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What Is the Stack Effect?: Research Shows Its Impact on Energy and Mold

It’s pretty common for homes in Alabama, Tennessee, and Kentucky to have crawl spaces; in fact, 15% of homes across the U.S. have them. They have numerous benefits, such as easily accessible plumbing and wiring as well as additional storage space.

However, crawl spaces have their weaknesses too of which the “stack effect” is one of the biggest.

This phenomenon refers to the naturally occurring phenomenon of air climbing in temperature as it rises higher through your home. The stack effect impacts both temperature and moisture levels which, in the worst case, can ruin your indoor air quality from mold growth and raise your energy bills.

In this article, we’ll tell you all about what the research says about the stack effect, it’s impact on your home, and what you can do about it.

What is the Stack Effect?

crawl space stack effect

The stack effect is a natural occurrence due to the temperature difference inside and outside a building. During cold months, warm indoor air rises and escapes through higher openings, creating a vacuum that draws in cold outdoor air. This continuous airflow cycle intensifies with building height and is particularly dominant when indoor heating is frequently used, as the indoor-outdoor temperature difference is larger.

The cycle causes temperature loss, particularly when there are cracks in your crawl space and attic. Consequently, you can end up using more energy for heating or air conditioning.

What the Studies Show

In this article, we’re really looking at what academic research show about the Stack Effect in order to give you the best possible insights into what you can do about it. Here’s a topical overview of what the most recent studies from across the world are showing about the stack effect. At end of each section, we give you key consumer insights that can help inform your decision-making for a possible solution.

I. Associated Problems of the Stack Effect

Studies have shown the problems caused by the stack effect are:

  • Poor Indoor Air Quality: Increased infiltration brings more outdoor pollutants, dust, and allergens, compromising indoor air quality.
  • Sticky Doors and Windows: Air pressure imbalances due to the stack effect can make doors and windows tough to operate.
  • Drafts and Noise: Air seeping through gaps and cracks can cause noticeable drafts and wind noise.
  • Higher Humidity and Moisture Problems: The upward movement of warm air can cause indoor humidity, condensation, and moisture damage, increasing the risk of mold growth and structural damage.
  • Increased Heating Load: The influx of colder outdoor air due to increased infiltration requires more energy to maintain the desired indoor temperature, leading to higher heating loads and energy consumption.
crawl space moisture reading

Key Insights:

  • The Stack Effect can cause health issues:: It can introduce allergens, dust mites, and microorganisms into your home, leading to a range of health issues from minor irritations like a runny nose to serious conditions like respiratory infections.
  • The Stack Effect circulates mold through your home: The cold air circulating from your crawl space carries tiny mold spores that can attach to damp walls and proliferate, especially in high-humidity conditions.
  • The Stack Effect raises utility costs: A compromised crawl space can boost your energy costs by up to 25% as your heating or AC system strains to manage temperature fluctuations. Additionally, the moisture buildup may necessitate a dehumidifier.
  • If not addressed fast, the Stack Effect can require costly repairs: If left alone, undiscovered, or ignored, then these issues can lead to expensive damage—moisture-laden air can cause wood rot, impacting your home’s structural integrity and resulting in costly repairs.

II. Causes & Contributing Factors

A study by Chinese academics shows that air leakage through building walls, particularly through air cavities like those around air conditioner pipes, can significantly contribute to mold growth by creating conditions conducive to condensation.

  • Air Cavities: Unsealed or poorly sealed openings in your home can lead to air leakage, allowing warm, humid outdoor air to enter the wall cavity. This is especially problematic if you have vents in your crawl space. The warm air can circulate through your entire home and exit through the attic, leading to moisture issues.
  • Temperature Differences: During cooler seasons, warm outdoor air can condense upon contact with colder indoor air, creating a damp environment within the wall cavity.
  • Exterior Moisture and Humidity: Outdoor moisture sources like humidity, rain, soil moisture, and plumbing leaks can exacerbate the stack effect.
  • Airborne Particles: Studies suggest that around 50% of your crawl space air may circulate into your living area, carrying moisture and harmful airborne particles such as allergens, dust mites, and mold spores.
  • Insufficient Insulation: Poor or inadequate insulation can’t mitigate the stack effect, contributing to temperature imbalances.
  • Roof Leaks: Damaged or missing roofing materials, ice dams, or blocked gutters can cause roof leaks, contributing to the stack effect and potential moisture problems.

