Terms You Should Know

Thermal conductivity
Thermal conductivity is the K-value. It is the measure of a material’s ability to conduct heat. A low K-value indicates that the material does not allow heat energy to easily pass through. According to the National Roofing Contractors Association (NRCA), a material must have a K-value of 0.5 or less to qualify as thermal insulation.

Thermal resistance
Thermal resistance is the R-value. It is the measure of a material’s resistance to conductive heat flow. It is the most common reference used in the insulation industry. A high R-value indicates that the material resists the flow of heat well. It is the inverse of the U-value.

Thermal transmittance
Thermal transmittance is the U-value. It measures how much heat is lost or gained by a material. A low U-value indicates that a certain material gains a small amount of heat in a given time. It is more common in the fenestration industry than in the insulation industry.

British thermal unit (BTU)
A British thermal unit (BTU) is a unit of heat. It is the amount of heat needed to raise the temperature of 1 pound of water by 1º Fahrenheit.


Photo courtesy of InterNACHI®


Basic Concepts of the Thermodynamics of Thermal Insulation

The primary purpose of insulation in a roof system is to reduce the flow of heat, or thermal energy, between the external and internal environments, thus providing thermal resistance. The key to understanding how thermal insulation works is to understand how heat transfers from one body to another. It does so by one (or more) of three methods. These are:

  1. Conduction is the process by which heat transfers through a solid material. Scientifically, it’s the transfer of heat from molecule to molecule. One molecule becomes energized, and it then energizes adjacent molecules through a material medium. For example, as the sun heats a roof, heat is transferred through the adjacent building materials.
  2. Convection is the process by which heat transfers within a gas or liquid. Convection actually consists of two mechanisms occurring simultaneously. The first is the transfer of heat from molecule to molecule, and the second is via fluid, consisting of a large number of molecules, moving as a result of an external force. The external force is considered forced convection when it is created mechanically, such as by a pump or fan, and it’s considered free convection when a density gradient is present, such as when the sun heats warm air and it rises.
  3. Radiation is the process by which heat transfers through electromagnetic waves and is absorbed by a surface. Heat is transferred by direct rays. It travels in a straight line from the source of heat to a body. Radiant heat leaving a surface depends on the surface’s ability to emit long-wave infrared radiation (emissivity), and the temperature gradient between the warm objects emitting radiation and the colder objects absorbing it. Emissivity values range from 0 to 1. A low-emittance value means the material emits low levels of radiant heat. A low-emittance value also indicates a highly reflective surface. For example, aluminum foil has a low-emittance value and it is used in reflective insulation.

These three methods outline the first law of thermodynamics, also known as Law of Conservation of Energy, which states that energy can be neither created nor destroyed in an isolated system; rather, it’s transformed or transferred from one form to another. Although these methods are described separately, heat is usually transferred through a combination of the three methods and is relevant in roofing science as related to thermal insulation. The second law of thermodynamics is also pertinent, and it states that whenever a temperature gradient exists, heat will flow from a mass or area having a higher temperature to a mass or area having a lower temperature by the path of least resistance.

Thermal insulation materials must have low thermal conductivity. In most cases, this is achieved by trapping air or some other gas inside small pockets in a solid. Hence, thermal insulation materials use the low conductivity of gases – compared to liquids and solids – to impede heat flow. But heat can also be transferred by free convection inside the gas pockets, and by radiation between the materials. All of this contributes to the total resistance to heat flow within the roofing system. However, conduction is the primary method of heat flow through building materials, and that’s what insulation impedes most effectively.

The Thermal Resistance of Thermal Insulation

As mentioned above, the function of insulation is to provide resistance to the flow of heat. Insulation is rated according to the effectiveness of its resistance to the conductive heat flow, which is measured in terms of R-value, or thermal resistance. This value depends on the type of insulation, its density, and its thickness. Age, temperature, and moisture can also play a role in the R-value of insulating material. The greater the R-value, the greater the efficacy of the insulation. Typically, with multiple layers of insulation installed, you can just add the R-values of the individual layers to determine the overall R-value of the installation. The insulation requirement will vary depending on the jurisdiction, climate, HVAC system, building use, and effectiveness of the roofing assembly as a whole.

Multiple factors must be taken into account when designers consider the R-value as a means to ascertain the thermal resistance of a building component.  Proper installation is most important, as the R-value is accurate for use as directed. Smashing two layers into the space designed for one will not double the R-value, as each layer needs its own thickness space to provide its rated R-value.  As an inspector, keep in mind that proper installation of the insulation in the roof system is key to the product’s optimal performance. Layers of insulation that have been compressed due to exceeding their load capacity, or through damage to the roofing system, will have diminished thermal resistance.

Another factor to consider is that penetration to the roof system breaks the thermal barrier. These are areas to consider when performing an energy audit. The equation for calculating R-value may be useful for inspectors, as it can be used to help calculate heat loss.  The equation for determining R-value is:

R-value = temperature difference x area x time ÷ heat lost

Where the area is stated in square feet, a temperature difference is in degrees Fahrenheit, time is measured in hours, and heat loss is in BTUs.

Building and energy codes specify a required minimum R-value for roof assemblies. The adopted building and energy codes vary from state to state, and even city to city, and should be considered during the initial design phase. The following chart by Carlisle Syntec System lists the applicable code and minimum R-value by state.

Insulation manufacturers usually post technical data sheets on their website that provide information about the R-value of their product. Technical data sheets posted by manufacturers also contain vital information about the proper handling of their products, which may be useful during a job the requires some sort of project oversight. Refer to the example below.


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Thermodynamics plays a crucial role in the efficiency and operation of a commercial building’s roof system. Understanding how conduction, convection, and radiation affect thermal insulation is vital to understanding how the building envelope functions and whether it is doing so efficiently. The most relevant measure to thermal insulation is its R-value. It measures the resistance to the conductive heat flow, which is the purpose that insulation has in the roofing system. Commercial property inspectors can refer to technical data sheets posted by manufacturers for information about specific thermal insulation products.