natevenn · October 20, 2014 00:31
diff --git a/Complex topic b/Complex topic
 My topic is on the sciences of buildings, in particularly the envelop of the building and the physical forces acting on the air boundary of the house. The physical forces that I will talk about are `convective heat transfer` and `air pressure`. Both play a part in how air movement happens in a building and the associated energy losses.
 	Houses have to breath, meaning the moist, stale air inside the house needs to be exchanged with fresh outside air. The national standard for the natural air change rate per hour (NACH) is about 3. This roughly means that a house with a NACH of 3 exchanges 30 percent of the air volume of the house every hour (occupancy level will change this number). More than this is considered a waste or what is commonly called a “leaky” house. This is were we as energy auditors focus most of our attention when doing an audit on a home. The reason is because the air infiltration of a “leaky” house can lead to  30 percent of your energy bill. The other reason is because air sealing is the quickest and the most cost effective energy retrofit that can be done. 
 	Imagine a cold January day where the outside temperature is 10 degrees and the inside temperature is 70 degrees. This gives a temperature difference of 60 degrees. According the the laws of thermodynamics, warm flows to cold. The heat transfer processes acting on the house are a combination of `thermal radiation`, `conduction`, and `convection`. Heat loss or transfer through apertures such as cracks or holes in the house is due to the `convective` forces. The greater the delta in temperature the faster the movement of air. 
 	In addition to convection, `air pressure` differences also play a role in air movement. Again in our winter time example we are heating the house from the inside. As heat rises and escapes through the cracks in the higher points of the house it creates a negative pressure in the house. This negative pressure then draws the pressurized cold outside air from lower points in the house. In building science this is called the `stack effect`. This `stack effect` can be so significant that when skyscrapers were first being developed they had to also redesign how people would get in and out of the building. The exit doors were too difficult to open due to the pressure created by the cool air rushing in. That's how the revolving door came to be. 
 	It was common practice to rely on these holes and cracks to provide the occupants with fresh air. Now we are starting to learn that this is a bad strategy. It leads to not only increased energy bills but also poor air quality. Who wants to breath air filtered by the dead mice in the crawl spaces? The new motto is build it tight and mechanically ventilate it right.
	My topic is on the sciences of buildings, in particularly the envelop of the building and the physical forces acting on the air boundary of the house. The physical forces that I will talk about are `convective heat transfer` and `air pressure`. Both play a part in how air movement happens in a building and the associated energy losses.
	Houses have to breath, meaning the moist, stale air inside the house needs to be exchanged with fresh outside air. The national standard for the natural air change rate per hour (NACH) is about 3. This roughly means that a house with a NACH of 3 exchanges 30 percent of the air volume of the house every hour (occupancy level will change this number). More than this is considered a waste or what is commonly called a “leaky” house. This is were we as energy auditors focus most of our attention when doing an audit on a home. The reason is because the air infiltration of a “leaky” house can lead to 30 percent of your energy bill. The other reason is because air sealing is the quickest and the most cost effective energy retrofit that can be done.
	Imagine a cold January day where the outside temperature is 10 degrees and the inside temperature is 70 degrees. This gives a temperature difference of 60 degrees. According the the laws of thermodynamics, warm flows to cold. The heat transfer processes acting on the house are a combination of `thermal radiation`, `conduction`, and `convection`. Heat loss or transfer through apertures such as cracks or holes in the house is due to the `convective` forces. The greater the delta in temperature the faster the movement of air.
	In addition to convection, `air pressure` differences also play a role in air movement. Again in our winter time example we are heating the house from the inside. As heat rises and escapes through the cracks in the higher points of the house it creates a negative pressure in the house. This negative pressure then draws the pressurized cold outside air from lower points in the house. In building science this is called the `stack effect`. This `stack effect` can be so significant that when skyscrapers were first being developed they had to also redesign how people would get in and out of the building. The exit doors were too difficult to open due to the pressure created by the cool air rushing in. That's how the revolving door came to be.
	It was common practice to rely on these holes and cracks to provide the occupants with fresh air. Now we are starting to learn that this is a bad strategy. It leads to not only increased energy bills but also poor air quality. Who wants to breath air filtered by the dead mice in the crawl spaces? The new motto is build it tight and mechanically ventilate it right.
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