For the Instructor

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Student Reading: Efficiency and Conservation - Building Insulation

Household energy use accounts for a significant portion of the nation's energy use; therefore, an important aspect of energy conservation is having buildings that are energy efficient. In this activity students will compare the how well different types of insulation perform and suggest ways of making a structure more energy efficient.

Learning Goals: Students will be able to:

Describe some of the materials used to insulate modern homes and buildings.
Collect data on the ability of different insulation types to retain heat, and calculate their heat loss.
Assess the efficiency of different insulating materials.
Assess a structure's resource use and propose ways that will make it more efficient.

Background

The cheapest fuels are conservation and efficiency—that is, using less energy, and making sure what you do use is done with the least waste. An important area of energy conservation concerns the heating and cooling of buildings because this accounts for the largest portion of energy use in a building or home. According to the US Energy Information Administration's 2009 Residential Energy Consumption Survey (Figures 1 & 2), most of the energy use in our homes goes to space heating (41.5%), followed by powering appliances, electronics and lighting (34.6%), and heating water (17.7%).

The better the buildings are at keeping heat in during winter and staying cool during summer, the more energy will be saved. Building design must also consider the need to exchange inside air with the atmosphere. This is important because building ventilation provides fresh air and prevents mold, mildew and other problems.

Heat and Temperature

Heat is energy that is produced by the random vibrations of atoms or molecules. One unit for measuring heat is the calorie and is based upon the amount of energy needed to increase the temperature of 1 g of water 1 degree C. A second unit of heat is the BTU, the amount of heat needed to raise one pound of water 1 degree Fahrenheit. Recall from above that the joule is a measure of work. This is another way to look at heat (the flow of thermal energy between objects) therefore 1 calorie = 4.184 joules.

Temperature is an object's response to the input or removal of heat, or how warm or cold an object is. It is measured in degrees using either the Fahrenheit or Celsius scales (see graph below). Some substances will change temperature with the addition of very little heat, while others require lots of heat to change temperature. This property is referred to as "specific heat." For example, one gram of water (1 ml) has a specific heat of 1 cal/g, meaning it takes one calorie of heat to raise the temperature of the water 1 degree Celsius (Table 1).

Energy in an Object

The total amount of energy in an object is the sum of the five different kinds of energy it could have where:

Total Energy = Kinetic Energy + Potential Energy + Thermal Energy + Chemical Energy + Electrical Energy.

Think of a fully charged D battery that is resting on a table. Because it is not moving, it has no kinetic energy, but it does have potential energy as would be seen if it fell off the table. If it is at room temperature or anything above absolute zero, it has thermal energy. It is charged so it has potential electrical energy, and it is the chemical reaction in the battery that makes the electricity flow once the circuit is closed, so it has chemical energy.

Heat Flow

Heat flows from a substance of a higher temperature to one of a lower temperature, never the other way. The heat can flow in three different ways. Radiation is the flow of heat through space or air. Conduction is the flow of heat between two substances in direct contact with each other. Convection occurs when the heat is carried by a flowing fluid, usually in moving air or water. Passive convection results from the heated fluid (water or air) becoming less dense and floating up. Active convection requires the addition of energy via a pump or fan to move the fluid. Substances that readily transfer heat are called good conductors. Substances that are poor conductors are also called good insulators.

Putting physics to work to make buildings more efficient

Heat enters and leaves buildings via 1) convection (air flow); 2) conduction (through walls, windows, doors, ceilings, floors, and foundations); and 3) radiation (same list as for conduction). Now that you know something about the basic physics of heat transfer, let us look at ways we can make buildings more efficient.

Heating Recovery Ventilation (HRV) system to reduce heat loss by convection

A first step in conservation is to reduce the flow of air into and out of the structure, as the convection of air carries heat. However the downside of a tightly sealed building is that the internal air becomes stagnant. Oxygen levels drop and CO₂ levels increase. Did you know that air inside of houses can be more polluted than air on the outside? Off-gassing of formaldehyde from furnishings, household cleaning products, and gases associated with cooking all contaminate the air. Hopefully nobody smokes tobacco indoors as that is another major contaminant. So how do we keep the warm (winter) or cool (summer) air in the house while keeping it fresh? The answer is a Heat Recovery Ventilator (HRV). This device blows fresh air into the building through thin metal tubes that exchange heat with the air exiting the structure. In areas with high humidity, like in the southeast US, one would install an Energy Recovery Ventilator (ERV) which works the same as an HRV but dehumidifies the air. In these ventilators the inside and outside air never mix, but heat is transferred through the metal tubes. This is convection and conduction at work! View this video: How an HRV works to save energy.

