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The Physics of Good Ventilation

By HHI Staff

For air to move into or out of a house, two basic requirements are necessary. First, there must be a path through which the air travels, and second, there must be an air-pressure difference (wind often causes an air-pressure difference) to push the air molecules through the pathway. In other words, for air to move from the outdoors to the indoors (or indoors to outdoors), there must be an opening in the house (the pathway or hole), and the air pressure indoors must be different than the atmospheric pressure outdoors.

 

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Air can't move into or out of a house with only openings—or with only an air-pressure difference—both are necessary. Therefore, to cut down on air movement, you have a choice: You can either close up the holes or reduce the air-pressure difference. To increase air movement, you either add more holes or increase the air-pressure difference.

 

Furthermore, at least two holes in a house are necessary for air to move through it—an entry hole and an exit hole. To understand why, try to blow air into a bottle, or suck air out of it. You can’t do it. The best you can do is to pressurize or depressurize the air inside the bottle—but you can’t blow air through it. With a house, you can’t have a certain amount of air entering unless a proportionate volume of air is also leaving. If one hundred cubic feet of air enters a house, generally one hundred cubic feet of air must leave the house somewhere else. Scientists sometimes call this conservation of mass—the mass of the air going in must equal the mass of the air coming out. While you can’t blow air through a bottle, it is easy to blow air through a piece of tubing because it has two holes—one at each end.

 

While most houses have quite a few holes in them, they may not be distributed evenly around the house so there may not be much air movement through the holes. For example, if all of the holes are on one side of a house, and wind blows on that wall, it will try to push air into all the holes at once. With no holes anywhere else in the house, there will be no exit holes, so it will be like trying to blow air through a bottle. The result will be very little air movement through the house. A house with holes on only one side probably isn't very likely, but mobile homes usually have many more holes through the floor than they do anywhere else, and this has an effect on how much air enters and leaves.

 

A tight house is one that either doesn’t have very many random holes, or the random holes it does have are very small. In either case, it takes quite a bit of pressure to blow very much air through a tight house. This is why tight houses often have poor air quality—there aren't enough holes for the available pressures (such as the wind) to blow air through. As a result, tight houses are often underventilated. This has led many people to recommend building looser houses. Actually, there are some excellent reasons to build a house as tightly as possible—tight houses are less drafty, more comfortable, and more energy-efficiency than loosely built houses. The real answer is not loose houses—it is tight, energy-efficient, comfortable houses with mechanical ventilation systems.

Holes in Houses

One of the largest “holes” in a house is an open window, but an open window can be an invitation to a burglar, so people rarely leave their windows open for extended periods. Even when all the windows are closed, there are still holes in houses—they just aren’t as noticeable. There are hidden gaps between the window or door frames and the 2x4s holding up the wall. There are even narrower gaps between the floor and the walls, and even smaller gaps around electrical outlets. There are also many hidden holes inside the structure that were cut through studs, floor joists, and rafters by plumbers, electricians, or heating/cooling contractors.

 

Sometimes, when you start looking around in an attic or basement, it is possible to locate holes in the structure that are quite large—large enough to poke your leg into. It is through these big holes that most of the air enters and leaves a house. Weatherization contractors have learned in recent years that when you caulk around windows and doors, you really aren’t tightening up a house very much. To do an effective job of weatherizing a house, you usually need to get into the attic and basement and plug up the big holes.

 

If it were possible to combine all of the small holes and gaps in a very tight house into one single hole, you would end up with an opening several square inches in size. In a very loose house you might end up with an opening several square feet in size. So, just because you can’t see any holes doesn’t mean there aren’t any. Most houses have quite a few.

 

The way house construction has evolved over the years has had an effect on house tightness. For example, earlier in this century, when houses were constructed with solid-wood 1x8 boards, there were gaps between each of the boards. When builders switched to using sheet goods such as plywood, there were still gaps between the sheets. However, because of the large size of the sheets, the number and size of the gaps were less and, thus, houses became tighter. The holes we are talking about are called random holes, because they are randomly found throughout a house and they weren't created for the purpose of supplying the occupants with fresh air. If a hole is created on purpose, specifically to provide a pathway for air to travel through, obviously that's a deliberate hole. The installation of a controlled ventilation system requires one or more deliberate holes.

