The Art of Building Science

Transcription

1 The Art of Building Science

2 What is Building Science? Building Science seeks to apply the scientific fundamentals of building dynamics to the functional relationships between the house s components and the environment. Central to this approach is the concept of the Whole-House System, which comprises the building envelope, the mechanical systems, and the house s occupants. According to Building Science, all of these components are interrelated so that even a small change in one component can have dramatic and unexpected effects on the entire house. Adopting a Building Science approach helps builders to provide their customers with homes that are: Healthy Comfortable Durable Energy Efficient Environmentally Responsible The Building Envelope: The building elements that define the envelope include exterior walls, foundation, windows, doors and ceiling. The building envelope allows the house to maintain a comfortable temperature and humidity level while allowing adequate ventilation inside the home regardless of outside conditions. The Mechanical Systems: This typically includes the heater, air conditioner, water heater, washer and dryer and ventilating fans. Most mechanical units have an obvious purpose (e.g., adding heat or moisture to a home), but they can also increase the airflow into or out of the home. The Home Occupants: This includes all living things, such as people, pets and plants. People directly affect the flow of heat, air and moisture in a home by the way they operate appliances, adjust the thermostat, do laundry, bathe and cook. Occupants also change the efficiency of the building envelope by opening and closing windows and doors. And since people and plants release heat and moisture into the home, they also affect the flow of heat, air and moisture. Understanding How Building Science Works Building dynamics are driven by physical, chemical and biological reactions among the home s components. In the case of home comfort, the study of heat, air and moisture flows is crucial to understanding those reactions and avoiding issues relating to mold, indoor air quality, water leakage and other problems. In fact, small changes in one component can have dramatic and unexpected effects on the entire house.

3 Heat Flow Heat that flows in and out of a home is a major factor in determining the home s comfort level and operating costs. Heat flows from areas of high temperature to areas of low temperature. The greater the temperature difference between the two areas, the faster the heat flows. In winter, a heated home loses heat to the colder outside. Conversely, in summer, an air-conditioned home gains heat from outdoors. Homes lose heat in three ways: conduction, convection and radiation heat transfer. In a home, these modes of heat transfer occur at the same time. Conduction refers to the movement of heat through a material, such as a wall or insulation. During the winter, warm air inside the house is separated from cold air outside by the home s walls. Because heat moves from areas of high temperature to areas of low temperature, the inside surface of the wall warms as it tries to reach the same temperature as the warm air inside the house. As the inside wall surface heats up, adjacent material also tries to warm. Over time, heat from the inside of the house transfers through the wall to the outside. Because the outside air is cold, the heat that travels through the material is lost to the outside. The rate of this heat loss is directly affected by the temperature difference between inside and outside air and the nature of the material. Some materials called conductors transfer heat very well. Glass, concrete and all metals are examples of good conductors. Other materials called insulators are very poor at transferring heat, including fiber glass and foam sheathings. Convection refers to the movement of gas (or liquid) over a surface. Wind blowing against a house is an example of a gas moving over a surface. This movement can be either natural or forced. Natural convection occurs when the movement of liquid or gas is caused by density differences. (Warm air rises because it has a lower density than the surrounding cool air.) For example, when warm air inside the house comes into contact with a cool exterior wall, some of the heat is lost to the wall, causing the surrounding air to cool. Since cool air has a higher density than warm air, the air drops. The warmer exterior surface of the wall heats the air next to it, decreasing the air s density and causing it to rise. The movement of air along the surface of the wall increases heat transfer and creates convection loops adjacent to both the interior and exterior surfaces. Convection also can take place inside of empty cavities. One example is the movement of air in a double pane window. In winter, air is heated on the inside surface of the window cavity, causing the air to rise. The air adjacent to the outside surface cools and drops. This creates a convection loop inside the window cavity that transfers heat from the inside to the outside. In forced convection, the movement of the liquid or gas is caused by outside forces. If the wind is blowing, the air movement across the outside of the wall will be higher, increasing the rate of heat transfer. This rate of transfer depends on the temperature difference, the velocity of the liquid or gas and what kind of liquid or gas is involved. For example, heat transfers more quickly through water than through air. Radiation Heat Transfer refers to the invisible electromagnetic waves that pass from one object to another (again, from areas of higher temperature to areas of lower temperature). For example, if you stand by a window on a cold day, your body radiates heat to the cold surface of the window, making you feel cold. In the summer, radiant energy from the sun enters the home through windows. The walls and contents of the room absorb energy, while at the same time, various objects in the room release their own radiant energy, causing the room to heat up.