Key Insights:

  • Perform Regular Home Maintenance: clean your gutters, check your roof, and make sure ground water is draining away from your home. If yard drains are clogged, make sure to clean them out.
  • Keep a consistent indoor temperature: Warm air holds more moisture than cold air. Condensation occurs when warm humid air comes into contact with a cold surface.
  • Install moisture and humidity control solutions: Localized sources of humidity, such as clothes dryers, should be directly vented to the outdoors. Air conditioners and/or dehumidifiers can be used to lower indoor humidity.

III. The Impact of Building Height

Research on high-rise buildings show the impact of the stack effect at a larger scale than in residential homes. Still, the insights we can take from these studies show us some of its finer details that require mitigation:

  • Temperature-Driven Pressure Differences: That the stack effect notably increases air infiltration rates, especially during heating seasons. The effect, driven by indoor-outdoor temperature differences, creates a vertical pressure difference. This pressure prompts warm indoor air to rise and escape through higher openings, leading to more infiltration on lower and potentially upper floors.
  • Increased Infiltration on Lower Floors: The pressure difference created by the stack effect pulls more air into the building on lower floors, leading to higher infiltration rates compared to middle floors.
  • Varied Infiltration Rates Based on Height: Infiltration rates are not uniform throughout the building. They vary depending on the floor and its relative position within the vertical pressure profile generated by the stack effect.

Key Insights:

  • Multi-storied homes are at a higher risk of the negative impact of the stack effect.
  • The Stack Effect might impact some parts of your home more than others.
  • Keep an eye on your HVAC system: While increasing airtightness can help mitigate the stack effect’s influence, it’s essential to consider the building’s overall ventilation strategy to address potential indoor air quality issues that may arise from reducing infiltration.

IV. Seasonal Changes and Indoor-Outdoor Temperature Difference

The greater the indoor-outdoor temperature difference, the more pronounced the stack effect.

A study on the stack effect in Swedish school buildings showed a correlation between outdoor temperature and pressure difference across the first floor of buildings. The temperature difference between environments plays a significant role in energy consumption and contaminant transport (ie. mold growth and radon gas infiltration) within a building, especially one with a crawl space.

The study found that the stack effect was particularly noticeable during the colder months when the the temperature disparity between your home’s interior (especially in your crawl space) and exterior, and your crawl space is at its highest. However, it also found that mild days with little wind are more critical than colder days in terms of the level of contaminants in circulation.

A common practice is to maintain an indoor temperature of approximately 70ºF by preheating the supply airflow to approximately 50°F.

Key Insights:

  • Maintain a stable temperature in your crawl space: ground thermal properties, and building thermal resistance determine the crawl space temperature.

V. Energy Consumption & Mold Growth

We’ve seen above how some studies explore how the stack effect significantly impacts a building’s energy consumption and indoor air quality.

It causes increased air infiltration, forcing heating systems to work harder to maintain indoor temperature, leading to energy wastage, especially in buildings lacking airtightness.

The stack effect can also contribute to mold growth within buildings. Warm air rising due to the stack effect carries moisture. When this humid air comes into contact with cooler surfaces like exterior walls or windows, it condenses. Continuous condensation creates damp conditions conducive to mold growth, especially in spaces with limited ventilation, such as behind furniture or within wall cavities.

crawl space mold on insulation

Mold growth is further exacerbated by:

  • High outdoor humidity levels: Periods of high outdoor humidity, such as during rainy seasons, increase the amount of moisture entering the wall cavity, making condensation and subsequent mold growth more likely.
  • Air conditioner operation: When the air conditioner is running, it cools the pipes and surrounding surfaces within the wall cavity. This can further lower the temperature of these surfaces, increasing the potential for condensation when warm outdoor air enters the cavity.

