Building Science

Building Science seeks to apply the fundamental laws of physics and building dynamics to the functional relationships between the building's components and the environment.  According to Building Science, the building envelope, mechanical systems, and occupants are interrelated so that even a small change in one component can have dramatic and unexpected effects on the entire building.

Building Science entails the careful consideration of how energy and moisture flow into and out of a home or commercial building, coupled with the careful application of materials and methods to optimize and manage those flows. The end result is a structure that is safe, healthy, durable, energy-efficient, environmentally-friendly, affordable and comfortable.

The Building Envelope

The building envelope is the shell that protects us from the elements; it comprises the basement walls and floor, the above-grade walls, the roof, the windows and the doors.

We expect a lot from the envelope: it must provide structural support for the walls and roof, protect the structure from deterioration, allow for natural lighting of the interior and serve as a means of getting in and out. The envelope must also separate our warm and comfortable controlled indoor environment from the weather outside.

To maintain our indoor environment, the envelope must control the flow of heat, air and moisture between the inside of the structure and the outdoors.

House as a System

Any structure is made up of components that work together to form an integrated system. It is more than just the sum of its individual components. The performance of one component depends on its relationship with other components in the same system. Your home's ventilation and heating components, construction materials, their assembly and the behavior of occupants all interact. For better or for worse, a change to one can affect all others.

If one part is performing poorly or out of balance, it can compromise the efficiency of the entire house and cause serious and ongoing comfort and energy problems. It is important to understand the intricate principles of building science, the dynamics of air flow, and how household equipment and appliances interact with each other and with a home’s construction.

For example, few contractors and sub-contractors – even building inspectors - understand how the components of a building interact with each other. In new construction, every trade may do its job properly, but if nobody is paying attention to the issues of moisture sources and ventilation, a house can end up with mold and mildew problems. Every aspect of the house may meet the prevailing building codes but there can still be carbon monoxide spillage into the living space. The construction supervisor may do a fine job of managing all the sub-contractors. But, if no one considers the interactions of the individual parts of the building, or think about how the building perform when occupied, all this hard work could lead to some serious health, safety, comfort and potential structural issues.

Once the house or building is constructed, diagnosing the source of air leakage, heat flow and moisture issues become more complex. Wall and ceiling coverings can hide large cavities and transition points in the shell where air, heat and moisture can flow freely from inside to outside the home, and visa versa. Remodeling projects (like additions or major renovations) can cause major changes in air flow distribution within the structure. A once-comfortable environment can suddenly leak warmth like a sieve.

Building Performance Principles

There are many principles at work in our homes.  Here are four principles that will help better understand how our homes or commercial buildings perform. These are:

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

Uncontrolled airflow through the envelope can be a major source of heat loss and can lead to other problems. Since warm air can carry large amounts of water vapor, airflow is also the main means by which moisture is carried into the envelope.

Under winter conditions, air is forced through the building envelope. Air moving out of the building carries heat and moisture, while air moving into it brings uncomfortable drafts and dry winter air.

For air to move from one side to the other there must be a hole in the envelope and a difference in air pressure between the inside and outside. The difference in air pressure can be caused by any combination of the following:

• Wind effect. When wind blows against the house, it creates a high-pressure area on the windward side, and air is forced into the house. There is a low-pressure area on the leeward side (and sometimes other sides) where air is forced out. Wind can also strip heat out of gaps and cracks as it passes by an exterior wall surface, much like an open window in a moving car.
  
  
• Stack (or chimney) effect. In a heated home, the less-dense warm air rises and expands, creating a higher-pressure area near the top of the house. Air escapes through holes, gaps & cracks from the ceiling into the attic. The force of the rising air creates lower pressure –or suction - near the bottom of the house. Outside air rushes in through cracks and openings around the lower floors, especially the areas around the basement or crawl space walls. The higher the building, the greater the pressure difference across the wall and roof.
• Combustion and Ventilation effect. Appliances that burn fuels such as wood, oil or natural gas need air to support combustion and provide the draft in the chimney. Open chimneys and fireplaces tend to exhaust lots of air. This air is replaced by outside air drawn in through the envelope. This is why a room often becomes drafty when there's a fire in the fireplace. Ventilation fans in the kitchen and bathroom, central vacuum systems, stove-top grills, clothes dryers and other exhaust fans also cause this effect.

