Air fills all spaces, and the inside your house is no exception. We need to exchange this air for fresh air regularly to keep the environment healthy for the occupants of our homes.
However, modern home-building guidelines favor houses that keep cold air inside in summer and warm air inside in winter, which can affect the air quality.
How long does it take to exchange the air in a house? To exchange the air in a house, the average is 1 to 2 air changes per hour or between 60 and 30 minutes. This is known as the air exchange rate and that amount of time needed can vary. With open doors and windows, the rate is about 4 changes per hour, meaning that it takes only 15 minutes.
The air exchange rate will vary depending on the room, how many people are in it, and how you use the space.
It’s essential to understand how long it takes to replace the air in every room to prevent the quality of the air you breathe inside your own home from being compromised.
In this article, we will discuss factors that affect the air exchange rate of your home as well as ways that you can improve the quality of that air.
History of Air Distribution
Architects have recognized the need for proper air distribution within buildings for many centuries. The purpose is essentially to manage the quality of air inside by replacing it with fresh air from outdoors.
We call this form of air circulation ventilation, a word that derives from the Latin ventilatio, which has its root in the word ventus, meaning wind.
Humans learned early that their homes needed ventilation when open fires were used inside to heat living spaces. Various civilizations adopted the use of chimneys and vents to allow smoke to exit their buildings.
In the 17th century, King Charles I of England recognized that poor ventilation was causing health problems, and he decreed that the ceilings in homes must be at least 10 feet high and that windows must be higher than they are wide to allow for ventilation.
In modern times, ventilation systems within buildings have evolved significantly. The shift in design is especially noticeable in the 1970s when research and technology focused on good indoor air quality and comfort.
Later on, professional organizations and government agencies established environmental air quality levels and guidelines for various environments.
Air Exchange Rates
The air exchange rate is the speed at which outdoor air replaces indoor air within a room. Organizations use the air exchange rate to determine air quality, particularly in workspaces, where there are legal requirements that employers have to meet.
Pollutants in the air can contaminate air quality if there isn’t sufficient airflow, and that is why this calculation is so essential.
Organizations calculate the air exchange rates within a room as follows: Air exchange rate (air changes per hour or ACPH) = fresh air supply (ft³/h) / volume of the room (ft³)
Therefore, if a room was 10ft³ and was delivered 60ft³ fresh air supply every hour, then the ACPH air exchange rate would be 6. That would mean that the volume of air is wholly changed within that room six times per hour.
There is legislation that requires a certain degree of air exchange in public spaces as well as providing recommendations for other environments.
The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) provides guidelines for recommended ACPH for various spaces (source).
Smoking rooms: 15 – 20
Restaurant kitchens: 14 – 18
Conference rooms: 8 – 12
Churches: 8 – 12
Entrance halls: 6 – 8
Factories: 8 – 10
Hospital wards: 6 – 8
Public lavatories: 10 – 12
Offices: 6 – 8
Bedrooms: 5 – 6
Bathrooms: 6 – 7
Living rooms: 6 – 8
Kitchens: 7 – 8
Laundry: 8 – 9
Why Do We Need to Exchange Air in a House?
Homebuilders construct houses to minimize energy losses, and they generally weatherize homes. This approach means that most new homes are well sealed, and there is very little natural exchange of air happening within the house.
Having such tightly sealed homes can be a problem as allergens, irritants, and pollutants can affect the quality of air within the house.
A house with poor ventilation can lead to issues with retained moisture and can make indoor temperatures uncomfortable. These issues can affect the comfort and health of the occupants, and homeowners need to be aware of such risks.
In the past, occupants most often simply opened windows to ensure air exchange, but modern homes often require more sophisticated methods.
ASHRAE recommends that homes receive at least 0.35 air changes per hour. The US average is between one and two air exchanges per hour in existing homes.
This number is dropping as homebuilders construct more tightly-sealed homes. These modern homes require mechanical ventilation to ensure fresh outside air enters the house.
For more in-depth information on this topic, check out our article on how often you should air out your apartment.