The HRV unit uses a principle called countercurrent exchange. The idea is that if two fluids move adjacent to each other and in the exact opposite direction, it will maintain a gradient of concentration that will maximize exchange of a substance (or heat) from one channel of flow to the other. We see examples of this in biology, such as in the legs of birds that wade in cold water. The warm blood from the artery descending in the leg transfers heat to the cold blood in the ascending vein. By the time the blood from the vein in the leg joins the circulation of the rest of the body, it has absorbed most of the heat from the artery.

Insulation

A good insulating material reduces both conduction and radiation. For centuries, people built well-insulated buildings using natural materials. The effective insulation of those structures was perhaps an accident of coincidence, for they had to build thick walls in order to support the building. Thick walls of logs, adobe, masonry, straw and mud, and sod were needed to support roofs. The very thickness of these walls made good insulation. During the Industrial Age, structural engineers figured out how to build structures with much thinner walls, often reinforced with steel or a skeleton of slender wood beams. While this made for less expensive and stronger buildings, the thin walls did little to control the loss and gain of heat. It was not until the middle of the 20th century that builders began to pay serious attention to insulating buildings. Energy was cheap, so prior to this designers counted on furnaces to keep the occupants warm rather than worrying about conservation. Read History of insulation to learn about the evolution of insulation. Note that with the use of modern materials it is easy to construct a new well-insulated and air-tight building. The larger challenge is to refit existing buildings that lack or have inadequate insulation. Visit this Department of Energy web page to explore different types of insulation.

The system of R-value ratings was developed to quantify the effectiveness of various types of insulation materials and thicknesses. The R-value is a measure of resistance to heat flow, so the larger the R, the better the insulation. Most communities today specify in their building codes a minimum R-value of 15 for the walls of new construction. Note that the system of R-values used in the United States is different from that used in most of the rest of the world, where they use the SI R-value system. To convert to SI R values, multiply the US R-value by 0.176. A list of R-values for a variety of building materials can be viewed at R-value table. Note that some materials like plywood sheathing have R-values of about 0.6, and the best windows achieve values of only about 4.0. Most exterior doors have R-values near 2, but a thick foam-filled steel door can be as high as 15. So if one builds a house with many windows and doors that include windows, they should expect significant loss and gain of heat.

What are the challenges of insulating old buildings? First, one must find a way to force the insulation into the empty spaces of walls, on attic floors, and under bottom level floors. Taking all the siding off a building is costly, but the easiest way to add insulation. To avoid this, one can drill may holes in the walls and blow in cellulose (fire-retardant treated newspaper), fiberglass, or various types of foams. The problem with blowing in fiberglass or cellulose is that there is no vapor barrier to keep the insulation dry. So moisture from the house will seep through the porous walls and condense on the cold insulation in the winter. This makes the insulation mushy, ineffective, and invites damage to walls from mold and insects. Blowing in a chemical foam that is sticky and waterproof provides the needed vapor barrier and seals the house from loss by convection. However, if not done properly, the foam will not cure correctly and will contaminate the air in the house by constant off-gassing. Generally it is easier to insulate open-access attics and basements, but a vapor barrier may still be required.

To insulate against radiant energy materials have been developed that are referred to as radiant barriers. Think of the shiny material you may have seen on spacecraft which is designed to prevent the transfer of the sun's energy to the interior of the craft. Radiant barriers work because they have a low emissivity (Ɛ) which is the ability of a material to emit thermal radiation. It is measured as the ratio of a material's radiant energy to that of a blackbody (a perfect emitter where Ɛ = 1) at the same temperature (using the Stefan-Boltzmann law). An example of a material with low emissivity would be aluminum foil (Ɛ = 0.03) whereas water has an emissivity of 0.96.

Energy efficient windows and doors

Windows and doors make a building livable. The doors allow access, and the windows let in natural light and provide ventilation when open. Thin glass is a poor insulator, easily allowing radiation and conduction. An early solution was the use of wooden shutters on the outside of the windows. If they fit well, the shutters trapped some dead air to improve insulation, and also served as a barrier to some radiation of heat. Storm windows became popular in the 20th century. These were second windows placed outside the regular windows. Like the shutters, they provided some additional insulation by trapping a layer of air, but they also allowed light to shine through. Modern high-efficiency windows are constructed of two or three layers of glass separated by gas such as nitrogen or argon that is a poor conductor of heat. The windows must also form a tight seal when closed to reduce convection. In addition, the windows may have a coating that can reflect radiant energy. These are called low emissivity (low-e) windows where the coating serves as a barrier preventing the transfer of heat through the window. They reduce the amount of ultraviolet and infrared light passing through the window. Just as insulation is rated by an R-value, window systems are rated using a U-value which is a measure of its ability to transfer heat. It also takes into account heat losses due to conduction and radiation. The lower the U-value the more energy efficient the window. For example, a doubled pane wood-framed window may have a U-value of 0.48 whereas the same window with argon gas between the panes and a low emissivity coating may have a U-value of 0.34.

