Convection, radiation, and conduction work differently on solids, liquids, and gases. It is important to understand the easiest and most common ways for heat to be lost.

While the transfer of energy is an important factor in home energy efficiency, the resistance of heat transfer is just as influential. There are ways that thermal energy can be conserved and the transfer of thermal energy resisted. This idea is known as insulation, which a concept that nearly everyone has experienced in daily life. When it is cold outside, we put a coat on, and if we are removing something hot from the oven, we use oven mitts. These everyday items are all forms of insulation. Residential buildings are no exception. Homes are designed to utilize insulation for the sole purpose of containing thermal energy. In general, materials with a low density make better insulators than materials with high density. Heat and energy flow through the molecules in objects, which means that the closer the molecules are, the easier the heat will flow. Objects with insulating properties control heat flow and minimize the transfer of heat between two objects. Insulation works by slowing the time it takes for two objects to reach thermal equilibrium. By imposing a thermal insulator, it is possible to prevent the temperature of an object from changing to that of the objects surrounding it.

Effective insulation will significantly reduce the rate at which energy transfers, allowing the object to contain its thermal energy for a longer period of time. This is especially important for residential buildings because of the need to create a consistent and comfortable environment for everyday living. There are specific reasons why some types of insulation proved to be more effective than others. Energy.gov says that “An insulating material’s resistance to conductive heat flow is measured or rated in terms of its thermal resistance or R-value -- the higher the R-value, the greater the insulating effectiveness. The R-value depends on the type of insulation, its thickness, and its density.” Every type of insulation responds differently to each of the energy transfer processes.

Conduction is a typical energy transfer process that requires the use of thermal insulation. The effects of conduction are almost nonexistent when the two objects are separated. For example, conduction would occur if you were to grab a hot pan out of the oven. An object as simple as a potholder greatly reduces this risk by interrupting the connection between the two solids. Although conduction is still being used to transfer heat, we do not feel the full transfer of energy because the potholder restricts the amount of heat transferred. However, this process is only reliable for a certain amount of time. Like all objects with a difference in temperature, the potholder will eventually be heated to the temperature of the pan it is in contact with. This demonstrates that no physical material is able to perfectly insulate thermal differences. When it comes to energy transfer by convection, a physical thermal insulator can be employed in the same way.

Convection uses the movement of fluid to carry thermal energy. Materials that restrict these movements of fluid from reaching an object provide insulation and keep the flow of thermal energy to a minimum. An everyday example of this would be the use of a windbreaker to combat the effect of windchill. Physical barriers prevent the motion of the fluid from coming in contact with an object, insulating it from heat transfer by convection. Thermal insulators that are used to combat heat loss from convection and conduction work differently than insulators meant to control radiative energy transfer. It is understood that all objects with any sort of heat radiate that thermal energy in pursuit of absolute zero. In order to contain this process of energy transfer, thermal insulators that control radiation rely on reflecting thermal energy back towards the object.

A statement given in an article by Ron Kurtus on thermal insulation provides an insightful example of insulation from radiation. He said, “A thermos bottle not only has an evacuated lining to prevent heat transfer by conduction, but it also is made of shiny material to prevent radiation heat transfer. Radiation from warm food inside the thermos bottle is reflected back to itself. Radiation from warm outside material is reflected to prevent heating cold liquids inside the bottle” Reflective insulation is an important way to contain heat and minimize thermal transfer caused by radiation. This type of insulation only works to radiate heat back towards a warmer object, meaning that the use of reflective materials is ineffective at keeping colder thermal energy from impacting a warmer object. Understandably, no system relies on one type of energy transfer to achieve thermal equilibrium. Thermal insulation must be effective at controlling all three of the processes of energy transfer. Now that we have established how heat is transferred and ways that it can be retained, we can discuss how the typical residential building uses energy and where that energy goes. Energy is an important part of daily life. It is what powers our phones, cooks our food, and keeps our lights on. Our homes consume energy in a multitude of ways. Despite this, we often forget about the amount of energy that is used to keep us warm.