How Air Moves through a House
All houses exchange indoor and outdoor air in some way. There are always some air leaks — even in newer, sealed homes, but especially in the case of older homes — that connect the indoor and outdoor spaces.
These usually occur around window frames, pipes, chimneys, or vents. Also, because the temperature and air pressure between inside and outside is generally different, air will naturally move from a high-pressure area to a low-pressure area. (source)
Ventilation of a house can occur naturally through open windows and doors or with mechanical assistance. If air movement is accidental — i.e., through cracks or other air leaks — it is called infiltration rather than ventilation.
Consider how an average house functions during winter. Warm air tends to rise, and therefore the air within the house will be warmer near the ceiling than in the basement.
This air is at a higher pressure than the cold air outside and will try to escape through the various air leaks or when a window or when someone opens a door.
Conversely, low-pressure, colder air will replace the air in the basement as it rises.
This type of air movement is called a “stack effect” and is the same principle that causes smoke to rise up a chimney and escape. In warmer weather, the stack effect is weaker or could even reverse in very hot weather.
Many US houses use a forced-air system to distribute air throughout a home.
These systems use air to transfer heat throughout the house by pulling colder air into the ductwork and pushing it to the furnace, where it is heated and then distributed through air vents to various rooms around the house.
The system communicates with a thermostat to keep the air temperature at the desired level.
Either fans or open windows, if there is air movement outside, will also work to distribute air around a house. Rooms without windows or vents need to have air mechanically circulated, or they will quickly become stale and unhealthy.
Indoor Air Pollutants
Indoor pollution sources are the primary cause of reduced air quality within American homes, according to the US Environmental Protection Agency (EPA) (source).
These pollutants release particles or gas into the air, and they are exacerbated by inadequate ventilation as there isn’t sufficient fresh air to dilute the impurities and to carry them out of the home.
Indoor air pollutants affect the health of a home’s inhabitants as well as the comfort of the house and can have both immediate and long-term effects.
Immediate effects include reactions such as irritation to the eyes, nose, or throat, as well as headaches and fatigue.
Most of these are short-term and treatable. Long-term effects after repeated exposure can include serious outcomes such as respiratory and heart diseases as well as cancer.
Pollutants include tobacco products, excess moisture, combustion products, radon, outdoor sources, and cleaning products, which we outline in more detail below.
The relative damage caused by each of these sources depends on how much pollutants they emit and how dangerous those emissions are.
It’s also important to consider whether they are releasing pollutants continuously or intermittently and how long their effects stay in the air.
Tobacco products produce a raft of harmful gases and particles.
Secondhand smoke contains more than 7000 substances and is classified by the EPA as a Group A carcinogen. Secondhand smoke can move between rooms in a house, and ventilation does not negate this.
Excess moisture is a common problem affecting air quality. Moisture can enter the house by rain or other leaks, water vapor in humid air, or through porous materials such as concrete or wood.
Excess moisture also originates from inside bathrooms, kitchens, and plants.
The most common issues with excess moisture originate when warm, moist air comes into contact with a cooler surface, causing it to condense in droplets.
This condensation often results in mold, mildew, and dust mites, which can trigger asthma and allergies and lead to decay of wooden products and rusting of metal items.
All fuel-burning combustion appliances, such as water heaters, furnaces, and ranges produce carbon monoxide, carbon dioxide, nitrous oxides, and water vapor.
When these appliances are not correctly vented or are overly worn, these pollutants can enter the house.
The most dangerous of these is carbon monoxide (CO), which is colorless and odorless, but is highly toxic and can be fatal.
The effects of carbon monoxide gas on individuals will differ according to the amount of their exposure. Other factors include the individual’s age and health
Radon is a radioactive gas that is generated underground and can enter a house from the ground. It is the leading cause of lung cancer in the US after smoking.
Fortunately, once you’ve had your home tested for it, you can manage radon levels in your home.
Usually, you can have a system installed that pulls the radon-rich air from under the foundation slab and vents the gas harmlessly into the atmosphere and away from the house.
High radon levels have appeared in all US States, and levels vary from home to home and within neighborhoods. It is, therefore, necessary to test for radon levels, and this is easy to do, as test kits are cheaply available from local hardware stores.