4 Air Flow Air moves because of pressure differentials from areas of high pressure to areas of low pressure. Air infiltration refers to the unintentional flow of air from the outside to the inside of a home, while air exfiltration refers to the flow of air from the inside to the outside. The overall amount of air in a home remains fixed, so that the amount of air that leaks out is always equal to the amount of air drawn in. There are five primary ways that air pressure increases or decreases within a home. These effects can create a number of occupant comfort problems such as drafts and cause surface staining around windowsills and at carpet edges. They contribute to poor indoor air quality due to airborne contaminant transfer. And, they can cause surface condensation due to the transport of airborne water vapor. The five effects are: Stack Effect: The stack effect occurs because heated air is lighter than cold air. During the winter, warm air inside the home rises, causing the pressure in the top areas of the home to increase and causing the air to exfiltrate through holes in the ceiling, attic and other high areas to the lower pressure outside. Because the air is being lost from the top of the home, it must be replaced, so air is drawn in from other areas (i.e., lower areas of the home). This causes the pressure in lower areas of the home to drop below normal, which causes (cold) air to be drawn into the lower areas to compensate. Somewhere in the elevation of the house, the pressure Air infiltration and exfiltration vary all day long. That means the pattern of air movement within the house is constantly changing and the neutral pressure plane is constantly moving up and down. In most homes, the neutral pressure plane is often located fairly high in the home (close to the ceiling). In houses without chimneys, the neutral pressure plane is usually lower (somewhere about midheight). Flue Effect: This occurs in all homes with an open chimney. Here, air that s heated by internal combustion appliances (furnace, hot water heater, wood stove, etc.) rises rapidly up the home s chimney and out of the house. As the hot gases leave, they create a low pressure in the area where the appliance is located. In turn, outside (cool) air infiltrates to take the place of the air that went up the flue. The flue effect tends to reduce the pressure throughout the house. Ventilation Effect: Homes have numerous devices (kitchen and bathroom fans, clothes dryers, etc.) that ventilate air to the outside of the home. These devices cause air pressure imbalances because they intentionally force air out of the home while causing infiltration of outside air. Wind Effect: As the wind outside blows, it increases pressure on the upwind side of the house and decreases pressure on the down-wind side. This causes an increase in infiltration on the up-wind side and exfiltration on the down-wind side. System Effect: Forced air heating and cooling systems use powerful fans to distribute conditioned air throughout the home. These powerful fans greatly influence pressure distribution within the house. In a well-designed air distribution (duct) system, air is pulled from all rooms in the house via the fan. The air is then heated/ cooled and returned back to the individual rooms. However, consider a case of poor duct system design. In this case, we have a bedroom with a supply grill, but no return air grill. When the bedroom s door is open,

5 supply air enters the room through the supply duct system. It returns to the furnace by flowing through the open door and through the halls until it reaches a return air grill. However, when the bedroom s door is closed, warm supply air continues to enter the room, but there s no way for the air to leave. This pressurizes the room, causing the warm air inside to exfiltrate to the outside. Moisture Flow Many normal, daily family household activities cooking, bathing, washing dishes, and drying clothes produce airborne water vapor. Plants and people also give off water vapor. Moisture also enters homes from basements, crawl spaces or leaks. In general, moisture will follow the path of least resistance, moving from warm to cold. In cold climates, moisture from the interior conditioned spaces attempts to get to the exterior by passing through the building envelope. In hot climates, moisture from the exterior attempts to get to the cool/ conditioned interior by passing through the building envelope. Moisture is one of the most important factors affecting a home s durability and the ability of a home s materials to resist deterioration. There are two mechanisms for moisture movement: Diffusion refers to the movement of moisture through a substance. In areas where vapor pressure is different from one side of an object (e.g., a wall) to another, moisture is diffused through the object (wall) to equalize the pressure. High permeability materials, such as brick, gypsum board and fibrous insulation, allow moisture to move freely. Low permeability materials, called vapor barriers or vapor retarders, resist the flow of moisture. Warm air holds more moisture than cold air. The temperature at which the moisture starts to condense is called the dew point (the point at which the air is completely saturated with moisture and can t hold any more). Relative humidity is a measure of the amount of moisture in the air, relative to the amount of moisture it can hold at that temperature. The dew point equals 100 percent relative humidity. If air at 100 percent relative humidity is heated, the relative humidity decreases. That s because at the higher temperature, air can hold more moisture.