 

Today, some energy-efficient builders are purposefully using special techniques to build houses that are almost hermetically sealed. Their goal is to create an airtight house with no random holes. When this is done, no amount of naturally occurring pressure can possibly provide enough air to supply the needs of the occupants—unless you install a ventilation system.

Pressures in Houses

Houses can be pressurized or depressurized in a variety of ways. For example, “naturally” occurring pressures—the result of Mother Nature—are quite common. They result in natural ventilation, something that varies considerably from day to day. This is primarily the result of pressures caused by the wind and by temperature differences (warm air exerts a small upward pressure as it rises up into cooler air).

 

Pressures in houses can also be caused by mechanical equipment. Some mechanical devices aren’t specifically designed to cause air to move into and out of a house for the purpose of ventilating the house, but they do so anyway. For instance, the fan in a clothes dryer blows air out of a house, so it has an effect on the air pressure in a house. A clothes dryer’s main purpose isn’t to ventilate—it is to dry clothes—so it contributes to accidental ventilation. Accidental pressures only cause air movement when the mechanical device is operating. Because these pressures are rarely continuous, they are sporadic in causing air movement through a house. Thus a number of different factors cause accidental ventilation.

 

Houses also contain mechanical devices, such as window fans, that are deliberately designed to exchange air in a building for the purpose of supplying fresh air or expelling stale air. This is what ventilation is really all about. This is called controlled ventilation—ventilation that is created “on purpose.”

 

So, the air pressures that push air through the holes (either random or deliberate holes) in houses fall into three categories: natural, accidental, and controlled. The direction the air moves (outdoors to indoors, or indoors to outdoors) depends on which way the pressure pushes. Air will always move from an area of high pressure, through a hole, to an area of low pressure.

 

If air movement is directly caused by a fan, it is said to be active. If air movement isn't directly caused by a fan, it is said to be passive. For example, a window fan actively blows a certain amount of air out of a window, but in doing so an equal volume of air will enter the house passively somewhere else.

 

If something causes the air pressure inside a house to be higher than the atmospheric pressure outdoors, the house is said to be pressurized, or experiencing a positive pressure, and indoor air will passively leak toward the outdoors through any random holes. If something causes the air pressure inside a house to be lower than the atmospheric pressure outdoors, the house is said to be depressurized, or experiencing a negative pressure, and outdoor air will passively leak toward the indoors through any random holes. It isn't unusual for part of a house to be pressurized and another part of the same house to be depressurized.

The Relationship Between Pressure and Holes

There is a close interrelationship between 1) the pressures in a house, 2) the number and size of the holes (house tightness), and 3) the volume of air moving through the holes. Often, if you change any one of these three items, you affect the other two. This is an important concept because pressurization/depressurization, house tightness, and air movement are very basic to an understanding of how ventilation systems work—and what problems can be caused by them.

Changing the Volume of Air Passing Through a House

If you have a house with a certain number of random holes, and you depressurize the house by blowing air out of it (e.g. with a window fan), then a certain amount of air will be sucked into the house through the random holes in the structure. If you increase the volume of air leaving (e.g. by turning an out-facing window fan up to high speed), more air will be sucked in. But the house will also have slightly more depressurization because it will be more difficult for the increased amount of air to enter through the limited number of random holes in the structure. If you reduce the volume of moving air (e.g. turn the fan down to low speed), then less air will move through the holes and there will be slightly less depressurization.

 

This same relationship holds true if you turn the fan around and blow air into the house and pressurize it. If you increase the volume of air, you will also increase the pressurization slightly. If you decrease the volume of air going in, you will decrease the amount of pressurization slightly. In this example, the number of holes remains the same, so the tightness of the house itself is unchanged.

 

The tighter the house, the easier it is to change the pressure in it. For example, in a tight house (one with few random holes) you may only need to speed up the window fan a little bit to measure a big change in pressure, but in a loosely built house (one with many random holes), you may need to speed up the fan considerably to get the same change in pressure.

 

[This article was adapted by permission from John Bower's book, Understanding Ventilation, published in 1995 by The Healthy House Institute.]

 

(Note: The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)

 

 

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The Physics of Good Ventilation:  Created on September 10th, 2008.  Last Modified on February 28th, 2011

 

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