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How Closing Vents in Unused Rooms Affects Indoor Air Quality

There seem to be two prevailing topics of everyday life that are increasing in popularity: energy efficiency and health. People are making changes towards sustainable living and mitigating their home’s carbon footprint. I, myself, am obsessed with water conservation, and I recycle or compost almost everything. Health, too, is becoming a major focus; cutting out additives from food, living an active lifestyle, and using air purifiers and diffusers to improve indoor air quality.

When it comes to trying to save energy, many Americans believe that closing vents in rooms that are not used is a logical and easy solution since the room does not require conditioned air. However, this is untrue, and, furthermore, the practice can negatively affect the air quality of this room as well as your entire home, compromising the other goal of increased healthy living.

Closed vents in unused rooms create temperature extremes. These damage the room and items therein, can pull conditioned air into the room, or cause unconditioned air to be sucked out. Mold and mildew can grow. Dust and bug carcasses and feces collect and become airborne. The HVAC system functions sub-optimally.

Defining Indoor Air Quality

When the average person thinks of indoor air quality (IAQ), they may focus on smoke, cooking fumes, outdoor-originating pollutants, and odorants. These absolutely affect IAQ, but scientifically, the definition extends to include things as minor as dust as well as heat, humidity, and airflow.

Temtop M10 Air Quality Monitor PM2.5 Formaldehyde(HCHO) TVOC AQI Real-Time Indoor Air Pollution Detector

Temperature, humidity, and airflow impact air quality because IAQ is not only measured by how healthy the air is but also how comfortable the air is. Hot, humid, and or still air can make living uncomfortable, as can cold and dry air. 

Taking into consideration pollutants, thermal conditions, and airflow, the factors that influence indoor air quality include (but are certainly not limited to) the residents of the home, the HVAC system, pollutant pathways, and sources of contamination that are possible. 

Effects on the Room IAQ

Temperatures Can Get High

When you close off the vents to a room and stop providing it with conditioned air, then the temperatures can rise in the room.

If this room contains hardwood flooring and closing the vents results in high enough temperatures, it is likely that the flooring will warp and/or buckle. 

Hot air can hold more moisture, allowing the humidity levels of the room to rise, which further causes the floorboards to swell. With this swelling, the flooring becomes uneven and eventually, it could disconnect with the subfloor entirely. Not only will the flooring be uneven and therefore difficult to walk on, but flooring disconnected from the subfloor often squeaks

Businessman sweating in his home office

Flooring is not the only thing in a closed-off room that could be damaged. Items made of glass, stainless steel, and fabric can often withstand high temperatures but any item stored in this room made of any other material, such as paper, leather, or wood, could be damaged.

Often-stored items like photo albums, books, and older furniture (especially if it’s antique or leather) are likely to warp, crack, or discolor in this overheated room. 

Temperatures Can Get Very Low

In the winter, an unused room with its vents closed can experience very low temperatures—at least compared to the rest of your home. 

As said above, heat can cause floorboards to expand, but the opposite is true for the cold. When floorboards shrink in the cold, the floor can heave and cause squeaking problems due to subfloor detachment. 

Hand holding a thermometer with low temperature

On top of this, if the room is put through a rapid and large temperature change, the walls and ceilings of this room may crack in extreme cases.

When temperatures are low enough, water can undergo a phase transition from gas to liquid. The gaseous water in the air, when subjected to the cold temperatures of this room, may liquefy and cause damage to the items you store, furniture, or the flooring. 

For example, when the water condenses and accumulates in areas with stored paper items, the items can warp and disintegrate. 

Mold and Mildew Thrive in Unconditioned Spaces

A room with its vents closed experiences a lack of conditioned air. Unconditioned air means that the temperature and the humidity are not being regulated. It is therefore likely that unconditioned spaces will have high humidity levels, which are ideal for mold and mildew to grow. 

Not only do mold and mildew produce unattractive stains on surfaces, but they can cause allergic reactions since they produce allergens.

Close-up Of A Shocked Woman Looking At Mold On Wall

Allergic reactions to mold and mildew usually manifest as fever-like symptoms including a runny nose, irritated eyes, and skin rashes. 

Irritation of the eyes, nose, throat, and lungs can occur in both those who are allergic and also those who aren’t. Furthermore, mold and mildew can trigger asthma attacks for those with asthma.

