From The Healthy House Answer Book: Answers to the 133 most commonly asked questions. Questions 72-80.
72. Exactly what is ventilation anyway?
Simply put, ventilation is the exchange of air in a house. In other words, it means bringing in fresh air and expelling stale, polluted, moisture-laden air.
73. My attic is vented, so my house is adequately ventilated, right?
Actually, no. Attic ventilation is for attics, and crawl-space ventilation is for crawl spaces. What we’re talking about is ventilation for the living space, because that’s where people are.
We’re often asked about ceiling fans. They’re designed to circulate air within a room—not to exchange the indoor and outdoor air. So, they aren’t considered ventilation fans. They may help make you feel comfortable, but they don’t remove pollutants or moisture from the house.
74. Why is ventilating houses so important?
Nearly all homes contain building materials, furnishings, decorating items, and cleaning products that outgas pollutants into the indoor air. Ventilation can dilute the concentration of those pollutants. But, even if totally inert, safe alternatives are used, there’s one pollution source that can’t be eliminated—people. Because of normal, everyday human metabolism, we all release various gases such as carbon dioxide, water vapor, ammonia, methane, etc. If allowed to build up indoors, these can make a house odorous and uncomfortable. Plus, we all need a supply of fresh air indoors to replace the oxygen that gets used up.
If you have an unhealthy house (one built, maintained, and furnished with unhealthy materials), you’ll probably need a powerful (and expensive) ventilation system to dilute the concentration of all the indoor pollutants. But if you have a healthy house, the ventilation system’s primary job will be to dilute the metabolic pollutants released by the occupants—so it doesn’t need to be as powerful. In other words, most houses need ventilation systems, but healthy houses often get by with less powerful, inexpensive ones.
75. Can’t a house breathe naturally—like people do?
This is a real common question. In fact, we do think houses should breathe—but they can’t possible do it the same way as human beings. People breathe through specific orifices (nose and mouth) because certain muscles (the diaphragm) cause lungs to expand and contract. Houses have no muscles and they can’t expand and contract, so they must breathe differently. Airplanes fly differently than birds, and houses breathe differently than people.
We’re strong believers in mechanical ventilation. That means using one or more fans (usually small ones) to exchange the indoor air for outdoor air.
76. Why not just build a loose house?
In loose houses, there’s a certain amount of air exchange caused by Mother Nature. For example, when the wind blows on the side of a house, it pushes air through the cracks into the living space. At the same time, air will be leaving through cracks in the opposite side of the house.
Sometimes a loose house works just fine, but there are several disadvantages. First of all, with a loosely built house, you have no control over how much air is entering and leaving—or where it’s entering and leaving. If the wind isn’t blowing, there may be almost no air exchange. At other times, there may be far more than you need. With this feast-or-famine approach, you typically get far too much ventilation in the middle of winter when the incoming air is very cold, uncomfortable, and expensive to heat.
When air is leaking in through the cracks of a loose house, it will be unfiltered. Therefore, whatever pollutants exist outdoors (smoke, overspray from lawn chemicals, pollen, etc.) will be brought inside. In addition, particles and gases from insulation can be pulled into the living space, and moisture can leak into building cavities—contributing to a hidden mold problem or rot.
There have been a number of studies that have measured how much air enters houses naturally due to certain factors, such as the wind. What they’ve found is that houses are much tighter than they used to be, and as a result, they often don’t get enough natural ventilation to satisfy the basic needs of the occupants.
The solution is not to build looser houses, because loose houses are drafty, uncomfortable, too dry in the winter, and expensive to heat and cool. There are good reasons to build tight houses (#37), and if they’re ventilated mechanically, you can have comfort, energy efficiency, and health—the best of all worlds.
77. Are there different kinds of ventilation?
Yes, there are two basic kinds—local ventilation and general ventilation—and most houses will benefit from both. Local ventilation is designed to remove large amounts of pollution or moisture quickly. Many homes already have local ventilation in the forms of kitchen-range exhausts and bathroom exhaust fans. These fans are typically used only occasionally. Bath fans are turned on to remove moisture after a steamy shower, or to eliminate toilet odors. Kitchen fans expel moisture and cooking odors. By using local ventilation as necessary, excess moisture and pollution are prevented from spreading throughout the house.
General ventilation is designed to exchange the air in the entire house slowly. It’s important because people don’t confine themselves to one room—they move throughout the house. And wherever they go, they need oxygen. They also need fresh air to dilute their metabolic by-products. If your house is reasonably healthy, there are some basic guidelines that have been established to determine how much general ventilation you need. In most cases, 15 cubic feet of air per minute (cfm) is enough for each person. Another rule-of thumb suggests you calculate the volume of the house (all the indoor space) and use a ventilation rate that will exchange 1/3 of the air every hour.