2.  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.

3. 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 dewpoint 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.

The following graph helps explain dewpoint and relative humidity data as they apply to our homes. Assuming we have a dewpoint temperature of 40 degrees and start with an interior temperature of 70 degrees and 40% relative humidity, our home would not see any condensation on surfaces. If we start to lower the temperature, our RH goes up, and at about 62 degrees we would start to see condensation, let's say on our windows. We could then raise the temperature or lower the relative humidity to get the window surfaces above the dewpoint, and eliminate the window condensation. 

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.

n 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.

There are two control strategies to consider:, holes and pressures. For any given hole, the amount of air moving through will be a function of the characteristics of the hole and pressures. And remember, if we stop the air from leaving our home, we will automatically reduce the air we feel entering our home. Forced air heating systems use pressure to move air across a heat exchanger, and into our conditioned spaces. The same system creates pressure across a furnace filter drawing air through the filter, which in turn catches the particles in the air. If the forced air system were not operating, a home would not have any filtration no matter how efficient the filter.

5. 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. Many people believe that because hot air rises, most heat loss will be through the ceiling. This is not necessarily so. Heat moves in any direction – up, down or sideways – but always from a warm spot to a colder one. A heated room over an unheated garage will lose heat through the floor. Similarly, heat loss can occur through walls in the basement or crawl space, as well as above the ground. Heat moves to the cold. It's the job of the envelope to control the flow of heat between the indoors and outdoors.

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. Heat can be transferred directly from one part of an object to another when particles bump into each other. For example, the heat from a cast iron frying pan is transferred to the handle and eventually to your hand. Some materials conduct heat better than others. Insulation works by reducing heat flow with tiny pockets of air, which are relatively poor conductors of heat.
• Convection. Heat can also be transferred by the movement of a fluid, such as water, or air. In an un-insulated wall space, for instance, air picks up heat from the warm wall and then circulates to the cold wall, where it loses the heat. Some heat is also transferred by the mixing of warm and cold air.
• Radiation. Any object will radiate heat in the same way that the sun radiates heat. When you stand in front of a cold window, you radiate heat to the window and so you feel cold, even though the room temperature may be high.

Remember pressure and holes? Even with insulation, if we have a hole and pressure across the hole, we can move an enormous amount of heat through insulation (convection). These same holes and pressures can move substantial amounts of moisture through the insulation. If we try to move moist air through insulation, the dewpoint and the resulting condensation can occur in closed wall or ceiling systems. Unlike window condensation, this will not be as obvious and may result in ceiling staining, biological contamination (e.g. mold), and in severe cases, structural degradation.

Moisture Issues

Moisture problems are the number one source of residential concerns. Interior relative humidity (RH) is what is most frequently discussed. Relative humidity is a percentage that indicates the amount of moisture in the air relative to the maximum amount the air can hold at that temperature. Based on both building and occupant characteristics, optimum RH levels may be quite different. For the building, depending on climate, windows may start showing heavy condensation at interior RH levels as low as 25%, while hardwood floors may start showing gaps between boards below 35% RH. For homeowner comfort, levels of 40%-60% are generally recommended, although on a seasonal basis controls to maintain 40%-50% RH would be ideal.

Dewpoint is another important consideration when discussing relative humidity. Simply stated, dew point is the temperature at which water vapor starts to condense into liquid water. Most frequently, this is seen on windows of a home in cold weather conditions. This also occurs in walls and attic systems, although this is not as readily visible as on windows. When it starts getting cold outside, and windows are starting to show signs of water condensing on the interior surface, this can be a problem.