Dust Collects

The cycling of air into and out of a room can carry away dust that has settled in your home. Without moving air, though, the dust that settles and collects is not disturbed and, therefore, just keeps accumulating. 

Similar to mold and mildew, when dust settles on surfaces, it is not only unattractive and damaging to the surfaces on which it sits, but it can also be harmful to residents’ health.

Surprised housewife find dust under furniture

Dust can come from a variety of sources, from outdoor origins to indoor through dust mites and more. 

Like mold and mildew, dust can lead to allergic reactions as well as asthma attacks and other respiratory problems due to irritation of the airways. 

Bugs Move in and Release Particulate Matter

When a room is undisturbed by both air flow and by people, it is likely for bug infestations to occur, especially since an unconditioned space is more likely to be humid, which bugs favor.

Not only is a bug infestation a rather gross occurrence, but bug infestations can also affect your air quality. 

As bugs procreate, defecate, and eventually die, they leave behind carcasses and feces. Not only do these things not smell great in large amounts and are rather gross, but their small sizes and weight allow them to become airborne. 

The thought of breathing in bug corpses and feces is disgusting and can also have adverse effects on your respiratory health. These rather large particles can irritate your airways similar to allergens. 

Effects on the Whole House IAQ

HVAC System Functions Sub-Optimally

Not only does closing vents in an unused room affect the air quality of the room itself, but the whole house’s air quality is affected. One way this occurs is the changes that your HVAC system undergoes when supply or return vents are closed

A closed supply vent increases the air volume throughout your ducts. Increased air volume in ducts that have their own fixed volume means that the pressure inside the ducts increases. The HVAC motor must combat this static pressure with velocity pressure, which means the motor must work harder. 

Modern interior HVAC ventilation
Whole house air ventilation and cleaning system. Ventilation pipes in silver insulation material on the attic

Closing a return vent is also harmful to your HVAC system since too low of pressure reduces the movement of the air and therefore your motor must work harder to move the same amount of air. 

No matter the vent that is closed, your HVAC will have to work harder to supply air to the rest of your home. 

This decreased ability of the HVAC system to transport air means the rest of your home will not receive adequate airflow and your desired temperatures may not be met.

Conditioned Air Lost to Unused Room

Although the point of closing off an unused room to your HVAC system is saving energy, closing this room off with the vent doesn’t mean the room doesn’t receive any conditioned air. 

In the winter, a room with its vents closed will be colder than the rest of the heated rooms in your home. Unfortunately, this creates a heat sink in your home. This is because hot air preferentially moves to cold areas.

Since this unused room is a cold area, heat from the rest of the home will move into it through cracks and other openings, essentially being wasted since the room is not used. 

In addition, as mentioned above, closing a supply vent means a build-up of pressure in the ducts of your HVAC system. This pressure leads to duct leaks, which means conditioned air being lost in unconditioned spaces like the attic, crawl space, and ceiling as well as this unused room. 

Stale and Humid Air Pulled out of Unused Room

The stale and humid air in this unused room is not truly confined here and can migrate to parts of the home that are occupied, making for bad indoor air quality throughout the entire home.

This is especially true in the summer. It is likely that the unused room is much warmer than the rest of the home since it is not being cooled. As said before, heat moves preferentially from hot areas to cold areas. 

Young Indian woman in saree opening window to get fresh air

Since this room is not perfectly sealed, the warm air can move through seams of the door as well as cracks and uninsulated areas of the walls. This warms up your home and increases the burden of your HVAC unit to keep your house cool on warm days.

Unconditioned air infiltrating the rest of the home does not only occur during the summer. Cold air is denser than warm air so it sinks. This can cause cold air, in any season, to sink and travel through the gap under the room’s door, and sink to the floor below it. This would be the most troublesome in the winter.

Airborne Particulates Can Move Through House

So, stale, humid air can infiltrate the rest of the home from the unused room, but this unconditioned air can also contain harmful particulate matter, which can negatively impact the air quality of the rest of your home. 

Mold, mildew, dust, bug carcasses, and bug feces can be carried along with moving air as well as their allergens. 

Not only are these particles unpleasant in smell and appearance, but they can also trigger allergic reactions, skin and airway irritation, as well as asthma attacks. 

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