10. Infiltration Heat Load and Weatherizing

Most older homes and some poorly constructed new ones have numerous cracks, holes, and openings through which cold outdoor air exchanges with the warm air inside a building in winter, and vice versa in summer. This uncontrolled entry of outside air into a building through unintentional openings is called infiltration, and it wastes a significant amount of energy since the air entering must be heated in winter and cooled in summer (Fig. 56). The warm air leaving the house represents energy loss. This is also the case for cool air leaving in summer since some electricity is used to cool that air. In homes that have not been properly weatherized, the air leaks account for about 30-40 percent of the total heat lost from the house in winter. That is, about one-third of the heating bill of such a house is due to the air leaks. 

The rate of infiltration depends on the wind velocity and the temperature difference between the inside and the outside, and thus it varies throughout the year. The infiltration rates are much higher in winter than they are in summer because of the higher winds and larger temperature differences in winter. Therefore, distinction should be made between the design infiltration rate at design conditions, which is used to size heating or cooling equipment, and the seasonal average infiltration rate, which is used to properly estimate the seasonal energy consumption for heating or cooling. Infiltration appears to be providing “fresh outdoor air” to a building, but it is not a reliable ventilation mechanism since it depends on the weather conditions and the size and location of the cracks.

The air infiltration rate of a building can be determined by direct measurements by (1) injecting a tracer gas into a building and observing the decline of its concentration with time or (2) pressurizing the building to 10 to 75 Pa gage pressure by a large fan mounted on a door or window, and measuring the air flow required to maintain a specified indoor–outdoor pressure difference. The larger the air flow to maintain a pressure difference, the more the building may leak. Sulfur hexafluoride (SF₆) is commonly used as a tracer gas because it is inert, nontoxic, and easily detectable at concentrations as low as 1 part per billion. Pressurization testing is easier to conduct, and thus preferable to tracer gas testing. Pressurization test results for a whole house are given in Fig. 57.

Despite their accuracy, direct measurement techniques are inconvenient, expensive, and time consuming. A practical alternative is to predict the air infiltration rate on the basis of extensive data available on existing buildings. One way of predicting the air infiltration rate is by determining the type and size of all the cracks at all possible locations (around doors and windows, lighting fixtures, wall–floor joints, etc., as shown in Fig. 58), as well as the pres-sure differential across the cracks at specified conditions, and calculating the air flow rates. This is known as the crack method.

A simpler and more practical approach is to “estimate” how many times the entire air in a building is replaced by the outside air per hour on the basis of experience with similar buildings under similar conditions. This is called the air-change method, and the infiltration rate in this case is expressed in terms of air changes per hour (ACH), defined as

The mass of air corresponding to 1 ACH is determined from m = pV where p is the density of air whose value is determined at the outdoor temperature and pressure. Therefore, the quantity ACH represents the number of building volumes of outdoor air that infiltrates (and eventually exfiltrates) per hour. At sea-level standard conditions of 1 atm (101.3 kPa or 14.7 psia) and 20ºC (68ºF), the density of air is

However, the atmospheric pressure and thus the density of air will drop by about 20 percent at 1500 m (5000 ft) elevation at 20ºC, and by about 10 percent when the temperature rises to 50ºC at 1 atm pressure. Therefore, local air density should be used in calculations to avoid such errors.

Infiltration rate values for hundreds of buildings throughout the United States have been measured during the last two decades, and the seasonal average infiltration rates have been observed to vary from about 0.2 ACH for newer energy-efficient tight buildings to about 2.0 ACH for older buildings. Therefore, infiltration rates can easily vary by a factor of 10 from one building to another. Seasonal average infiltration rates as low as 0.02 have been recorded. A study that involved 312 mostly new homes determined the average infiltration rate to be about 0.5 ACH. Another study that involved 266 mostly older homes determined the average infiltration rate to be about 0.9ACH. The infiltration rates of some new office buildings with no outdoor air intake are measured to be between 0.1 and 0.6 ACH. Occupancy is estimated to add 0.1 to 0.15 ACH to unoccupied infiltration rate values. Also, the infiltration rate of a building can vary by a factor of 5, depending on the weather. 

