CBP Magazine: Articles

Controlling heat transfer within the building envelope is crucial to energy efficiency and occupant comfort.

Seasonal changes in temperature have a huge impact on a building's energy use and the overall comfort of its occupants. For this reason, it is quite important to give adequate attention to the concept of thermal control during the building design process and how it influences the building envelope.

In our previous article, we gave an introductory overview of the study of commercial building science, briefly discussing the various elements and factors that influence sustainable building design. One of the factors mentioned was heat flow, also known as heat transfer, the passing of thermal energy from a hot body to a colder body.

In this article, we will discuss the three modes of heat transfer and how certain building materials can be used to impede heat transfer, making a more thermally efficient building. A thermally efficient building that manages moisture is a more durable building, requiring less maintenance over the years.

To fully grasp the concept of thermal control in a building we must first understand heat transfer.

Modes of heat transfer
Heat flow in and out of a building is a major factor in determining its comfort level and operating cost. Heat has a natural tendency to flow from an area of high temperature to one of lower temperature. The greater the temperature difference, the greater the heat flow through an assembly. During winter, a heated building will lose heat to its colder exterior. In the summer, an air-conditioned building will attract heat from the exterior, which is now much warmer.

There are three different ways by which heat transfers in and out of a building-conduction, convection, and radiation. In a building, these modes of heat transfer all occur at the same time and play an important role in the heat balance of a building.

Conduction
Conduction takes place when a material, such as a wall, separates an area of high temperature from an area of low temperature. During the winter, the inside is warm and the outside is cold. Only the wall separates the two extremes. The inside surface of the wall warms and tries to reach the same temperature as the air inside of the building. As the inside wall surface heats up, the adjacent material also warms. After a while, heat from the inside transfers through the wall to the outside of the building. This results in heat loss from the building to the colder temperatures outside.

The rate of heat transfer through the wall depends on two things: the temperature difference between inside and outside, and the makeup of the material. Some materials transfer heat very well and are called conductors. Glass, concrete, and all metals are examples of good conductors. Other materials, such as fiberglass and foam sheathings, transfer heat very poorly and are referred to as insulators.

Convection
Convection is the second most common mode of heat transfer. Heat transfer by convection occurs as a result of the movement of liquid or gas over a surface, such as wind blowing against a building. There are two types of convection: forced and natural. Natural convection occurs when the movement of liquid or gas is caused by density differences. For example, warm air rises. This happens because it has a lower density than the surrounding cool air, and that's also what causes a hot air balloon to rise. Cool air does the opposite and falls. This heating and cooling of air creates convection loops adjacent to both the interior and exterior surfaces of a wall.

Convection can also take place inside empty cavities. One example is the movement of air in a double-pane window. For example, in winter air is heated on the inside surface of the window cavity causing the air to rise. The air adjacent to the outside surface cools and drops. What results is a convection loop inside the window cavity that transfers heat from the inside to the outside.

A second type of convection is known as forced convection. Here, the movement of the liquid or gas is caused by outside forces. If winds are blowing, the air movement across the outside of the wall will be higher, increasing the rate of heat transfer. The rate of heat transfer by convection depends on the temperature difference, the velocity of the liquid or gas, and what kind of liquid or gas is involved. For instance, heat transfers more quickly through water than through air.

Radiation
Radiation involves the transfer of invisible electromagnetic heat waves from one object of higher temperature to another of lower temperature. One common example of radiation heat transfer is from the sun. When you walk outside on a sunny day, you immediately feel the warmth from the sun, even if the air is cold. Heat from the sun is being transferred through space by radiation to warm you.

Radiation also plays a heat-transfer role in a building. If you stand in front of a window on a cold day, your body radiates heat to the cold surface of the window and you feel colder. Likewise, if you stand in front of a window with the sun streaming in, you will feel warm as a result of the incoming solar radiation. Solar radiation is primarily short-wave radiation. Glass is nearly transparent to this short-wave radiant energy from the sun, and as a result, once sunlight enters a room, the sun's energy is absorbed by the walls and the contents of the room and is converted to heat. At the same time, the warm objects in the room also emit radiant energy.

To make a building more energy efficient and comfortable, we need to impede these modes of heat transfer. Though it is impossible to stop these processes, it is possible to significantly slow them down by placing obstacles in their path. This is referred to as "breaking the thermal bridging."

