The basic physics of insulation is that heat energy shifts from warmer to cooler areas until they are the same temperature. Insulation slows this transfer which keeps buildings warm in winter and cool in summer.
Good insulators contain very few solid materials and discontinuous fibres. The voids in insulating products inhibit convective heat transfer.
Conduction
Conduction is the transfer of thermal energy through molecular transport within a material or from one material to another. The rate of conduction depends on the materials’ temperature and their state: metals are good conductors, liquids are moderate, and gases are the worst. Insulation works to inhibit conduction by preventing the flow of thermal energy between different areas.
For example, feathers and fur are great insulators because they don’t allow the movement of molecules that cause heat to pass through them easily. Similarly, sea mammals like seals and whales rely on blubber to keep them warm – a natural insulation that prevents the movement of molecules.
A material’s ability to resist conduction is known as its thermal resistance, or R-value. This is determined by dividing its thermal conductivity (lambda value) by its thickness and is expressed as a decimal figure or percentage, e.g. R-value is calculated as a product’s thickness/thermal conductivity, where the lower the figure, the better the insulation performance.
When it comes to insulation, the best materials have low thermal conductivity and high density. Increasing the density reduces convective heat transfer, but it also increases the material’s conductive properties, which can impact the R-value. The industry has developed products that use vacuum technology to suppress convection and reflect radiant heat, as well as low-conductive materials to reduce conductive transfer and improve the R-value.
Convection
Convection is the transfer of thermal energy through the actual physical movement of molecules within liquids and gases. For example, the heat transfer in a pot of boiling water is convection. Heat transfer by convection is common in your home as well, and it’s often exacerbated when you open and close windows or doors. Insulation is effective at preventing this form of heat transfer by trapping air that doesn’t easily conduct heat and slowing down or stopping the motion of fluids that do.
Less dense materials, such as air and wood, insulate better than metals and other solids. This is because the atoms of those substances are closer together and don’t readily transmit thermal energy. Metals, on the other hand, have high thermal conductivity.
When choosing insulation, be sure to look at its R-value, which reflects the material’s ability to resist all three mechanisms of heat transfer—conduction, radiation and convection. The higher the R-value, the more effective the insulation.
FHB associate editor Rob Yagid takes a look at the different types of insulation available, and how their R-values relate to the amount of heat they can prevent from escaping your home through those three mechanisms. Then, he helps you determine the best type of insulation for your needs by examining the factors that affect the R-value of each material.
Radiation
Insulation keeps temperatures steady, provides acoustical comfort and helps systems run at maximum efficiency. While it may seem like a simple technology, in order for insulation to function correctly there is an enormous amount of science and engineering at work. From the materials used to their design and installation, the proper application of insulation requires skills from science, technology, engineering and math (STEM) disciplines.
Any material that keeps electricity, heat or cold from easily transferring through it is an insulator. Insulators can be made from wood, plastic foam, rubber or even Swiss cheese – as long as it keeps the transfer of energy to a minimum. In the world of insulation, surface emissivity is often overshadowed by thermal conductivity – but it plays an important role in certain circumstances, particularly when infrared radiation is a significant transfer method.
Radiation is the transmission of infrared energy from a hot surface to a cold surface through air or a vacuum. This type of energy travels through space without heating anything it passes through – but when it comes into contact with something that can absorb the radiant energy, it will be converted to heat. This is why insulators with small pockets of air trapped inside them are so effective at stopping the transfer of heat – because the radiant energy is stopped by the air pocket and doesn’t reach the desired cold surface.
Thermodynamics
A basic understanding of thermodynamics is helpful to understand how insulation works. This is because the first law of thermodynamics states that energy cannot be created or destroyed, but can only be transformed from one form to another. When a material acts as an insulator, it stops heat energy from flowing from warmer to cooler areas, or between hot and cold objects.
This is accomplished through the mechanisms of conduction, convection, and radiation. In order to stop the flow of energy, insulators must be made from materials with low thermal conductivity and emissivity. These properties are important to evaluate when selecting an insulating material for a specific application.
Thermodynamics is a difficult subject to learn, but it’s an essential component of building science and energy efficiency. Students should know that heat moves from hot to cold objects, that it takes the path of least resistance, and that insulators slow down this transfer of heat energy.
An easy way to get kids interested in this topic is by testing the insulating capabilities of different materials. In a simple experiment, students can compare how long it takes ice cubes to melt in various materials. This activity teaches students about heat transfer and helps them understand why engineers take the insulating qualities of materials into account when designing building structures.