Building Science Basics

Introduction to Building Science

A house is much more than four walls and a roof. It is an interactive system made up of many components, structure, ventilation, and filtration. Each component influences the performance of the entire system. Buildings perform in very predictable fashion. These performance characteristics are based on four simple principles of physics. Although solid design is the starting place for a healthy home, proper maintenance is important for both interior and exterior components to function as designed.

Stack effect is a natural phenomenon whereby heated low pressure air rising in a multistory building escapes through holes in the building envelope. This process creates low pressure in the home and draws unconditioned (cold in winter / hot in summer) dirty air in from outside and in the crawlspace to fill the void left by rising air.

Building Performance Principles

There are many principles at work in our homes. Here we will address four principles that will help us better understand how our homes perform. These are:

1. Moisture movement
2. Dewpoint temperatures
3. Pressures
4. Heat flow

1. Moisture movement:

Moisture levels in a home depend on a variety of different factors such as lifestyle (showering and cooking), number of occupants, leaks, and ground/atmospheric moisture. Moisture wants to move from areas of high vapor pressure to areas of low vapor pressure. Vapor pressure is the pressure exerted by water molecules in a mixture of air. An example: when the home is being heated, moisture wants to move to the outside, and when it is being air conditioned, moisture wants to move from the outside to the inside of our homes.

One of the most common ways we use to discuss moisture in homes is relative humidity (RH) levels. RH is a percentage that indicates the amount of moisture in the air relative to the maximum amount the air can hold at that temperature. Warm air can hold more moisture than cool air, so the RH of a sample of air will change as the temperature changes, even though the actual amount of moisture in the sample does not. If we raise the temperatures, we lower the RH and if we lower the temperature, we raise the RH.

2. Dewpoint temperatures:

Dewpoint is the temperature where water vapor will change to liquid water. This is a function of both temperatures and the amount of moisture in the air. If we have a dew point of 40 degrees, any surface in the home that reaches this temperature will have liquid water on it. To prevent this condensation, we can either raise the surface temperature or lower the relative humidity.

3. Pressures:

Pressure moves from areas of high pressure to areas of low pressure.

Pressure and holes are one of the biggest concerns in residential construction, and they are tied into much of what we need to understand about how our homes function. Pressures can be caused by external conditions (wind and temperature), internal conditions (exhaust fans, air handlers, chimneys and vents, and clothes dryers). In order for pressures to influence how a house performs, there needs to be either an intentional or unintentional hole associated with pressure. If you feel cold air entering your house, this is a result of both a hole and pressure. If you take either of these away, the hole or the pressure, the air will not move. An important point to remember is that cold air entering your home may be replacing warm air leaving your home. In other words, we tend not to notice air leaking out of our home as much as air leaking into our homes, although they can be equal amounts. Air leaking out can generate problems with attic and wall condensation in cold climates and ice dams in climates with heavier snow loads.

4. Heat flow:

Heat moves from areas of higher temperature to areas of lower temperature. When heating, your home's warm air is escaping to the outside, and while cooling the opposite is happening. Insulation is designed to resist heat flow, so the higher the R-value (R-value is the resistance to heat flow: the higher the number, the better an insulation material is able to slow heat flow), the slower heat will move into or out of a home. To understand how we slow heat movement in our homes, we need to discuss the mechanisms of heat flow. There are basically three types of heat flow we need to be concerned with.

Conduction is the transfer of heat energy between objects that are in contact (touching a hot iron is one form of conduction)

Convection is a mechanism for heat transfer in gases and liquids; it requires air or liquid movement to transfer heat (a hair dryer moves heat this way)

Radiation is the transfer of heat in the infrared spectrum, and will occur even in the vacuum of space (that's how the sun's warmth reaches us)