Perhaps the most common misperception regarding PHs concerns airflow. With the past problems of poor indoor air quality of “too-tight” houses built in the late 1970s and 80s, many builders balk at airtight homes. Even though a PH is “tight,” it does breathe. However, rather than breathing unknown volumes of air through uncontrolled leaks, PHs breathe controlled volumes of air by mechanical ventilation, which circulates measured amounts of fresh air through the house and exhausts stale air. The health and comfort of the occupants come first, and good indoor air quality is indispensable.
A PH is ventilated using an energy-efficient, balanced mechanical ventilation system. PHs use energy recovery ventilators (ERVs) or, in cold, dry climates, heat recovery ventilators (HRVs), which incorporate air-to-air energy recovery to transfer from the exhaust air to the incoming fresh air, significantly reducing the energy needed to heat incoming air. State-of-the-art ventilation systems have heat recovery rates of 75% to 95%.
The ventilation system exhausts air from the rooms that produce moisture and unwanted odors, such as the kitchen and bathrooms. Humidistats monitor when moisture levels are elevated, initiating an increase in the ventilation. The exhaust air is drawn through the ventilator, passing through a heat exchanger to transfer the heat to the incoming fresh air. Exhaust air is not mixed with the incoming air—only its heat is transferred.
The ventilator filters the fresh air and removes excess moisture. The system is generally very quiet and draft-free. The PH Planning Package (PHPP; see below) recommends an ACH of 0.3 to 0.4 times the volume of the building, and a guideline for supply air of 18 cubic feet per minute (cfm) per occupant.
The main difference between an HRV and an ERV is that the HRV conserves heat and cooling energy, while the ERV does both and transfers humidity as well. In summer, an ERV helps keep the humidity outside; in winter, it helps prevent indoor air from becoming too dry. For in-between seasons, when no conditioning is needed, a bypass can be installed for either system to avoid heating the incoming air. Alternatively, the ventilation system can be turned off altogether, and windows can be thrown open to bring in fresh air.
Either system’s efficiency can be increased by prewarming or precooling the incoming air by passing the incoming air through earth tubes. Since the ground maintains a more consistent temperature throughout the year than the outdoors, passing the air through tubes buried in the earth either preheats or precools the air, depending on the season. Preheating and precooling can also be accomplished indirectly, by circulating water in an underground pipe and using it to heat or cool the air with a water-to-air heat exchanger.
PH designers choose windows and doors based largely on their insulating value. Low-emissivity (low-e) coatings have significantly affected the heat conductivity of windows. These coatings are microscopically thin, transparent layers of metal or metallic oxide deposited on the surface of the glass. The coated side of the glass faces into the gap between the two panes of a double-glazed window. The gap is filled with low-conductivity argon or krypton gas rather than air, greatly reducing the window’s radiant heat transfer. Various low-e coatings allow for high, moderate, or low solar gain to provide a range of options for houses in all climates, from heating-dominated to cooling-dominated. Builders can choose triple-pane low-e-coated, argon-filled windows with special low-conductivity spacers and insulated frames with little thermal bridging. These windows eliminate perceptible cold radiation or convective cold airflow, even in periods of heavy frost.
Not only must PHs minimize energy loss, they must also carefully manage a home’s energy gains. The first step in designing a PH is to consider how the orientation of a building—and its various parts—will affect its energy losses and gains. There are many issues to be considered: Where should the windows be for maximum sunlight when sunlight is wanted, and minimal heat gain when heat gain is unwanted? The more natural lighting there is, the less artificial lighting will be needed. Designers can enhance residents’ enjoyment of available sunlight by orienting bedrooms and living rooms to the south, and putting utility rooms, closets, etc., to the north, where sunlight is not needed.
PH windows are oriented to take advantage of the passive solar energy, but the goal is not simply to have as much solar gain as possible. Some early superinsulated buildings suffered from overheating, because not enough consideration was given to the amount of solar gain the home received. Good design considers solar gain within the home’s overall conditioning needs—and within the budget. Even very efficient windows can lose more heat over a year than they gain, depending on their location, and large windows are expensive.
In the northern hemisphere, windows on the north side receive very little direct solar heat gain, while those on the south can receive a great deal of it. In summer, and especially in cooling-dominated climates, preventing excess solar heat gain is important. This is accomplished by shading the windows, either with roof eaves of the proper length, which block the high-angled summer sun but allow the lower-angled winter sun to enter. Deciduous trees or vines on a trellis can filter out sunlight in the summer but allow it in the winter when the vegetation has been shed. In climates with significant cooling loads, unshaded east- and west-facing windows should be limited and those used should have low-solar-gain, low-e coatings. Otherwise, during the morning and late afternoon, low-angled sunlight can generate a great deal of heat through these windows.
Another, less obvious source of heat gain is internal. Given the exceptionally low heat loss in a PH, heat from internal sources can make a difference. Household appliances, electronic equipment, artificial lighting, and people can all have an effect on heat gain. While designers may not be choosing how many or which appliances will be installed, they often select lighting, and must take into account this heat gain when calculating the overall internal energy gain.