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Buildings are essentially a manifestation of the basic laws of physics. What
holds them up, what keeps them dry, and what makes them comfortable are all
just applied physics. Buildings fail when we ignore these laws.
Heat Movement: Thermodynamics
Energy is basically the “go” of things. Without energy the planet would be at rest, and nothing would ever happen. It takes energy for everything we know to exist. We rarely think about energy because it is the mainly invisible: only the results of energy in action are apparent to us. Energy has two basic laws that determine its behavior and the results we can achieve by using it.
The First Law of Thermodynamics
The first law of thermodynamics says that energy can neither be created nor destroyed, only changed from one form to another. We are familiar with various forms of energy on a daily basis. Atomic or nuclear energy is what powers the sun and nuclear power plants, and is the basis of nuclear bombs. Basically the energy that holds atoms together is broken and energy is released, called radioactivity.
Electrical energy occurs when electrons are released from an atom and they flow from a higher concentration to a lower concentration. A battery has more electrons on one end than the other, and when a light bulb or a motor connects them, the electrons flow from one end to the other. That is why a light switch works: we connect the concentrated electrons in the power lines to the “ground” and the light bulb is in the middle of the flow, creating resistance — which causes it to glow. When we flip the switch, the flow stops and the bulb goes out.
Chemical energy is released every time we eat. Digestion breaks the chemical bonds in food and releases energy. Petroleum is stored chemical energy. The energy is stored in the bonding of molecules and released when those bonds are broken by refining the oil or by burning it.
Mechanical energy is the energy of anything in motion. The chemical energy in petroleum is burned and released to create the mechanical energy of our cars going from place to place.
Potential energy is stored in differences in altitude. Hydropower is based on gravity pulling water down from higher elevations to lower elevations spinning a generator to produce electricity in the process.
We constantly change energy from one form to another every day.
The Second Law of Thermodynamics
The second law of thermodynamics says that energy can be changed from one form to another, but something is always lost in the process. In other words, there is no such thing as a free lunch when we convert energy — just as when you translate from one language to another, something is lost in the process. When you burn petroleum, you lose some of the chemical potential energy in the form of heat.
Heat is the final or lowest form of energy. At the end of the day, all the energy we use turns into heat that is radiated from the earth to the universe, an infinite heat sink. The more times energy is converted from one form to another, the more heat is lost in the process. The difference between what we started with and what we have to work with in the end is called “entropy.” Entropy measures how much is lost in conversion. The higher the entropy, the less efficient the energy conversion process was. The lower the entropy, the more efficient the process. Understanding energy use requires an understanding of how entropy works.
Different forms of energy have different concentrations, or the ability to do more or less work. Uranium has many times more potential for doing work than a hot rock. Electricity is a much higher form of energy than the heater in your car. You can do some things with one that you can't do with another, so we change it back and forth to do the work we want to accomplish.
Sunlight is the only form of incoming energy we have on the planet. Everything else comes from stored energy like oil, coal, or natural gas. When we use solar energy to heat our homes, there is very low entropy because we are going from the energy source to a direct application, making us comfortable. Very little is lost in the process of sunlight coming through our windows and heating our homes in the winter.
On the other extreme, if we use electricity generated by an oil burning power plant then there is a whole string of high entropy processes that take place. Oil exploration is often in remote places, like Alaska or under water. It takes energy to do the exploring. When it is found, it then takes energy to manufacture massive equipment to extract the oil. The equipment must then be transported to the site, using more energy. It takes more energy to run the pumps to extract the oil. Once the oil is extracted it must be transported to another location for conversion to useable products like diesel oil. It takes a lot of energy to reconfigure the chemical bonds in petroleum to make other products at refineries. The oil is then transported to a power plant that burns the oil to create steam to turn the big generators that create electricity. The electricity is then distributed through power lines that lose energy in the process, until it is converted by a transformer from high voltage to a form that you can use in your house. At the outlet, you can plug in a computer that uses electricity very efficiently and does amazing work, or you can plug in an electric baseboard heater that converts electricity to heat to make you comfortable for a few minutes. Energy, or entropy, is lost at every step of the process. The final usable energy available is called net energy.
Efficiencies multiply. Net energy from a coal or oil-fired power plant is only 15 percent by the time it reaches your home! This is an example of high entropy energy use.
All energy moves from higher concentration to lower concentration. So that means that warm air always moves from hot to cold. The difference is called the temperature differential. The higher the differential the more heat moves from hot to cold. In the winter, your house is warmer than outdoors so heat moves from your house to the universe. The only thing that stops it is your insulation or reducing the conduction of heat through the walls and ceilings.
Hot air is also lighter than cold air so hot air rises like a hot air balloon. Often, the second floor is warmer than the first floor or the basement. That is from the process of convection, warm air rising and cooler air falling.
Also, warmer objects radiate heat to cooler objects. That is the experience of standing in the sun on a hot day. We feel warmer in the sunlight because of the radiant heat from the sun. That is why you feel the heat from a teapot even though your hand may be several inches away.
So heat transfer has three characteristics:
Conduction is the process whereby heat flows through a material: Thermal conduction is analogous to electrical currents; if it conducts electricity, it will conduct heat. Insulation slows the rate of conduction and is a better insulator than wood. Wood is a better insulator than metal. Would you rather pick up a hot frying pan with a cast iron handle or a wood handle?
Convection is the heat transfer in a gas (air) or liquid by the circulation of currents: Convection is based on the fact that warm air (or water) rises and cold air falls. A chimney works because of convection. Drafts form at single-glazed windows because the windows cool the air, which gets heavier and moves down the window and across the floor.
Radiation is energy radiated or transmitted as rays or waves (or in the form of particles for the subatomic physicists in the family): Radiation is how the sun works to heat the earth. On a warm day it is hotter in the direct sun than in the shade, even though the temperature is the same. Warm surfaces radiate toward cold objects.
Our homes use these principles all the time to keep us comfortable — or not. The intention with green building is to use all of the laws of thermodynamics to our best advantage. We can incorporate as much insulation as possible to reduce the conduction of heat to the environment; reduce drafts by sealing the house well and incorporating ventilation where and when we want it, by directing and controlling convection; and we can take advantage of the radiant energy from the sun in winter through passive solar design.
Conventional approaches often don’t take full advantage of the natural laws. Homes are oriented any which way, regardless of where the sun’s heat is; they incorporate only as much insulation as they are required to by building codes, and too often they are drafty or hotter in some areas than others because natural convection has been ignored. We overpower the natural laws of thermodynamics by using more energy in heating and cooling equipment than we need to, resulting in unnecessary energy costs.
Ventilation is the way we manage the air inside the house. In bathrooms or kitchens we have exhaust fans to eliminate the humid air at the point of highest concentration. In other parts of the house, we typically move air around with the furnace fan. Ultimately, what we want is to control how much air enters the house, where it enters the house, and what we do with it once it is there.
Grandma’s house never had ventilation problems because it probably wasn’t insulated and it exchanged air with the outdoors all the time. Air sealing houses was never even a consideration. If water got into the wall cavities, it dried right out because there was nothing to keep it from evaporating. In today’s homes, the intention is to make them as airtight as possible with as much insulation as possible and to control air movement either by designing for natural convective ventilation or by using mechanical means.
If the house is very tight, it is important to bring in fresh air, especially if there are gas appliances such as furnaces, water heaters, gas ovens, or clothes dryers. A fireplace makes it even more critical to bring in “make-up” air to replace the air that is used in combustion. There are many ways to address this issue, using varying degrees of technology. The first is to keep a window open in the area where fuel is burning, for instance, in a room with a fireplace (although it is not so romantic to have a cold draft wafting over the bearskin rug while you are lying there with a glass of wine). The second is to have ventilation built into the combustion appliance, such as a vent that supplies combustion air right to the fireplace. Sealed combustion furnaces and water heaters are designed that way.
Other ways to bring in fresh air are to have a fresh air vent into the return air duct in a forced air system. Many commercial buildings use this approach. On the high end, heat recovery ventilation systems are tied into exhaust fans in the bathroom and kitchen. ‘When these fans are used or when a timer activates the equipment, fresh air is drawn in as air is exhausted. The air streams pass through an air-to-air heat exchanger that transfers the heat from the out going air stream to the incoming fresh air. This allows you to have fresh air without paying the energy penalty for exhausting conditioned air and reheating incoming cold air.
Good heating, ventilation, and air conditioning design creates balanced supply and return ducts so that there is no positive or negative pressurization in the home. Just as a balloon expands because you pressurize the air inside it, a home can be pressurized one way or the other. When you turn on an exhaust fan, you pull air out of the house creating a slight negative pressure.
