In this lesson, students explore the impact of a building’s material properties on energy consumption. Choosing materials that minimize the energy consumed is an essential step in green building design.
Students will learn about two key properties of building materials—the R-value and the U-value—and explore how the choices they specify for the walls, roofs, and windows that comprise the building envelope affect the predicted annual energy costs for operating the building.
Thermal Properties of Building Materials
A building material’s ability to resist the transfer of heat (or insulate) is often described through a single number—the R-value.
The R-value is a measure of thermal resistance. Materials or assemblies with higher R-values are better insulators and help to moderate the effect of changes in outside temperature.
R-values are computed as the ratio of the temperature difference across an insulator to the heat that flows through that material, and the values for most typical building materials are readily available in reference books and online tables.
R-values are often given per unit of material thickness. So if a material is described as having an R-value of 1.0 per inch (25 mm), and a wall made solely from this material is 5-inches thick (0.13 m), the R-value for that wall would be 5.0 (1.0 per inch x 5 inches). More often, R-values are defined for entire wall assemblies.
U-value is the name of another property that is used to describe the thermal performance of building materials that are more conductive (less resistive) to transferring heat, such as windows and skylights.
While the R-value measures a material’s resistance to transfer heat, the U-value does the inverse—it describes a material’s ability to transfer heat (U-value = 1 / R-value).
Materials or assemblies with higher U-values are better conductors. When comparing U-values for different window or wall types, lower numbers indicate better insulators that transfer less heat per unit of thickness.
Thermal TransferWe can estimate the amount of heat that will be transferred between the interior and exterior of a building by considering:
- The thermal characteristics of each of the surfaces and assemblies that make up the building envelope.
- The temperature differences between the interior and exterior of the building for the project location at different times of the day and throughout the year.
- The range of temperatures that occupants of the building will consider to be comfortable during the times that it is being used.
Using this estimate, we can compute the amount of energy that will need to be consumed to heat and cool the building and the associated total energy cost.
While energy efficiency is an important green design goal, it is important to balance this with the comfort of the occupants and users of the building. If a building’s users perceive the environment as too hot or too cold, they will be unhappy and unsatisfied with the design.
The issue of thermal comfort is complex—depending on a number of factors that all impact the perception of the users—and subjective. No two humans are exactly alike in their preferences. Some of the factors that affect the perception of comfort include:
- Environmental factors—for example, air temperature, radiant temperature, air velocity, and humidity
- Personal factors—for example, clothing worn and personal metabolism
Analysis tools such as Autodesk® Ecotect® Analysis software’s weather tool provide calculators that help assess the environmental factors for a given design and location and explore the ways that changing one affects the other. These tools, however, do not take personal factors into consideration.
Many different measures can be used to describe thermal comfort, calculating the thermal stresses on users of a space and translating those stresses into a number. The measures available in Ecotect Analysis include:
- Predicted mean vote (PMV)—a single digit number that estimates the average rating that would be given if a large group of people were asked about the comfort of a space. It ranges from +3 (for hot) to 0 (for neutral) to -3 (for cold).
- Predicted percentage dissatisfied (PPD)—a predicted percentage of people who would be dissatisfied with the comfort. As PMV increase or decreases (indicating that most people are feeling too hot or too cold), PPD also increases. Unlike PMV, which estimates the average rating given by a large group of people, PPD indicates the range of the individual responses.
Using PMV and PPD, we can quickly evaluate the impact of design decisions and get simple measures that consider the combined effect of many different thermal comfort factors. Although these measure are useful for evaluating the overall comfort of a space, other local factors that affect human comfort should also be considered, including:
- Radiant temperature and vertical air temperature differences
- Floor surface temperature
- Cyclic variations in temperature
Factors such as the heat or cold radiated through a window or the glare created by direct sunlight can make spaces uncomfortable or unacceptable.