Lab 5 - Heat and Insulation

As a child, you were probably told to turn off the lights when you left a room and to turn off the television when you were not watching it. It definitely makes sense to turn lights and appliances off when they are not in use; however, lights, televisions, computers, and other appliances only account between 10% and 30% of the energy used in an average American home. On the other hand, heating and cooling (collectively known as space conditioning) devour between 50% and 70% of the energy used in most homes. Unfortunately, most houses in the United States are not well insulated, and inadequate insulation is a major cause of energy waste.

When thermal energy is transferred from your house to the outdoors we say that heat is transferred or that there was a flow of heat. Thus we will reserve the word heat for the cases where thermal energy is being transferred from one place to another. We will not speak of the heat ʺinʺ a body but only of the thermal energy in a body though we say that energy is transferred as heat from one body to another. This is similar to the way we use the word work. We never speak of the work ʺinʹ a body but only of the work done in transferring energy from one body to another.

Insulation decreases heat flow between the outdoors and a house interior by providing a resistance to such flow. There are many different types of insulation: fiberglass blankets, loose mineral fibers and fiber pellets, polyurethane foam, fibrous or plastic boards, plastic fiber made mainly from recycled milk bottles, cement‐based foam, adobe (clay), natural fibers such as cotton and straw, and more. Different insulation materials have different degrees of thermal resistance, or resistance to heat transfer.

You and your partners will examine the thermal resistance of four different insulators in an effort to determine the characteristics of an effective insulator. You will measure the temperature change of hot water in a pop (soda) can that is wrapped in an insulator, and this change will be divided by the thickness of the material in order to compare the effectiveness of the different materials.

Pre‐lab reading¶

Textbook section 6.5

Equipment¶

• 5 – empty pop cans
• 5 – alcohol thermometers
• Set of various insulators (bubble wrap, felt, can cozies)
• 1 – digital calipers
• hot tap water

Predictions/preliminary questions¶

1. Would a substance with high thermal resistance be considered a good insulator or a poor insulator?
2. a) Rank the following substances according to their effectiveness as an insulator: aluminum, first aid gauze, foam “cozy” for a pop (soda) can, rubber sheet, and bubble wrap. List the best insulator first. (Note: These comparisons are to be made in terms of effectiveness per the same unit of thickness.) b) On what reasoning did you base your decisions about ranking?

Procedure¶

As previously mentioned, you and your partners will investigate the thermal resistance of four insulators to establish the characteristics of an effective insulator. You will measure the temperature change of hot water in a can that is wrapped in an insulator, and this will be divided by its thickness to compare the effectiveness of the different insulators.

1. Make a table in which to record your groupʹs temperature measurements. You will be measuring the temperatures of the water in five cans each minute for 15 minutes.
2. Determine the mass of each can.
3. Take the five cans to the restroom and fill them with hot water. Be sure each can has the same amount of water ‐ almost filled to the top ‐ and bring them back to the lab.
4. Determine the mass of each can when filled with water and record it in your notebook.
5. Leave one can unwrapped. Put one can in the “cozy” and wrap the other cans with a layer of the various insulators: bubble wrap, first aid gauze, and rubber sheet. Secure the insulators around the cans with some tape.
6. Tape a thermometer in each can, so that it does not touch either the sides or bottom of the cans and so there is no heat loss through the hole in the can.
7. Record the temperatures of the water in five cans each minute for 15 minutes.
8. While you are taking your temperature measurements, use the calipers to measure the thickness of the insulator around each can.
9. Determine the temperature in the room with another thermometer and record it in your notebook.

Analysis¶

• For each can for which you have recorded data, graph the temperature minus the room temperture as a function of time. You can put the data for all five cans on the same graph for comparison. Based on your data, rank your materials from “best” insulator to “worst”.
• Is the water cooling at the same rate during the entire experiment for one of the cans? If not, when is it cooling fastest? slowest? Can you explain why it cools faster sometimes than others? (Hint: use the expression for rate of heat transfer by conduction on p. 107 of your book.)
• Calculate the rate of cooling during the first minute, during the time between the sixth and seventh minute and during the time between the 14th and 15th minute for one of the cans. How much thermal energy left the water in the first minute?
• How much thermal energy left the water between the 6th and 7th minute? (Use the expression: change in thermal energy=(specific heat)x(mass of water)x(change in temperature))
  • The specific heat of water is 4186 J/kg/^oC .)
• Now use this formula that incorporates the “R-value” of insulation.
  • ${\rm rate~of~cooling~}~=~(1/R)~{\rm (area~of~can~surface)~x~(temperature~‐~room temperature)}*

Hint: To calculate the $R$ value of the insulation around this can. (You need to measure the height $h$ and radius $r$ of the can to calculate its surface area. The surface area is the area of the cylinder plus the two circular top and bottom faces. $A=2\pi rh + 2\pi r^2$. You need to then solve the above equation for $R$ and use $A$ and the rates of cooling that you determined to get $R$.) Do this both for the first minute and for the the period between the 6th and 7th minute. Are the answers the same? Should they be? Compare the $R$ value with those in this Wikipedia article. Would this insulation be adequate for a house?

Conclusions¶

1. • a) What material was the best insulator? The second best? The worst? The second worst? How does your measured ranking compare to your predicted one?
   • b) Based on your results, what are the properties of a material that you think will make it a good insulator?
2. Why do most houses in Minnesota have double‐ or triple‐pane windows? Is it the glass that is an efficient insulator, or is something else involved?
3. How can the Inuit construct igloos out of snow and still expect them to remain warm inside? Why do forest animals burrow into snow to find shelter from the winter cold?
4. The following materials (at the stated thicknesses) have the same thermal resistances: 13 cm of polyurethane foam, 58 cm of white pine, 5.5 meters of window glass, and 2.25 km of silver. How does this fit with your results in this exercise?
5. Consider the insulation with which we who live in Minnesota are terribly familiar: winter clothes.
   • a) What kind of clothing do people usually wear in winter?
   • b) What about these winter clothes makes them better insulators than our summer clothes? Relate your answers to the observations in this exercise.
6. What heat transfer mechanism or mechanisms were involved in this exercise? (Radiation? Conduction? Convection?) Draw an illustration to help explain your answer.

Additional notes¶

Research at Oak Ridge National Laboratory, which started in 1943 as a nuclear weapons laboratory, now includes the development of new insulation materials as a part of the Building Envelopes Program. Insulation materials differ and must be chosen to fit several factors: the local climate; the size and construction of the home; the type and efficiency of the heating and cooling systems; and the fuel used. This is because different insulation materials have different degrees of thermal resistance.

In construction, the measurement of thermal resistance is called the R‐value. The greater the R‐value, the greater the insulation effectiveness. The R‐value depends on the type of material and its thickness. The U‐value, which is simply the reciprocal of the R‐value, is another common measure of thermal resistance. Physicists typically use a quantity called the thermal conductivity to report a material’s resistance to heat transfer.