We have all felt the effects of thermal energy. When we walk by a camp fire we feel the heat. In winter we may feel a chill indoors when we walk by a window. That window is cold and our bodies radiate at the window and make us cooler. In extreme cases, people can die of hypothermia in the desert because their bodies radiate at the cold sky of night.
All warmth on Earth comes from the sun and that warmth is Solar Infrared Radiation. It warms the land, lakes and oceans and drives the weather we experience. In winter, the amount of thermal radiation we receive in the Northern Hemisphere can be as much as 95 percent less than what we receive in summer. As a result, the radiating environment we live in is much colder in winter.
This first article will show how our radiating environment affects our homes and how the heat losses occur. In order to better understand this heat loss engine we will start with a few rules of heat loss:
1) 60 to 80 percent of heat losses in homes and buildings is Radiated Heat Loss. Heat losses occur through three heat transfer methods: conduction, convection and radiation. In our homes, conduction and convection can drive this heat loss until the lost heat arrives at the exterior wall. At this point the heat will be radiated away from the home.
This home is a newer home which was built to code. However after construction was completed, the builder had to return to replace a defective window. The builder failed to insure the integrity of the 'building envelope' and did not completely repair the insulation and vapor barrier. As a result there is a noticeable loss of heat from the window. The heat travels up the interior of the siding which is heated and radiates to the external environment. The temperature scale on the right side of this thermograph shows the different temperatures of the the siding of this house.
Home interiors can be 'heat sinks' when building envelope is compromised. That means that the radiating temperature of the home can radiate at a cold spot in the walls and transfer energy out of the house.
This homeowner decided to instal a walkway in the attic. The homeowner mistakenly moved the insulation under the boards and the resultant cold spot is evident from that mistake. The home's heating system heats the interior walls and the walls will radiate at this cold spot. That heat will be lost to the home.
In these first two thermographs it is important to note that had these homeowners had the ecoEnergy sponsored blower door test, these faults would not have been detected. There was no evidence of air movement near these envelope breaches. The only way they were found was with an infrared camera. In a later article, we will see how these heat losses can be quantified so the homeowner will have a better idea how much these faults will cost. Now we shall learn about air temperature.
2) Air temperature has almost nothing to do with heat losses. I have now made an assertion which everyone will reject. However, we must look at how air affects our radiating environment and not just at its convective nature. In infrared, air is a transmitter of radiation. What is important is the radiating environment that every wall in the home is subjected too. On the interior of the home, most walls will be at or near room temperature. However, outside walls will all be different because they will each radiate at different objects that they face. Some might be facing other homes, trees or landscapes, Each wall will have a different profile.
This next series of thermographs and pictures will show how much our environment changes in infrared.
These thermographs were taken at about the same time of day. The day was sunny. The air temperature was 0 celsius.
SOUTH FACING WALL(Sunny Side)
The pie plate is used to reflect the radiating temperature of the environment that the wall is radiating at. The wall is heated by the sun to almost 45 degrees Celsius but the pie plate is reflecting an environment that is about -6 degrees Celsius. The environment powers the heat losses on this wall (remember air temperature is about freezing). So we can ask what is this environment?
As can be seen, this side of the home is radiating at shrubs, earth, snow and sky all of which add up to a temperature of -6 Celsius.
NORTH FACING WALL(Shaded side)
This thermograph was taken a few moments later. This wall faces north. Remember, the air temperature is at freezing. However the pie plate reflector has a temperature of about -16 degrees Celsuis. Our North wall is radiating at a completely different environment. This environment will power the heat losses experienced on this wall.
This wall faces a scene made up of water, a distant land mass and the sky and the swans.
In daylight on a sunny day, the sky is always cold (unless facing right into the sun). This thermograph and its radiating temperature is the composite of the warm land on the horizon (about +10 degrees Celsius), cooler water (about 0 degrees Celsius), even cooler clouds (about -15 degrees Celsius) and very cold sky (below -40 degrees Celsuis). The clear sky (not facing the sun) is always this cold in summer and in winter. Which brings us to the next rule.
3) Heat travels from warm to cold. This is the Second Law of Thermodynamics! Whether heat is conducted, convected or Radiated this always holds true. Finally!
4) All building materials used in homes are good to excellent Radiators. Because they are good radiators, when they are colder than the radiating environment they will also be good radiation Absorbers. Wood, fibreglass, glass, aluminium (anodized painted), vinyl, brick and concrete blocks, cast concrete walls and floors and latex and oil based paints can all radiate and absorb at very high rates.
The next article will look at the natural pressure dynamics in a building and how this will affect the radiation losses.
