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Building Envelope Applications

Increase the efficiency and structural integrity of commercial and residential structures by detecting and correcting defects.  Infrared thermography can be used to:

Infrared image showing heat loss through windows

 

  • Air infiltration is the leading cause of energy loss
  • Detect moisture problems
  • Inspect roofing and insulation
  • Detect Heating and Cooling Losses
  • Detect electrical problems

What Causes Convection Currents

Imagine a cool room with a radiator at one end and no fans or any other forced air systems to blow the warm air to the other side of the room. How does the radiator heat the entire room? The key is convection currents. The hot radiator sets up convection currents that transfer thermal energy to the rest of the room and eventually heat the entire room. How do convection currents work?

The hot radiator warms the air that is closest to the radiator. The warm air expands, becomes less dense and rises to the top of the room. When the air reaches the top of the room it is pushed sideways towards the far wall by the more recently warmed air rising from the radiator below. In this way warm air moves to the other side of the room. Once on the other side of the room the air drops down both because it has cooled a little and because the air behind it continues to push on it. The air then continues to circulate back to the radiator and repeat the process.

By continuing to circulate, the convection current transfers heat energy to the other side of the room and heats the entire room. This process can work in any fluid, whether a liquid or a gas. Because matter must circulate for convection currents to transfer thermal energy convection currents can not work in a solid. However they can efficiently transfer heat in a fluid.

When a fan or other mechanism circulates the fluid more rapidly, it is called forced convection. If the fluid circulation is not forced or helped in any way, it is called natural convection.

Examples of Convection Currents

The only things required for convection currents are a heat source and a fluid that can circulate to transfer the heat energy. The air heating a room is a small scale convection current. There are also many larger scale convection currents.

Wind patterns are large scale convection currents in the atmosphere. If there is a warmer spot on Earth, convection currents are set up resulting in wind as the air circulates. Ocean currents are the same effect but in the water rather than the atmosphere. Heat in one portion of the ocean sets up convection currents which result in ocean currents as the water circulates to transfer heat energy.

Beneath the Earth, the mantle seems solid but is really in a slowly flowing plastic putty-like state. Heat sources in Earth's core set up slowly moving convection currents in the mantle. According to the theory of plate tectonics, crustal plates float on the mantle and slowly drift from the convection currents in the mantle.

The Sun's surface has a mottled appearance called granulation. These granules result from convection currents transferring energy from the solar interior to the surface. The individual convection current cells form the granules observed on the Sun's surface.

For a liquid or a gas, which does not conduct heat well, convection currents are usually the most efficient way to transfer thermal energy.

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Energy Rules!

Sun

Section B. Energy Transfer

 

Energy Transfer
Energy can be transferred from one location to another, as in the sun's energy travels through space to Earth. The two ways that energy can be transferred are by doing work and heat transfer.

 

Doing Work

Work (Man Pushing Wheelbarrow)

Energy can be transferred from one object to another by doing work.  When work is done on an object, it results in a change in the object's motion (more specifically, a change in the object's kinetic energy). 

Energy is often defined as the ability to do work. Work equals force multiplied by distance. To learn more about work please visit What is Energy? - Section C. Measuring and Quantifying Energy if you have not already.

An illustration of how doing work is an example of energy transfer

Suppose that a person exerts a force on the wheelbarrow that is initially at rest, causing it to move over a certain distance. Recall that the work done on the wheelbarrow by the person is equal to the product of the person's force multiplied by the distance traveled by the wheelbarrow. Notice that when the force is exerted on the wheelbarrow, there's a change in its motion. Its kinetic energy increases. But where did the wheelbarrow get its kinetic energy? It came from the person exerting the force, who used chemical energy stored in the food they ate to move the wheelbarrow. In other words, when the person did work on the wheelbarrow, they transferred a certain amount of chemical energy stored in the person was transferred to the wheelbarrow, causing its kinetic energy to increase. As a result, the person's store of chemical energy decreases and the wheelbarrow's kinetic energy increases.

Wherever you look, you can see examples of energy transfers. When you turn on a light, you see result of energy being transferred from the sun to the plants to the coal to electricity and finally to light you see. During each of these transfers, energy changes form. There are two main forms of energy, kinetic energy (motion) and potential energy (position). To further classify energy, these forms are sometimes further described as thermal (heat), elastic, electromagnetic (light, electrical, magnetic), gravitational, chemical (food), and nuclear energy. See the What is Energy? - Section B. Two Main Forms of Energy for more information on kinetic and potential energy.  