Key Insights: Mitigating Mold Growth and Lowering Energy Bills

To effectively prevent and address mold growth related to the stack effect, several strategies can be employed:

  • Addressing the stack effect can lower your energy bills:
  • Condensation supports mold growth: The condensed water, combined with dust and other organic materials that might be present in the wall cavity, provides a favorable environment for mold spores to settle and grow.
  • Mold poses significant risks to human health: Mold spores present in the air can trigger allergic reactions, asthma, and other respiratory problems in susceptible individuals. Some types of mold can produce mycotoxins, which have been linked to a range of adverse health effects.
  • Mold growth and high energy bills are one and the same issue:
  • Moisture control: It is crucial to identify and address all sources of moisture intrusion into the building. This includes fixing leaks in the building envelope and addressing any plumbing issues.
  • Ventilation: Proper ventilation is essential to remove excess moisture from areas prone to condensation, such as kitchens and bathrooms. Exhaust fans vented to the outdoors can be helpful in these areas.
  • Insulation: Adequate insulation can help minimize condensation by preventing interior surfaces from becoming too cold.
encapsulated crawl space with dehumidifier, sump pump, and foam board insulation

VII. The Importance of HVAC Systems

Operating an air conditioner usually reduces humidity. As warm, moist air cools while passing over cold pipes and surfaces, moisture condenses, lowering the room’s humidity. This effect is most noticeable when the outdoor temperature and humidity are high, or when the air conditioner is used in the afternoon.

For instance, the study in China found that running an air conditioner lowered the surface temperature in a cavity to 41.9°F below the outdoor air’s dew-point. The resulting condensation would have reduced the relative humidity.

The previously cited Swedish study, however, showed that the stack effect caused an imbalance in the ventilation system, with greater exhaust airflow than supply airflow, often resulting in a slightly lower pressure in the classroom compared to the crawl space. The imbalance in the ventilation system, intended to prevent air leakage from the classroom to the outside and potential moisture problems, contributes to the lower pressure in the classroom.

However, turning off the air conditioner entirely can have even larger adverse effects. If windows are left open, outdoor air can quickly increase the room’s relative humidity, particularly during early evenings when outdoor humidity is often high.

Key Insights: Imbalanced Ventilation Systems and Pressure Differences

technician repairing floor vents in home

To minimize mold growth risk, consider the following:

  • Regulate moisture sources by carefully timing ventilation.
  • Use a dehumidifier to reduce indoor moisture, particularly when the air conditioner is off.

Conclusion: How Crawl Space Encapsulation Mitigates the Stack Effect

In case you’re looking to improve your situation, there’s no better way to do so than by contacting a professional crew to encapsulate your crawl space. Our Tennessee or Alabama team at AFS will provide you with a free inspection and offer solutions to keep your home safe, stable, and comfortable for years to come.

It’s pretty common for homes in Alabama and Tennessee to have crawl spaces. They have numerous benefits, such as easily accessible plumbing and wiring as well as additional storage space. However, crawl spaces have their cons too. Namely, the stack effect is the biggest issue homes with crawl spaces have. 

This phenomenon refers to the movement of air through your home. The stack effect impacts both temperature and moisture levels, leaving you with poor air quality and low energy efficiency. The latter means that your utility costs are going to skyrocket, and you’ll need to pay them with your hard-earned money. 

While home designers and builders understand the stack effect, homeowners don’t think much about it until they learn that it’s causing their high energy bills. Let’s delve into what this physical process is and how it affects your home by discussing the problems it causes as well as point out smart solutions you can employ with professional help. 

Stack Effect FAQs

Insulation typically lasts 15 to 20 years, but it can succumb to damage from mold and moisture if the crawl space is not properly encapsulated. AFS Foundation & Waterproofing Specialists recommend ExTremeBloc™ foam board insulation as a durable, lasting solution. Contact us to learn more and schedule a free inspection.

Without a vapor barrier, buildings are susceptible to moisture damage, which can compromise structural integrity, reduce insulation effectiveness, and promote mold and mildew growth.

Yes, by reducing air leaks and keeping insulation dry, vapor barriers improve the thermal performance of a building, leading to better energy efficiency.

Ted Dryce

Ted Dryce

Content Writer

Ted is an SEO Content Writer who has been with Groundworks since 2021. He’s covered home repair topics ranging from crawl space encapsulation to regional soil conditions. When he’s not working, Ted is performing improv comedy and working on his own creative projects.

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