Building Science Principles - How Energy Travels Through Your House

Convection is the fashion in which heat flows in fluids where molecules can move around, as in water or air. In houses, warm air rises to the ceiling depressurizing the lower level, and leading to infiltration of cold and dusty air from the crawl space. Air-tight ceilings and floors are necessary to prevent heat loss due to convection and stack effect.

Conduction is the most predictable type of heat flow. Heat conducts through solid objects and between objects touching one another. As an object becomes warmer, the molecules vibrate, bump, and vigorously rub against each other, passing heat through the material. Heat always flows from higher temperatures to lower temperatures. Materials with high thermal resistance (R-values), such as insulation, slow down conduction.

Radiation heat flies through space from one object to another. We can feel the sun's radiant heat on our faces. Objects within a line of sight of one another, exchange heat radiation continuously. In this exchange, there is a net heat flow from high-temperature objects to low-temperature objects as dictated by the second law of thermodynamics (The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines). The high temperature object gets cooler and the low-temperature object gets warmer as a result of radiant-heat exchange. During seasons houses are affected differently by the sun's radiation. Every house should be designed, or re-designed to maximize the benefit of solar radiation in winter time and minimize it in the summer time.

Heat transmission is a combination of conduction, convection, and radiation through the solid walls, roof, and floor of the building. R-value is used to measure resistance of walls, floors, and ceilings. U-factor is the inverse of R-value, meaning that: U=1/R. When R=2, U =0.5. Air exchange is a separate component of heating and cooling load consisting of air leakage and ventilation.

Relative Humidity and Dew Point
Homes have more or less moist air. Moisture moves from areas of high vapor pressure to areas of low vapor pressure. Vapor pressure is the pressure produced by water molecules in a mixture of air. When a home is heated, water molecules want to travel outside seeking to dominate colder air. On the contrary, in hot and humid climates, warmer water molecules travel from the outside to air-conditioned cooler space. Dew point is a temperature where water vapor changes to its liquid phase. Dew point is function of both concentration and temperature. Proper use of vapor barriers is crucial to prevent water condensation in walls and attic.