What is Thermal Control in Buildings?

Thermal control helps keep the occupants of a building more comfortable by moderating the temperatures of the interior surfaces of the building and reducing drafts. Additionally, it reduces the energy consumption of a building for heating and cooling to a fraction of what it would otherwise be.

Thermal insulation helps keep a building cooler in summer and warmer in winter by decreasing the passage of heat through the exterior surfaces of a building. A wide variety of materials are used today as thermal insulation. These include wood, plastics, and metal products.

There are three ways that heat is transmitted: through conduction, convection, or radiation. Conduction occurs when materials are in direct contact. Convection takes place in a fluid medium, such as air or a liquid. Radiation occurs between two objects, not in contact and not shielded from each other.

Buildings gain or lose heat through these processes. The purpose of thermal control is to slow down heat gain or loss in order to maintain comfortable interior temperatures while conserving energy.

Choice of Insulation

The choice of insulation material depends on its physical characteristics, resistance to the flow of heat, and cost. When choosing a specific type, R-Value is usually the most important consideration.

However, the application is important as well as some materials are better suited for certain parts of the building. Installations on walls differ from installations on roofs or from floors for example.

Location and Placement

Additionally, location and placement play a significant role in the type of insulation. For example, roof insulation can be placed above the roofing membrane, between the membrane and the roof deck, and below the roof deck. Each location requires a different type of insulation, and each has its advantages and disadvantages.

There are two advantages to placing the insulation above the membrane: the membrane is protected from temperature extremes and the membrane, being on the warm side of the insulation, acts as a vapor barrier.

Each element of a building that experiences heat gain or loss needs to be addressed in a manner specific to the condition. For example, cracks and openings can be addressed by the use of caulking and weather-stripping, while windows can be treated with tinted, reflective, or insulated glass. Walls and roof assemblies are typically treated using thermal insulation.

Thermal Insulation

Thermal insulation is composed of materials that have a high thermal resistance (R) or high reflectivity, such as aluminum foil. Enclosed air has excellent insulation properties, therefore insulation materials are typically made of air enclosed cells in foamed glass or plastic.

Some common types of insulation include loose fill, batt or blanket, board or sheet, reflective, and foam. Each has advantages depending on the use and placement. Insulation is measured using R-Value, the inverse of thermal conductivity (R=1/k) where R is resistance and k is thermal conductivity.

Loose Fill Insulation

Loose fill is composed of glass or mineral wool, vermiculite, and perlite. It is typically 4 inches thick with an R-Value of 3.90 and it is used in wall cavities and flat air spaces such as attics.

Loose fill insulation is blown into building cavities using special equipment. It is well suited for places where it is difficult to install other types of insulation, such as irregularly shaped areas, around obstructions, and in hard-to-reach places.

Batt or Blanket Insulation

Batt or blanket insulation is made from glass or mineral wool enclosed by paper or aluminum. Thickness is typically 3.5 inches and R values are about 11.00. It is used in air spaces in framed walls, floors, and ceilings.

Batt insulation and blankets are available unfaced, faced on one side with moisture resistant kraft paper forming a vapor barrier, and faced with aluminum foil forming a fire-resistant facing. Some types have a facing on both sides and are used for vertical applications on walls, and horizontal applications on floors and ceilings.

Board or Sheet Insulation

Board or sheet insulation is comprised of cork, glass, or mineral fibers, or paper pulp. It is typically 1 inch thick with an R-value of 2.75. It is typically used for wall sheathing and as rigid roof insulation.

Also known as rigid insulation, board or sheet insulation is used in all parts of a building. Wood and cane fiberboard are commonly used for exterior sheathing and shingle backer boards and are asphalt impregnated. Granulated cork rigid insulation is used for roof, wall, and floor insulation.

Reflective Insulation

Reflective insulation is made up of aluminum foil in combination with layers of paper and a 1-inch air space. The R-value for two reflective surfaces with a 1-inch air space is 1.39. It is typically used on roofs, walls, and floor insulation with a vapor barrier.

It is available in single thickness layers or in a multi-layer batt that has dead air spaces between the layers. The reflective foil uses the reflective properties to reject the passage of heat plus the effectiveness of the dead air spaces.

Foam Insulation

Foam can be either the spray type or rigid panels usually 1 inch in thickness. It has an R-Value of 6.00 and is commonly used as sheathing or in the case of spray foam in irregular spaces.

Foamed-in-place insulations are generally polyurethane or phenol-based compounds that provide excellent insulation. When mixed they are pumped through hoses into cavities, such as wall cavities, and sprayed in layers on flat and sloping surfaces such as roof and decks.

Vapor Barriers

Vapor barriers are often combined with insulation, and since condensation tends to form on the cool side of insulating materials, aluminum foil-backed insulation should always be installed with the foil facing the warm side.

A vapor barrier is used to keep water vapor generated inside a building, such as by cooking, from penetrating the wall and condensing as moisture on the building insulation. It can also be used to reduce the penetration of moisture from outside sources into the building.

Closing Notes

Heat loss and heat gain in buildings is a result of three conditions: conduction, convection, or radiation. In order to minimize the transfer of heat energy, various types of thermal insulation are used.

Each of these materials has advantages and disadvantages. Some types of insulation are more adequate for use in specific parts of a building. How they are placed in relation to other building materials and building elements can also play a role in their effectiveness.

In addition to the methods described here, there are other elements of a building which affect heat transfer. Openings, such as doors and windows are particularly susceptible to heat loss or gain. Likewise, roof attics are often a major source of heat transfer.

While building materials are evolving and improving in their performance capabilities, new methods are also being explored. These include green roofs, the use of light color materials to reduce heat absorption (heat island effect). In combination, these materials and construction methods can help improve human comfort and help prolong the life of a building structure.