Breaking thermal bridging
Thermal bridging is the name given to the path that offers smooth travel for heat transfer in poorly insulated buildings, usually built from concrete and metal with insufficient heat-flow resistance between the outside and the exterior walls. The best way to slow down heat transfer is to put insulators between the conductors. Commercial insulation consists of cavity insulation, which occupies space inside the wall cavity, and insulation sheathing, which is installed on the outside of the external walls. There are a variety of materials that can be used for cavity insulation, including fiber glass, mineral wool, cellulose, open- and closed-cell foam plastics, reflective insulation, and radiant barriers. Sheathing is usually made from expanded polystyrene, extruded polystyrene, polyisocyanurate (ISO board), or glass-fiber board. Before selecting insulation materials, it's best to check the ratings of their thermal properties.

Rating insulation thermal properties
As mentioned earlier, insulation materials and building envelope systems are characterized by their resistance to heat flow. Material performance can be rated according to thermal conductivity (k), thermal conductance (C), and thermal resistance (R-value). In the case of system performance, total thermal resistance is shown as RT and thermal transmittance is shown as U-factor or U-value. With material surface performance, emissivity ratings are indicated by the symbol "_" and reflectivity is indicated by the "_" symbol.

When it comes to measuring the thermal properties of building materials, the standard is ASTM C 518. Here, a heat-flow apparatus measures heat transfer through homogeneous materials, such as insulation. Several material properties, including thermal resistance, conductance, and conductivity, can be determined from temperature, heat flux, area, and thickness data. Another standard, ASTM C 1363 Hot Box, measures the thermal performance of building envelope assemblies. Measurements include the effects of thermal bridging due to structural components, as well as insulated cavities.

For calculating the heat flow of insulated building envelope assemblies, there are three different methods of varying complexity devised by ASHRAE. These methods can all be found in Chapter 25 of the ASHRAE Handbook of Fundamentals. The first and most simple is the isothermal planes method. This is used when cross-sections have continuous, homogeneous layers. The second method, the parallel-path flow method, is used when cross-sections have structural and cavity areas and when components have similar thermal resistance. The third method, the modified zone method, is used with steel-framed assemblies. These assemblies have cross-sections with both structural and cavity areas.

Calculating heat flow can be as easy as adding thermal resistance in a system with homogeneous layers, as with the isothermal planes method, or as complex and complicated as the modified-zone method. Most building professionals can do their own calculations in the isothermal planes method, but with the complexity of the modified zone method, it's best to use the free online calculator provided by the Oak Ridge National Laboratory. This will help to ensure accuracy.

Structural components are highly conductive and create thermal bridges. For example, metals conduct 300 to 1,000 times more heat than most building materials. The thermal impact of a metal stud in a framed cavity is greater than the actual surface area of the stud, so metal has an exaggerated effect on heat transfer, out of proportion to its physical size. Because of this, choosing the proper insulation assembly is crucial.

Types of insulation assemblies
Matching insulation assemblies with applications depends on the material used for the external walls of the building. External walls are typically concrete block or tilt-up, metal, curtain walls (no cavities), or masonry façade (brick, block, or concrete panels with insulatable cavities).

Concrete block and tilt-up walls
In the case of concrete block and tilt-up walls, insulating sheathings such as foam plastic insulation board can be installed either on the interior or on the exterior of the concrete. The location of the sheathing depends on the climate and the type of sheathing material.

Interior, non-load bearing, steel-framed assemblies can support cavity insulation. Since thick concrete has insulating value, many building codes have reduced insulation requirements due to the mass effect of the concrete. It's often advisable, though, to exceed code requirements to achieve optimum energy-efficiency and sustainability.

EIFS
EIFS systems (exterior insulation and finishing system) resemble traditional stucco. When installing an EIFS, it's important to follow the manufacturer's installation instructions, so moisture does not encroach behind the EIFS at windows, doors, and other fenestrations, where it becomes trapped.

Steel stud cavity walls
The most common wall assembly is the steel stud cavity wall, which includes a masonry façade. To improve the thermal performance and increase cavity condensation control in cold climates, the designer can specify exterior insulating sheathings, which increase cavity surface temperatures and improve energy efficiency as well; incorporate exterior air barriers, which also function as wind barriers to reduce air leakage; and specify interior air barriers, such as a "smart," breathable vapor retarder, to reduce the potential for convective loops and increase drying capability.