• Here are ways to resolve these problems. First, deal with the issue of dewpoint. Every relative humidity has a dewpoint. For this example, use a relative humidity of 48%, standard window glass of an R-2 and a dewpoint of 40 degrees F. With these values, we would expect to see condensation forming on the glass when the outside temperate goes to 0 degrees, with condensation getting heavier as the outside temperature gets colder. As we cannot realistically change the outside temperature, we must lower the relative humidity or raise the temperature of the glass.

Moisture can be solid, liquid or gas (water vapor). It can originate from outside as ground water in the soil or as ice, snow, rain, fog and surface run-off, or from inside in the form of water vapor produced by the people in the house and their activities, such as washing, cleaning, cooking and using humidifiers.

In its different forms, moisture travels by several means:

• Gravity. Water running down a roof or condensation running down a window pane shows how gravity causes water to move downward.
• Capillary action. Water can also move sideways or upward. Capillary action depends on the presence of very narrow spaces, as with lapped siding or porous materials, such as concrete or soil. (Think of how a paper towel absorbs water.)
• Diffusion. Water vapor sometimes moves directly through materials by diffusion, depending on the difference in water-vapor pressure and the material's resistance to this pressure.
• Air movement. As water vapor, moisture is carried by moving air. This can happen, for example, where there is air leakage through a crack in the house envelope. Far more moisture can be carried by airflow through a small hole in the envelope than by diffusion through building materials.

In general, we need to consider four sources of moisture in a home. These include:

1. Homeowner generated moisture (water vapor) - produced by bathing, cooking, breathing and wet cleaning. Although variable, this can be as much as 6-10 pints per day.
2. Building generated moisture (water vapor) - produced both by new building components drying out and seasonal drying of building components. This can be as much as 2-5 gallons per day.
3. Soil generated moisture (water vapor) - water vapor from air movement into the home from the surrounding soil, and capillary movement through the slab or foundation walls.
4. Bulk water (liquid water) - below ground and through building components such as windows, flashings and roof components. This can be as much as 20-100's gallons per occurrence.

Ventilation

Household air can contain high levels of pollutants and irritants, including dust, pet dander and chemicals as well as high moisture levels.  These can have a serious health impact on your family, along with causing problems for the house itself. Homes need effective ventilation for the occupants and the building.  Effective ventilation means both a controlled ventilation rate and a means of distributing the fresh air to habitable spaces where people live.  Ventilation systems are used to expel stale and polluted air from the house and bring fresh air into the home.  There are many different types of ventilation, as well as different installation techniques for each type of ventilation:

• Natural ventilation:  Homes that rely on windows and natural airflow through cracks in the building.  Even though a home may be drafty, it may not be a reliable source of fresh air, since natural ventilation depends on factors such as temperature and wind.  Since you can't control natural ventilation, there is no way to ensure that fresh air is coming in.
• Spot or source point ventilation:  This form of ventilation uses bathroom and kitchen exhaust fans.  These are generally operated on an intermittent basis for cooking and bathing, and are not necessarily a substitute for occupant ventilation.
• Balanced ventilation:  This type system requires both a ventilation fan to remove moist and polluted air and a fan for fresh air to replace the air being vented.  This can be achieved with or without heat recovery as part of the system.
• Exhaust ventilation:  This system uses an exhaust fan to remove air from the home.  An equal amount of make-up air enters either by way of intentional openings or through random holes in the structure of the home.  This type of ventilation maybe preferred to other systems during months requiring home heating.
• Supply ventilation:  This system uses either a supply fan or a forced air heating/cooling fan to deliver fresh air into a home and in some cases can pressurize the home.  This type of system maybe preferred to other systems during months requiring home cooling.

These systems either use neutral pressure, negative pressure or positive pressure techniques to provide ventilation.  Depressurization of your home can cause problems with combustion appliances backdrafting.  Backdrafting is the term used to describe the unwanted flow of combustion gases into your home by vented combustion appliances.  Pressurization of your home can promote moisture movement out of the home during heating seasons.  Careful considerations of the type of ventilation, as it relates to pressures, should be made prior to choosing a system for your home.