Insulation is one of the most basic applications of science in our lives. It slows the transfer of heat and cold, keeping a building at a consistent temperature in all seasons.
All insulation materials get their insulating properties from small cells or voids in the material structure. These voids inhibit convective transfer by reducing the surface area of the material.
Heat Transfer
For insulation to perform as intended, it needs to limit the transfer of heat from one object to another. This is accomplished by blocking thermal conduction, restricting convection and reducing radiation. The type of material used to create the barrier determines how well the insulation works.
The effectiveness of insulation is determined by its R-Value, which is a product of the thickness of the material and its thermal conductivity. It is important to consider both when choosing the right insulation for a project.
Insulation can be a great way to reduce energy costs and protect the environment. It can be incorporated into a new home or easily added to an existing home as part of a renovation project.
The best insulation materials have a low thermal conductivity value, are inexpensive and readily available. Cellulose is a good example, as it is an environmentally friendly and cost effective choice. It also has the added benefit of being fire resistant.
Any non-metal material is an insulator with the exception of graphite, which has a very high thermal conductivity. Plastic, rubber, wood and ceramics are some examples of good insulators. The human body is also an excellent insulator as its hair, skin and fat trap air close to the body and prevent its loss. This is how our ancestors kept warm in the snowy outdoors and why we wrap ourselves in blankets in the winter.
Materials
Insulation can be made from a number of different materials. The best material for a particular application will depend on a number of factors including cost, performance, availability and environmental impact.
The main function of a thermal insulation material is to frustrate the flow of radiant energy by creating a barrier that slows down the rate at which heat moves from hot surfaces to cold ones. This will slow down the loss of thermal energy from a building and similarly reduce the rate at which heat gains into a building. However, it will not stop the flow of energy completely.
For a material to be an effective insulator it must have a low conductivity that minimises convection. This means that the material should have a high proportion of void space within its structure. This can be achieved by having thin connecting walls, discontinuous or cellular construction or an inert gas filling the voids. The most common insulators are glass fibre and foamed plastic such as EPS (expanded polystyrene) and polyurethane.
Unfortunately, many of the plastic insulation materials used in buildings have very poor environmental profiles. Their manufacture uses unsustainable fossil fuels and releases greenhouse gases as well as toxic chemicals into the environment and workers at the manufacturing plants. Some of these chemicals are also flammable and require flame retardants in order to meet national and international safety standards.
Thermal Conductivity
Ultimately, thermal insulation is about preventing heat transfer. Heat transfers in three ways: conduction, convection, and radiation. While convection and radiation are less important, conduction is the most significant when it comes to insulating materials.
Basically, thermal conductivity is how easily a material can pass heat from one place to another without moving through the material itself. It is a property of matter and it varies depending on the type and state of the material. It is also a function of temperature, as described by Fourier’s law.
The most common insulating materials have low thermal conductivity, as explained by their physical structure and composition. Many are filled with pores, pockets, and voids that obstruct heat transmission pathways. Examples of insulating materials include expanded or extruded polystyrene (commonly known as Styrofoam), silica aerogel, and warm clothing. The insulating properties of air and other gases are also very effective.
However, a team of researchers at MIT has recently developed a device that can vary the thermal conductivity of materials on demand. This could open up new technologies for controllable insulated building materials like smart windows, walls, or even wearables that harvest waste heat from body movement. Their findings are published today in Nature Materials and were authored by Bilge Yildiz, Gang Chen, Qiyang Lu PhD ’18, Samuel Huberman PhD ’18, and six others at MIT and Brookhaven National Laboratory.
R-Value
The R-value of insulation is the ability to resist conductive heat flow, and the higher the number, the more effective it will be. R-values can be given to a single material or an assembly of materials such as a wall or roof. It is important for builders and contractors to know the proper R-value required for their project as this will help reduce energy requirements which can lower a building’s electricity bills.
In the past, resistance to heat transfer was referred to as a material’s U-factor. However, manufacturers found that it was difficult to market insulation with a low U-factor so the R-value was created as the inverse of the U-factor.
R-values are determined by a complex calculation, but they are easy to understand. To determine the R-value of a particular insulation, the material’s thickness and thermal conductivity must be known. Then, the insulation must be compared with other materials to determine its R-value.
Nominal R-values are the manufacturer’s advertised values, while effective R-values are the real R-values obtained under a specific set of conditions. For example, if batt insulation sized for 5 inches is stuffed into a 4-inch wall cavity, the effective R-value will be less than the nominal value because the insulation will not be as thick. Also, the condition of a construction assembly’s components must be considered when determining an R-value.