A minimum of 0.35 ACH is required to meet the fresh air requirements of residential buildings and to maintain indoor air quality, provided that at least 7.5 L/s (15 ft³/min) of fresh air is supplied per occupant to keep the indoor CO₂ concentration level below 1000 parts per million (0.1 percent). Usually the infiltration rates of houses are above 0.35 ACH, and thus we do not need to be concerned about mechanical ventilation. However, the infiltration rates of some of today’s energy-efficient buildings are below the required minimum, and additional fresh air must be supplied to such buildings by mechanical ventilation. It may be necessary to install a central ventilating system in addition to the bathroom and kitchen fans to bring the air quality to desired levels.

Venting the cold outside air directly into the house will obviously increase the heating load in winter. But part of the energy in the warm air vented out can be recovered by installing an air-to-air heat exchanger (also called an “economizer” or “heat recuperator”) that transfers the heat from the exhausted stale air to the incoming fresh air without any mixing (Fig. 59). Such heat exchangers are commonly used in superinsulated houses, but the benefits of such heat exchangers must be weighed against the cost and complexity of their installation. The effectiveness of such heat exchangers is typically low (about 40 percent) because of the small temperature differences involved.

The primary cause of excessive infiltration is poor workmanship, but it may also be the settling and aging of the house. Infiltration is likely to develop where two surfaces meet such as the wall–foundation joint. Large differences between indoor and outdoor humidity and temperatures may aggravate the problem. Winds exert a dynamic pressure on the house, which forces the outside air through the cracks inside the house.

Infiltration should not be confused with ventilation, which is the intentional and controlled mechanism of air flow into or out of a building. Ventilation can be natural or forced (or mechanical), depending on how it is achieved. Ventilation accomplished by the opening of windows or doors is natural ventilation, whereas ventilation accomplished by an air mover such as a fan is forced ventilation. Forced ventilation gives the designer the greatest control over the magnitude and distribution of air flow throughout a building. The airtightness or air exchange rate of a building at any given time usually includes the effects of natural and forced ventilation as well as infiltration.

Air exchange, or the supply of fresh air, has a significant role on health, air quality, thermal comfort, and energy consumption. The supply of fresh air is a double-edged sword: too little of it will cause health and comfort problems such as the sick-building syndrome that was experienced in super-airtight buildings, and too much of it will waste energy. Therefore, the rate of fresh air supply should be just enough to maintain the indoor air quality at an acceptable level. The infiltration rate of older buildings is several times the required minimum flow rate of fresh air, and thus there is a high energy penalty associated with it.

Infiltration increases the energy consumption of a building in two ways: First, the incoming outdoor air must be heated (or cooled in summer) to the indoor air temperature. This represents the sensible heat load of infiltration and is expressed as

where p_o is the density of outdoor air; c_p is the specific heat of air (about 1 kJ/kg · ºC or 0.24 Btu/lbm · ºF); V = (ACH)(V_building) is the volumetric flow rate of air, which is the number of air changes per hour times the volume of the building; and T_i = T_o is the temperature difference between the indoor and outdoor air. Second, the moisture content of outdoor air, in general, is different than that of indoor air, and thus the incoming air may need to be humidified or dehumidified. This represents the latent heat load of infiltration and is expressed as (Fig. 60)

where h_fg is the latent heat of vaporization at the indoor temperature (about 2340 kJ/kg or 1000 Btu/lbm) and ω_i = ω_o is the humidity ratio difference between the indoor and outdoor air, which can be determined from the psychrometric charts. The latent heat load is particularly significant in summer months in hot and humid regions such as Florida and coastal Texas. In winter, the humidity ratio of outdoor air is usually much lower than that of indoor air, and the latent infiltration load in this case represents the energy needed to vaporize the required amount of water to raise the humidity of indoor air to the desired level.