78. What kinds of ventilation equipment are available?
Ventilation equipment falls into three basic categories: depressurization systems, pressurization systems, and balanced systems. Bath fans and kitchen-range hoods are almost always exhaust fans. When they blow stale, polluted air outdoors, the house becomes slightly depressurized, and an equal volume of make-up air enters somewhere else. If you have a loosely built house, the make-up air will enter through the cracks. But in a tight house, you may need to poke one or more holes in a wall for the make-up air to enter. At first, this may seem ridiculous—build a tight house, then deliberately poke some holes in it. However, it’s really much better than having a loose house because, if you install the holes, you have control over how much air is entering, where it’s entering, and where it’s leaving. Plus, it’s usually more energy efficient.
You can also use an exhaust fan for general ventilation. For example, you might have a single fan hooked to some ductwork to pull stale air from several rooms at once. This approach is usually combined with deliberate holes in the walls of several rooms so the make-up air can enter. Several companies sell make-up air inlets (often called through-the-wall vents) which are designed to allow a specific amount of air to enter. They’re often placed in bedrooms, or other rooms, where it’s nice to have a continuous supply of incoming fresh air. If you opt for this approach, you should know that make-up air inlets are equipped with very inefficient filters.
All exhaust fans slightly depressurize a house, something that can interfere with chimney function. (Of course, if your house doesn’t have a chimney, this may not be a concern.) It’s also possible to use a fan to blow fresh air indoors—which will slightly pressurizes a house. In a tight house that’s pressurized, through-the-wall vents are necessary to allow stale air to escape. While pressurization systems are possible, they aren’t common. In cold climates, pressurization usually isn’t a good idea because it can push excess moisture from the living space into wall cavities. This can result in hidden condensation, mold growth, or rot.
Balanced ventilation systems use two fans—one to blow fresh air indoors and one to blow stale air outdoors. As long as the airflows are equal, this approach neither pressurizes nor depressurizes the house. The incoming and outgoing air is often ducted to different rooms. Because this approach requires two fans, the equipment can be more expensive to purchase and operate. However, it can give you the most control and provide better distribution of the air. In cold climates, there’s a specialized type of balanced ventilator that's become popular. It’s called a heat recovery ventilator . It’s actually the most common type of general ventilation equipment on the market.
79. What exactly is a heat recovery ventilator?
Although the terms have slightly different meanings, most heat recovery ventilators (HRVs), are also called air-to-air heat exchangers. They’re balanced ventilators. So basically, they consist of a sheet-metal cabinet with two fans. But, they also have a special heat-recovery core. During operation, the incoming fresh air and the outgoing stale air pass very close to each other inside the core. They don’t touch and contaminate each other. However, the heat from the warmer airstream passes through the core into the colder airstream, warming it up. So, in the winter with an HRV, the outgoing stale air warms up the incoming fresh air. This makes HRVs more energy efficient than other forms of ventilation.
There are different kinds of HRVs available. Some have different types of cores (rotary, heat-pipe, or flat plate) which are made of different materials (metal, paper, or plastic). Their efficiency varies depending on the specific design features. When shopping for one, ask about ease of cleaning, whether it has an automatic defrosting system, its capacity, control options, what special features it has, what materials were used in its construction, and cost.
80. Isn’t ventilation equipment expensive?
Well, it certainly costs something. If you want a healthy house, we feel ventilation is a necessary expense. Fortunately, the cost doesn’t have to be exorbitant. In fact, there’s a wide variety of equipment available. Some is inexpensive to install but more costly to operate. The actual operating expenses will depend on the cost of electricity, the harshness of the climate, how powerful the ventilator is, and how often you run it.
If you opt for an HRV you’ll spend more up-front on equipment, but your operating cost will be lower. The cheapest general ventilation system might only cost a couple of hundred dollars to install. On the other hand, a Cadillac system with all the bells and whistles could run as much as $3,000 by the time it’s all hooked up. There’s really something for every budget.
In most parts of the U.S., the operating cost of a general ventilation system is routinely in the neighborhood of $100 per year, and it’s often less. While HRVs can be costly to purchase, they can pay for themselves in energy savings in a few years—if you’re in a very cold climate. If you live in a mild climate, HRVs generally aren’t cost effective, so other strategies can make more sense. When all the costs are factored in, it’s usually cheaper to build a tight house with a mechanical ventilation system than a loose house that’s ventilated (often overventilated) haphazardly by Mother Nature.
(Note: This article is part of the original HHI Archives, and was believed to be accurate at the time of writing. The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)
