National Oilheat Research Alliance 31 Figure 3 Pressure Differences ing them, he or she can make better decisions, and be a more valuable source of information for Oilheat customers. The Building Shell as an Energy System As mentioned in the beginning of this chapter, to understand home performance it helps to think of the building as an envelope or shell. Remember, its function is to keep the inside air in the building and the outside air from entering the building. All of the components of the shell—walls, windows, doors, floors, ceilings and roof—comprise the envelope, which has two parts: the air barrier and the thermal barrier. The air barrier stops the flow of air through the envelope and the thermal barrier stops the flow of heat. Parts of the building are inside these barriers and parts can be outside the barriers. The flow of air through the envelope is affected by and affects the HVAC systems, the operation of the other appliances and the actions of the people in the building. That’s why building science also involves the study of the interactions in the building. It is a struggle to compromise between undesirable air leakage and the proper ventilation and moisture control needed to maintain a healthy environment in the building and the durability of the building components. The Building Envelope Four basic principles of physics dictate how the envelope works. • Heat flows from hot to cold. • High-pressure air flows to low-pressure. • Airflow carries heat through the walls. • Airflow also carries water vapor into the envelope. Airflow is caused by pressure differences caused by wind, the stack effect, the air for combustion and exhaust fans. These factors combine to create air pressure in the top of the building that is higher than atmospheric pressure outdoors and pressures less than atmospheric low in the building. Somewhere in the middle is the neutral pressure plane where the pressure indoors and outdoors are equal. Figure 3. Wind blowing on the building forces air into the upwind side and sucks air out of the downwind side (Figure 4). The Stack Effect (Figure 5) is hot rising and cold air falling in the building. The stack effect makes the air pressure higher than atmospheric pressure in the top of the building and below atmospheric pressure lower in the building. Air drawn into the burners for combustion and combustion gases vented from the building cause the pres
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