Always incorporate water resistive barriers, and provide ventilation and drainage space behind the masonry façade, to reduce wetting the substrate materials and to promote drying. This exterior wall configuration is a cost-effective way to achieve thermal performance while managing moisture.

Metal buildings
Metal buildings have their own set of installation and compliance recommendations. An authoritative publication covering ASHRAE 90.1 is available from NAIMA, the North American Insulation Manufacturers Association. It's available online at www.naima.org. NAIMA'S reference for flexible fiber glass insulation used in metal buildings, Standard 202-96, provides information on thermal performance of metal-building roof systems and wall systems. R-value and U-value data are listed for screw-down roofs and for sidewalls having varying cavity R-values and fastener spacing.

For more energy efficiency information, consult ASHRAE 90.1, "Energy Standard for Buildings Except Low-Rise Residential Buildings," which provides minimum insulation R-values and offers guidelines for overall building energy efficiency.

Commercial roofing guidelines
Roofs can also contribute to the thermal efficiency of a building, but there are various guidelines to follow during installation to ensure this efficiency. With low-sloped roofs, the first thing to remember is to protect insulating sheathings from moisture penetration by sealing every opening against rainwater. Next, to increase the roof's thermal performance, use low-e reflective roofing or cool roofing. Roof insulation materials should be sloped toward drains and roofers should thermally isolate parapets from roof-wall intersections. Flashing is critical when integrating parapets, access doors, elevator towers, penetrations, and so forth.

For more information on accurate roof-surface radiative properties, visit the Cool Roof Rating Council (CRRC) website.

In regards to metal building roofs, one can install thermal blocks or thermal breaks of foam insulation over the purlins to improve thermal performance. To further improve performance, a second layer of metal building insulation can be added. The NAIMA guide, 202-96, contains this and more information on insulating metal buildings.

Fenestration
Fenestration refers to any opening in a building envelope, including windows, doors, curtain walls, and skylights. The National Fenestration Rating Council (NFRC) offers a labeling and certification program for window and door products that permits building professionals to select windows based on thermal performance. Factors that affect window performance include frame type, glazing type, type of gas fill-argon versus air, for example-and low-emittance coatings. So, when you install high performance windows, look for the NFRC label.

U-factor
U-factor in windows is similar to R-value in insulation products: both are indicators of thermal performance. Energy Star U-factor recommendations are given by zone. With U-factor ratings, the lower the number, the greater the thermal resistance. This is just the opposite of R-value, in which higher the number is better.

Solar heat gain coefficient
Another indicator to look for is Solar Heat Gain Coefficient (SHGC). This number rating indicates a window's efficiency in preventing solar radiation from entering and heating a building.

Since windows are generally made from very thermally conductive steel or aluminum, it's important to select thermally broken windows with an air space between components. Remember that lower U-factor not only means increased energy efficiency, but also better condensation control on surfaces. Installing airtight systems will increase energy efficiency and reduce the potential for moisture accumulation. Be sure to insulate hidden mounting flanges and metal surfaces to reduce surface condensation, and don't skimp on installation details and proper flashing. The goal is to have an airtight, moisture-resistant installation. Many studies have shown that making systems airtight in colder climates can reduce energy use by as much as 30%.

The goal of the sustainable building design movement is to create a significant increase in energy-efficient, healthy, long-lasting buildings. Such buildings will provide more pleasant working environments for their occupants and make operations more efficient and economical for building owners. Following these thermal control design guidelines is a good first move toward achieving this goal.

Author
Stanley D. Gatland II is the manager of building science technology for CertainTeed Corp.'s, Valley Forge, PA, Insulation Group. He is responsible for generating and providing technical information to architects, engineers, builders, trade contractors, building envelope consultants, building scientists, and building code officials on the system performance of new and existing building envelope materials, as well as building science educational training. Stan has expertise in the areas of building science and architectural acoustics. He is a graduate of the Univ. of Massachusetts, Amherst, with a BS and MS in mechanical engineering. He is a member of ASHRAE, ASTM, ASME, and BETEC.

Thermal Control in Building Envelopes