Two other terms that are used in reference to windows are Visible Light Transmittance and the Solar Heat Gain Coefficient (SHGC). The Visible Light Transmittance is the amount of light that passes through a window while the Solar Heat Gain Coefficient is the fraction of incident solar radiation admitted through a window. A window with a SHGC of 0.25 would pass 25% of the available solar heat through the window. These terms plus the U-value can be found on labels developed by the National Fenestration Rating Council that can assist consumers in comparing the efficiency of different windows or doors.

Modern doors can be made of wood, steel or fiberglass. Wood doors provide a nice look but solid wood is not a good insulator. Steel or fiberglass doors are more energy efficient than wood doors because they can be filled with polyurethane foam insulation core. A source of heat loss for doors are the air leaks around the edge of the door. Doors should be fit into frames that seal well around the edges or weather-stripping can be added to prevent air leaks.

Location

The climate of an area has a large effect on the heating and/or cooling needs of a building. Selecting the appropriate type of insulation, the amount of insulation, the right energy efficient window or door depends upon the climate (see heating degree days in Passive Designs). For example, radiant barriers are more cost effective in warm southern climates than in northern ones. In general, the colder the climate the more insulation is needed but there is a tipping point where the cost of further insulation exceeds its benefits. The following figure shows the different climatic zones of the US and provides the recommended R-values for attic, ceiling, wall and floor insulation. A similar map is available for windows and doors and it provides recommended U-values and SHGCs.

Collecting your thoughts: Systems thinking and reflection

You have just learned about the different approaches used to improve the efficiency of energy usage in buildings. Take a few moments to consider how this all fits together. Think of the building as a system that requires inputs of energy to maintain a desired temperature. Draw a picture of a building and annotate it with arrows that show the direction of flow of heat into and out of the building through its various components (walls, windows, doors, attic, floors, etc.) in the winter. Repeat the diagram for summer.

What are some ways to improve the efficiency of the buildings on your college campus?

Experiment — comparing insulators

Houses are constructed from all types of materials including wood, straw, mud, rock, brick, bamboo, and sod. Some of these materials like sod and straw are excellent insulators. Modern construction incorporates non-structural materials to add insulation to the structural skin and skeleton of buildings. In the post-World War period, fiberglass or rock-wool became popular insulators added to the walls of buildings. Various kinds of solid foams came on the market in the 1970s. Now there are foams made of cellulose that can be blown into wall spaces. All of these insulators work by reducing conduction, convection, and radiation. Their effectiveness is measured in "R –values." R = d/k, where d is the thickness of the insulator and k is the material's thermal conductivity.

We will test effectiveness of several different types of insulators. There are pre-constructed boxes made of cardboard, foam core, or other material provided by your instructor. We will place in each box an Erlenmeyer flask filled with hot water and record the change in temperature over time to test the effectiveness of each type of insulation. We will also have an Erlenmeyer flask on the lab bench with no insulation. Here is a handout for recording the results: Data sheet & Questions (Microsoft Word 18kB Jul14 17)

Questions

Graph the results for the cardboard versus foam boxes, with time on the X-axis and temperature on the Y-axis. Discuss your findings.
Determine the average number of calories of heat lost from each bottle in the experiment using the average initial and final temperatures for each treatment. Discuss your findings.
Rank the effectiveness of the boxes from the best-insulated to the poorest-insulated. Using these same materials, how could you make them even more insulated?
Compare your home energy use to similar homes. Gather the following information: a) number of people living in your house; b) square footage of your home; c) the different fuel types used in your home (e.g., fuel oil, propane, natural gas) and the your utility bills from the last twelve months. Go to Energy Star's Energy Yardstick

What was your yardstick score? What are some things you could do to reduce energy use in your home?

References and Resources

Al-Houmoud, Mohammad (2005) Performance characteristics and practical applications of common building thermal insulation materials. Building and Environment 40:353-366.

Department of Energy's Insulation Fact Sheet

Gardner, G. T., & Stern, P. C. (2008). "The short list: The most effective actions US households can take to curb climate change." Environment: Science and Policy for Sustainable Development, 50(5), 12-25.

Insulation types: