Building Science

Click to open image! Click to open image!	A home is a complex interactive system made up of many components including the building envelope, heating and ventilation system, lighting and appliances. An understanding of the relationship between the system components, the environment and the occupants are imperative in understanding building science. Without this whole-house approach, it is likely the home components will fail and health, comfort, energy efficiency and the environment will suffer. There are many principles at work in a home. Here are three building science fundamentals to aid in understanding how a home performs.
Energy Movement: The Second Law of Thermodynamics: Heat Flow Heat is a form of energy. Heat flows naturally from warm areas to cold areas in all directions. In the winter, a home is being heated and warm air escapes to the outside. In the summer, a home is cool or being cooled and warm air infiltrates to the inside. When there is an effective air barrier, the only thing stopping heat from entering or leaving a home is insulation which reduces conduction. Typically the better and thicker the insulation material, the slower heat will move. To better understand how to control heat flow, one must understand the three characteristics of heat flow:

	Convection is a mechanism for heat transfer in a gas or liquid by circulation of currents. It is based on the fact that warm air rises and cold air falls. A chimney works this way.
	Conduction is the transfer of heat energy between objects that are in contact. Insulation slows the rate of conduction.
	Radiation is the transfer of energy as rays or waves. Radiation is how the sun works. Warm surfaces always radiate toward cold objects. Radiation heat loss in homes takes place primarily through single pane windows.
Air Movement Many of the most significant failures in a home are because of unintentional air flow. Air movement causes occupants to be uncomfortable, contributes to heating and cooling loss, and greatly affects the indoor air quality within a home. There are two mandatory characteristics for air to leak from a home: first, holes and cracks, and second, pressure differences. To control air movement, all potential flow paths in and out of the home must be sealed and blocked. If they are not, three major pressure differences will cause severe problems.
Click to open image! Click to open image!	The Stack Effect is when warm air moves upwards in a home because it is more buoyant than colder dense air. When the temperature inside the home is higher than the outside, a positive pressure is created inside along the upper areas, such as the ceiling or second story, and negative pressure is created along the lower areas such as the crawl space. This pressure difference allows cold air to enter the lower part of the home and warm air to escape into the atmosphere through cracks and holes within the building envelope. This effect is reversed in the summer.
Click to open image! Click to open image!	The flue and vent effect, also known as the mechanical effect, contributes to pressure differences when a fireplace or equipment such as fans are operating and forcing stale air and combustion products out of the home. While operating, air is exhausted through chimney, flues or vents, causing negative pressure within the home. This pressure difference allows cold air to enter the home through any crack or hole within the building envelope.
Click to open image! Click to open image!	The wind effect is when the building envelope experiences pressure differences due to wind speed and direction. A positive pressure on the windward side of the home blows air through penetrations in the building envelope. At this same time, a negative pressure is created and air is drawn out of penetrations on the opposite side of the home. Wind often times can carry allergens, moisture and contaminants into the home.
Water Movement: Hydrodynamics Moisture, like energy, moves from higher concentrations to lower concentrations and always wants to change states (ice, water and steam) depending on temperature. Any porous material will act like a sponge, including building materials, which is why items such as wood framing and sheeting are protected by non-porous siding and shingles. Water in its liquid form within the building envelope is a very destructive force to a home and has a major impact on air quality and comfort. Typical sources of water vapor are standing water, people and pets, plants, showers, cooking and unvented appliances. To minimize water vapor flow and its related damages, one must understand the sources of moisture and the mechanics of water vapor flow.
	Condensation is the physical change of water from vapor to liquid. This occurs at the Dew Point, which is the temperature at which air can no longer hold moisture and condensation will occur. Humidity pressure can affect the dew point. During cold weather and when the home is heated, moisture wants to move to the outside, and when it is being air conditioned, moisture wants to move from outside to the inside.
	Relative Humidity is a measure of the amount of water vapor that is held by air at a given temperature, expressed as a percentage of the maximum amount of water vapor the air can hold at that temperature. Warm air can hold more water vapor than cold air. Research indicates the optimal range of humidity for human health is between 40% and 60%. If humidity is too low, people experience respiratory irritations. If humidity is too high, condensation occurs on cool surfaces such as windows and walls and where mold, mildew and bacteria can grow.
	Capillary Action is the ability of a liquid to rise by absorption. Water in the ground can rise up through concrete used in foundations and be released into an area with lower vapor pressure. Capillary action can be a major contributor to moisture entering the building envelope through the foundation.
	Gravity causes water to flow downward. Surface runoff, ground water and driving rain can enter the building envelope through cracks and holes. This is prevented by proper flashing and rain gutters and properly sloped grade away from the building.