6 Air leakage also creates moisture movement due to the water vapor that s part of air. Moisture carried into or out of a home due to air leakage can be 10 to 100 times greater than moisture transferred by diffusion. The same pressure differential factors influencing air flow (stack effect, flue effect, ventilation effect, wind effect and system effect) also influence moisture flow in this way. Therefore, in an airsealed home, less moisture is released to the outside. How Can Builders Use Building Science to Reduce Risk? As an example of how heat, air and moisture flow interrelate to create problems, consider the case of one homeowner who complained about indoor moisture on the interior wood sills of his aluminum windows. The interior finish components were deteriorating, and black mold was beginning to grow. The homeowner was removing approximately 1 2 gallon of moisture each day from the windowsills with a sponge and bucket. Observations of the home uncovered EIFS installed improperly around the windows. Insulation had deteriorated in the wall cavities and a large amount of air leakage was occurring. Moreover, the house was constructed over a vented crawl space with no vapor retarder in place. Had the builder and his subcontractors used a Building Science approach to evaluating heat, air and moisture flow, they would have correctly flashed the windows and installed a vapor retarder under the house in the crawl space. These steps would have helped to reduce internal moisture and provide for better spot ventilation. Building Science helps builders understand how to put the concepts of heat flow, air flow and moisture flow into practice. For example: Ideally, mechanical equipment, ductwork and plumbing should not be located in exterior walls, vented attics or vented crawlspaces. All air distribution systems should be located within the conditioned space. Return air paths should be incorporated into the design of closed rooms to maintain comfort and uniform room temperatures. The only place air should be able to leave the supply duct system and the furnace or airhandling unit is at the supply registers. Return systems should be hard ducted and sealed with mastic in order to be airtight. Building cavities, stud bays or cavities, and panned floor joists should never be used as return ducts. Duct leakage can result in either pressurization or depressurization of entire conditioned spaces or specific rooms, thus increasing space conditioning energy requirements. Homes should be protected from wetting during construction and operation, and be designed to dry should they get wet. In cold settings, it s best for vapor retarders and house wraps to be installed toward the interior warm surfaces. Conditioned spaces should be maintained at relatively low moisture levels through the use of controlled ventilation and source control. Breathable materials used as exterior sheathings help to allow building assemblies to dry toward the exterior. In hot climates, air flow retarders and vapor diffusion retarders are installed on the exterior of building assemblies, and the assemblies are allowed to dry toward the interior by using permeable interior wall finishes, installing cavity insulations without vapor diffusion retarders and avoiding interior non-breathable wall coverings. Furthermore, conditioned spaces are maintained at a slight positive air pressure with conditioned (dehumidified) air to limit the infusion of exterior, warm and humid air. Foundation wall and slab assemblies must be constructed so they resist water vapor and water from getting in them, but they also must be constructed so that it is easy for water vapor to get out when it gets in or if the assembly was built wet to begin with (there can be thousands of pounds of water stored in freshly cast concrete and freshly laid masonry). If flooring, carpets, tile, interior insulation or finishes are installed before sufficient drying, or in a way that does not permit drying to the interior, the result can be buckled flooring and walls and mold growth. Conclusion Using knowledge of basic Building Science can help contractors and builders create homes that are more comfortable, more energy efficient and which provide better value than homes built just a few short years ago.

7 CertainTeed Corporation Building Science Department PO Box 860 Valley Forge, PA CertainTeed Corporation 4/04