The basic principle behind thermal insulation is simple to understand. The harder it is for heat to travel through a material, the better insulator that material will make. In this blog, I’m going to take a look at how that effectiveness is quantified, after a brief review of the three methods of heat transfer. The building and construction industry use a ratio called the R-factor to indicate how well a building material can insulate a space. Also called the thermal insulance or thermal resistance, a higher R-value indicates a more effective insulator.
Heat Transfer Methods
There are three methods through which a material can lose heat (energy) to its surroundings: conduction, convention, and radiation. The efficacy of each method depends on both the characteristics of the material and the environment around it.
Conduction is a simple form of heat transfer that takes place in solids, liquids and gases through the transfer of energy from high-energy molecules to their lower-energy neighbors. The molecules in an area of high temperature have a high energy compared to adjacent areas of lower temperature, and while vibrating will collide with those lower energy molecules and transfer some of their energy. Without any energy added to a system, conduction between all of the molecules will eventually equalize the temperature of all of the objects. For instance, your hot coffee transfers its energy to the “I Hate Mondays” mug holding it, and at some point the sides of the mug will be the same temperature as the now slightly-cooler coffee.
In general, conduction happens slowly and evenly across a material. The rate of conduction is governed by the temperature difference between materials and each material’s thermal conductivity, or k, in W/(m·K) or BTU/(h·ft⋅°F). Metals have a high k, ceramics and glass are lower, and wood, cloth and gases have the lowest values. Conduction is the most important heat transfer method when considering insulation.
Convection relies on the same mechanism as conduction, where high-energy molecules collide with and transfer energy to lower-energy neighbors. However, the difference in convection is that those lower-energy molecules are removed and replaced by unaffected molecules. Convection always involves a fluid (a gas or liquid) which can continually carry heat away from the hot object. Since the cooling fluid is always kept at the same low temperature instead of equalizing with the hot material, convection cools much faster than conduction alone.
Radiation operates differently than convection or conduction, instead involving the release of photons by excited (high-energy) molecules. For heat transfer, those photons are radiated in the infrared spectrum. Unlike convention and conduction, radiation heat transfer can occur in a vacuum without any surrounding medium. In fact, it’s the only method available for space vessels to dump the internal heat they create. Radiation is much slower than conduction or convection for most materials, and doesn’t affect building design as much.
Thermal resistance, the “R-value”, represents the ability of a material to slow down the conduction of heat from a hot environment on one side to a colder environment on the other. It is calculated equation (1)
R = R-value
L = thickness of the layer
k = thermal conductivity
As L increases, the R-value increases, meaning the material is more effective at slowing heat transfer. On the flip side, as the thermal conductivity increases, R-value decreases and the material is less effective at slowing heat transfer. The thermal resistance is analogous to resistance in a circuit, which impedes the current flow. Like resistors in series, when multiple layers of insulation are combined, their R-values are simply added together. Because of its simplicity, the R-value is a very useful method for comparing the efficacy of different insulation materials and thicknesses.
It should be noted that materials are sometimes specified in terms of their “U-factor” instead of the R-value. The U-factor is simply the inverse of the R-value, and is called thermal transmittance. A U-factor of 0 means a material allows no heat transfer through it. Increasing U-factors represent less-effective insulators that let more heat pass through.
R-Values for Walls and Windows
As I mentioned above, calculating the total R-value for a combination of materials is as simple as adding the various R-values together. Table 1 below shows the R-value calculations, using equation 1, for the wall of a typical residential home (cross section shown in Figure 2). The table includes the inside and outside air films, which each offer their own small amount of conduction resistance. Even if air is moving across a surface, the speed drops down very close to the surface due to friction, which this creates a small film of still air that acts as a thermal insulator.
Table 1: R-Values for a common residential wall
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An R-value of 21 is fairly typical for most older houses, though the US Department of Energy now recommends new or remodeled houses meet at R-value of 38 for southern climates and 49 for northern climates.
R-Values in Windows
However, the weak point in any building’s insulation is not usually the insulation in the walls, but the windows and skylights. Even the most energy efficient windows are only one third as effective at insulating as the simple wall shown in Table 1. Table 2, reproduced from Mechanical and Electrical Equipment for Buildings Appendix Table E.15, show the R-values for some standard window configurations. The single-pane window has an abysmal R-value of 0.77, while a triple-paned low-emissivity window reaches 6.67. R-values for windows are not calculated the same way as normal insulation. There is a more complicated method for determining the equivalent R-value for windows that takes into account radiation, convenction, and emissivity.
Table 2: R-Values for common window configurations (reproduced from Mechanical and Electrical Equipment for Buildings, 11th edition, Appendix Table E.15)
|Layers of glazing and spaces (outside to inside)||Total Window U-factor (W/m2·K) [BTU/ft2·°F·h]||Total Window R-factor (m2·K/W)|
|Single-glazed||3mm [⅛ in] clear||7.38 [1.30]||0.14 [0.77]|
|Double-glazed||3mm [⅛ in] clear|
13 mm [½ in] air
3mm [⅛ in] clear
|3.63 [0.64]||0.28 [1.56]|
|Double-glazed low-εf||3mm [⅛ in] low-ε 0.08|
13 mm [½ in] argon
3mm [⅛ in] clear
|1.7 [0.30]||0.59 [3.33]|
|Triple-glazed low-εf||3mm [⅛ in] low-ε 0.08|
13 mm [½ in] krypton
3mm [⅛ in] clear
13 mm [½ in] krypton
3mm [⅛ in] low-ε 0.08
|0.85 [0.15]||1.18 [6.67]|
There is a lot to be gained by upgrading from single-paned windows to double- or triple-paned versions, but they will still cause more energy loss than the walls around them. Figure 1 shows two houses; the house on the right is a darker blue, indicating better insulation. However, in both houses, the windows light up bright red, showing just how much energy they let escape.
Designing and Specifying Insulation Materials
In the current energy-conscious environment, more and more people are paying attention to insulation and glazing when designing a new building or refurbishing an existing one. Every year, more local and state governments are setting specific requirements for energy efficiency (such as California’s Title 24 Energy Efficiency Standards). Even beyond the legal requirements, correctly insulating a building and balancing the window area more than pays for itself over the long run. In the upcoming blogs, I will delve deeper into the specifics of certain energy-saving techniques and evaluate different methods and building materials.
- Thermal Conductivity of Materials and Gases, http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html. Retrieved on March 24, 2016.
- Grondzik, W. T., Kwok, A. G., Stein, B., & R. (2009).Mechanical and Electrical Equipment for Buildings, 11th Edition. John Wiley & Sons.