Air always wants to be the same pressure everywhere, so when you turn on the fan, air comes in through leaks around doors, windows, or other penetrations in the envelope. This is called infiltration, but we experience it as a draft.
When the house is tight and we turn on a fan or the dryer, there are few leaks around the penetrations so air must come from somewhere else. Air will always follow the path of least resistance so the furnace, water heater, or fireplace flue becomes a likely candidate for make-up air. The problem is that it can backdraft carbon monoxide (CO) from the combustion gas into the house. This then becomes dangerous because you can’t smell or see carbon monoxide — and it can be deadly.
The ideal situation is to have the house slightly pressurized. This helps keep out drafts, it creates resistance to external gas such as radon from entering the house and it reduces the risk of back-drafting. Heat recovery ventilators often create a positive pressure indoors.
Hydrodynamics covers the laws of water movement. Water and air both act according to fluid dynamics, but with different densities or viscosities. Water always wants to move or change states. Changing states means that it is converted from ice to water to steam depending on temperature. What we are most concerned about are the movements of water and humid air.
Moisture, like energy, moves from higher concentrations to lower concentration. Any porous material will act like a sponge. The drier sponge will absorb moisture until it is as moist as it can get. The same is true with building materials; wood is porous and will absorb moisture, which is why we protect our buildings with materials that don’t absorb water such as shingles or siding.
Water is very insistent, and will migrate through any material or flow through any cracks it can whenever it has the opportunity. So if your foundation is not waterproofed sufficiently, water will always find a way to migrate through the porous concrete and into your basement. Once it is inside it is a pain to deal with, so keeping it out in the first place is the most important thing.
A roof either works or it leaks. The same is true for your siding material, although we are much more aware of leaky roofs than leaky siding. Roofs have two membranes, roofing felt or tar paper and a protective covering such as shingles or roof tiles; the main waterproofing layer is the felt paper, while the shingles are secondary, to protect the felt paper from degrading in the weather or from ultraviolet sunlight.
Siding should have the same consideration as roofing for keeping water out. Siding has more penetrations in it such as windows and doors and every one has the potential for leakage. The problem with leakage around windows is that the water gets into the wall framing and often never gets inside the drywall, so we don’t see it. If the water has no way to evaporate and dry out, it causes rot or mold. Once it does become visible, it is often far too late, and the whole wall may need to be replaced. So once again, it is very important to keep the water out in the first place with good flashing around doors and windows and some form of house wrap to direct the water flow down and out away from the walls. This is called a drainage plane.
Moisture can also come from the soil around the house. If you live with a high water table, water can push its way up through basement floor. If you live with a crawlspace, the moist soil can release moisture that is then trapped in the floor framing or distributed through the house by the ducts running through the crawlspace. The dirt floor of the crawl space should be sealed with sheet plastic, taped at the seams and around the perimeter, and the ducts should be sealed with mastic.
Although it is easier to visualize how water flows than how moist air moves, humid air is another way moisture gets into walls. In most climates, air inside the house contains more moisture than outside air. This is because people have the bad habit of breathing! We also water plants, boil water for dinner, and take showers. All these activities put moisture into the air. If the walls and ceiling are not protected with a vapor barrier, then the moisture can migrate through electrical outlets, recessed ceiling fixtures, or through the paint into the drywall, and ultimately get trapped inside the walls. Once there it can cause the same rot or mold problems.
Alternatively, in a hot, humid climate, there is more moisture outdoors than indoors. In these situations, a vapor barrier does not make sense; you need to ensure the walls can “breathe,” or allow moisture to evaporate.
Still, we want some moisture in the air. Believe me, living in the high desert of Colorado, indoor moisture is a great thing — just not too much of it. Relative humidity is the measure of how much water is in the air at a given temperature. Warm air can hold more water than cold air. Most people are comfortable with a relative humidity of 30 - 50 percent. Above that, moisture problems can develop. Below 30 percent, hard wood floors or wood trim may shrink and crack (to say nothing of your nasal passages!).
This is just an overview of the wonderful world of building science. By having a basic understanding, you can make your home much more comfortable, have a better idea of what is going on when it isn’t, and be able to have constructive conversations with your design team and contractors to assure that you are getting the most for your money. Living with nature starts by obeying the laws of nature. Green building is derived from living more closely with the natural processes that surround us, and making our homes a natural part of the larger ecosystem.