Volatile Organic Compounds (VOCs) include several evaporated substances that many varied sources emit, including building materials, furnishings, pesticides, and some cooking processes.
The best known and most dangerous of these is formaldehyde, which is in resins used in composite wood products, glues, paints, preservatives, and fertilizers. It also comes from cigarette smoke and unvented appliances.
High exposure to formaldehyde can compromise health, and you should seek treatment.
How to Improve Indoor Air Quality
It is essential to be aware of pollutants because, although one cannot do away with them entirely, one needs to know how to manage them. The easiest way to manage the source of air pollutants is to address the source.
There are various ways to control the sources of indoor air pollutants. Some of these include banning smoking inside your house, using only sealed combustion appliances, and ensuring an airtight connection between your house and garage.
Another way is to control moisture by servicing gutters, grading the ground around the house, and venting wet areas. You should also use safe household products that are low-VOC.
If it isn’t possible to control the source of the pollutant, then the best approach is to exhaust locally from its source area.
This applies significantly to exhaust fans or vents in bathrooms or kitchens that will exhaust moisture or other pollutants directly outside before they mix with air quality in other rooms.
You cannot remove some particles by the methods above, such as soot or pollen.
In this case, it is possible to install systems that circulate and filter the air throughout the house. These are either mechanical or electrostatic filters that you need to clean regularly to allow them to do their job correctly.
Although the methods described above are more sophisticated, there is still value in diluting the air inside the house by simply opening windows and allowing the free flow of air.
In conditions where there is little airflow, you can achieve the same effect by using fans.
Air Quality Problems
Houses will generally provide clues as to where there are issues that may affect air quality. It is essential to be aware of these and to address them as soon as possible so that you can avoid any adverse health effects.
Excess infiltration is often evidenced by feeling drafts even when the windows and doors are closed or seeing tracks of dust near poorly sealed windows and doors. Leaks in air conditioning ductwork can also result in excess infiltration.
Inadequate Air Exchange
Persistent smells are often a sign of inadequate air exchange as they suggest that your ventilation system is not expelling the air carrying the odor.
Too Much Air Exchange
Sometimes the systems we install may be exacerbating outdoor conditions such as making an interior too dry or humid.
Strategies for New Buildings
When building a new home, one has the luxury of planning the ventilation and will generally build with as few leaks as possible and then ventilate mechanically. This is achieved by installing one of the three main ventilation methods.
1. Exhaust Only
The exhaust-only method refers to exhaust fans, generally found in bathrooms and kitchens, but sometimes throughout the house.
The exhaust-only system works by depressurizing your home – it exhausts air from your home, and new air enters through leaks and vents.
Exhaust systems work better in colder, dryer climates as they seldom draw in the hot and humid air. When this is the case, there is a risk of excess moisture. This system is simple and relatively inexpensive to install but may draw in pollutants.
2. Supply Only
The supply-only system is a forced-air system that draws clean air into the interior of the house. This system uses a fan to pressurize your home, forcing outside air in and allowing inside air to leak out.
It works well to keep moisture out in humid environments but may not work so well in cooler climates as it could create very damaging condensation.
This system is also relatively cheap and straightforward to install and gives better control over the air entering the house, thereby eliminating more pollutants.
The balanced-system is a combination of the two systems, which aims to keep a constant and balanced pressure inside the house. It is a more expensive system as they use more power, but these systems work in any climate (source).
Strategies for Old Buildings
They tended not to build old buildings as tight, and whole-house solutions may not be the best approach. It is best first to address these symptoms of low air quality. It can also be useful to make use of the following:
- Use exhaust fans and open windows when possible.
- Open screened windows, funnel breezes into the house, or use fans.
- Make use of humidifiers or dehumidifiers as necessary.
It’s difficult to calculate how many times a day air is exchanged in an average house because it depends on so many variables. What we do know is that many factors could be reducing the quality of air that we breathe inside our homes.
It’s worth considering what sort of air we’re breathing and whether we need to test our air quality and make some changes to how regularly and cleanly it’s exchanged.