Sun Bathing

 Heat Transfer 
Heat is given off when an object's thermal energy is transferred. Thermal energy (see below) can be transferred in three ways: by conduction, by convection, and by radiation.
 
1. Conduction

Conduction Example (Game of Pool)
Conduction is the transfer of energy from one molecule to another. This transfer occurs when molecules hit against each other, similar to a game of pool where one moving ball strikes another, causing the second to move. Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other. For example, metal is a better conductor than wood or plastic.
 

Convection Example (A Radiator Emiting Heat)

     2. Convection 
Convection is the movement of heat by a liquid such as water or a gas such as air. The liquid or gas moves from one location to another, carrying heat along with it. This movement of a mass of heated water or air is called a current.
 
3. Radiation Radiation Example (Sun Bathing)
Heat travels from the sun by a process called radiation. Radiation is the transfer of heat by electromagnetic waves. When infrared rays strike a material, the molecules in that material move faster. In addition to the sun, light bulbs, irons, and toasters radiate heat. When we feel heat around these items, however, we are feeling convection heat (warmed air molecules) rather than radiated heat since the heat waves strike and energize surrounding air molecules.
 
More about Thermal (Heat) Energy
Heat is given off whenever energy is being used. You can tell if a television has been on by feeling if it is warm. When you run up a flight of stairs you feel warm because you are burning food energy.
What exactly is heat? Heat is the transfer or flow of energy from a hot object to one that is cooler. When you feel a warm object, you are actually feeling thermal energy, which is the movement of molecules that make up the object. An object has more thermal energy when it is warm than when it is cool. 
 
The more thermal energy an object has, the faster its molecules move. These faster moving molecules bump into each other more frequently and spread out as they require more space (decreasing the density of the molecules). Think of people standing in an 
elevator. If they started moving around, they would start bumping into each other and need more space. This is essentially what happens when molecules get more energy and start moving around; they spread out.

Elevator
For the most part, the volume of an object increases as the amount of thermal energy it receives increases. In other words, the molecules in warmer objects are less densely packed than the molecules in cooler objects. (NOTE: Temperature is a measurement of how fast molecules move.) 
You can't see thermal energy, but you can detect evidence of heat transfer. You might see the air shimmering over a radiator (convection), put your hand on a warm spoon that's been sitting in a hot bowl of soup (conduction), or notice that the sun shine feels warm on your skin (radiation). If you need evidence of thermal energy or heat in  your life, just feel your arm. Your body generates heat 24 hours a day! (Taken from KEEP Activity Guide "Exploring Heat").
A note about thermal energy and heat. In strict scientific terms, there is a distinct difference between heat and thermal energy. A way to think about this distinction is objects possess thermal energy, while heat is transferred from one object to another. Wherever possible, we have tried to remain true to these distinctions. However, since heat is the more familiar term we often use that to facilitate understanding.

Infiltration of Outside Air in Winter

Very leaky houses are uncomfortable and have high energy bills, so finding and curing infiltration problems is a high priority for weatherization operations. The rate of air infiltration in a home depends on many factors, the most important being the size and location of holes in the thermal envelope and the difference in temperature between inside and outside. Warm air inside a dwelling gives rise to stack-effect infiltration as it tries to escape from the top of the envelope, sucking in cold air at the bottom. Wind and leaks in duct systems can also have a major effect on infiltration, but these effects are not usually as constant over the heating season as is stack­effect infiltration, which is at its worst on coldest days.

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The Weatherization Assistance Program enables low-income families to permanently reduce their energy bills by making their homes more energy efficient. During the last 32 years, the U.S. Department of Energy 's (DOE) Weatherization Assistance Program has provided weatherization services to more than 6.2 million low-income families.

Weatherization Works

Notice of Public Rule Making (NOPR) published by DOE in the May 21 Federal Register (PDF 84 KB) Download Adobe Reader.

Public meeting for commenting on the Weatherization NOPR; June 18, 2009, 11 a.m. to 5 p.m. Eastern Daylight Time; Room 83-069 of the DOE Forrestal Building, 1000 Independence Avenue S.W., Washington, DC 20085; or attend via Webcast (PDF 824 KB)
Download Adobe Reader.

By reducing the energy bills of low-income families instead of offering aid, weatherization reduces dependency and liberates these funds for spending on more pressing family issues. On average, weatherization reduces heating bills by 32% and overall energy bills by about $350 per year at current prices. This spending, in turn, spurs low-income communities toward job growth and economic development.

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Oak Ridge National Laboratory gives technical support and evaluations.

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The Weatherization Assistance Program Technical Assistance Center provides guidance for program operations and fosters community partnerships to advance weatherization.

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