Understanding the Stack Effect

The “Stack Effect”, or “Chimney Effect” as you may have heard it called, sounds confusing; but it’s really all about how the air flows throughout your home.

What is the Stack Effect?

how the stack effect works graphic
As air comes into the ground level entrance and rises up through the home, it leaves via exit points, such as gaps around windows, exterior doors, attics, basements, and crawl spaces. This continuous cycle is called the Stack Effect.

The Stack Effect occurs when air infiltrates the lower part of a structure & exfiltrates through the top. This happens only when there is an entry & exit point for outside air to flow through your home (both must be present for the Stack Effect to occur). When warm air moves in through entry points, up through the structure, and out of exit points, it creates a vacuum for cold air.

Due to warm air being lighter, heavier cold air settles at the bottom of the structure of your home. As warm air enters the lower part of a home from outside, it rises and reduces the pressure in the base of the building while forcing any cold air above it out open doors, windows, or any other openings.

The Stack Effect occurs commonly in winter, but can happen in the summer as well. During summer or in warmer climates, the Stack Effect is reversed. The hot air outside enters the upper portion of the cooler building and creates a draft down.

Here’s what Santa Fe™, a manufacturer of indoor air quality products such as crawl space dehumidifiers, has to say about the Stack Effect and how seasonality effects home air ventilation.

“Depending on how tight your home is, during cold winter weather usually enough fresh air exfiltrates/infiltrates the home that fresh air ventilation is not necessary. But during the summer, the wind is the only natural pressure on the home. Therefore, homes require more mechanical fresh air ventilation during the summer when the windows are closed. Of course, if you have a fairly tightly constructed home you may need mechanical fresh air year-round.”

Check out their Q & A explaining the stack effect here.

Looking for more in-depth information about natural home ventilation? Check out this resource.

The Stack Effect & Air Quality

Due to the Stack Effect, as much as 50% of the air you breathe inside your home can directly come from the crawl space. That is a significant percentage of the air you breathe in your home, just from one area.

Crawl spaces create perfect environments for undesirable conditions such as moisture, fungi, and pests. Often, this results in strange smells inside the home, high levels of indoor humidity, and insect invasions. It is important to learn how these conditions end up rising to the livable parts of your home if left untreated. Some signs to look for include: Decaying rotting beams & joists, rotten wood, mold spots (green, blue, or white), and water stains on baseboards.

Blog Article: How to Recognize and Resolve Mold In A Crawl Space

Why Does the Stack Effect Occur?

The Stack Effect occurs because there is a pressure difference between the outside air and the air inside your home caused by a difference in the air temperature. That pressure difference is the driving force for the effect to occur.

As hot air rises and escapes through the upper levels of your home, it creates a pulling effect that replaces the air in your home with unpleasant crawl space air, exposing you and your family to poor air quality in the living areas of your home.

How the Stack Effect affects Your Home

Mold & Condensation

As the cold air sweeps into the crawl space and up through your home, it brings with it tiny mold spores that are small enough to float in the air. That means they’ll spread around your home and even attach to your damp walls, where they will grow quickly. While these thrive in humid conditions, you’re likely to see them grow if the crawl space remains open.

mold on crawl space support beams
Mold on crawl space support beams

High Energy Costs

An open crawl space will drive up your energy costs by 15-25%. Unregulated airflow means you’re going to experience swinging temperatures and moisture buildup, which will force you to use your HVAC or dehumidifier. Come winter, you will find that your home is colder than usual. In the summer, it will be too hot. Both will again require excessive air conditioning.

Preventing the Stack Effect - Crawl Space Encapsulation

The most effective solution to avoid the Stack Effect is to air seal and insulate key areas, including the basement and crawl space. If you invest in getting your home air sealed and insulated, you can potentially see 30-50% savings on your utility bills.

According to the US Environmental Protection Agency, crawl space air pollution is to blame for 40% of indoor air quality issues. By isolating the space and ensuring enough airflow, encapsulation aids in preventing moisture build-up and enhancing air quality. Avoiding expensive repairs and lowering energy costs can save you money over time.

Crawl space encapsulation is a technique that involves sealing off your crawl space to prevent moisture and other harmful elements from entering your home. This method helps protect against structural damage and health hazards, and improves indoor air quality. Encapsulation is an effective way to ensure a dry and healthy living space.

Contact Crawlspace Medic today to schedule your free crawl space inspection.


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