How To Reduce The Stack Effect And Lower Your Energy Bills

The Stack Effect: Is It Working For Or Against You? 

Hot air rises and cold air sinks. This is a lesson that many remember from middle school science class, but though it sticks in their memories, few realize how significantly this concept affects their day-to-day life. Though there’s a little more to this principle than what you learned as a seventh grader, differences in temperature produce a phenomenon called the ‘stack effect,’ or ‘chimney effect.’ While ventilation is important for a healthy home, stack effect ventilation can result in significant energy loss, thereby impacting your monthly utility bill. 

A Kansas City insulation company can ensure that your home is optimally ventilated and insulated. Getting a professional assessment of your home’s energy efficiency will allow you to identify areas of improvement to ensure that your furnace and AC unit use as little energy as possible, without compromising your overall comfort. Understanding stack effect ventilation is necessary for understanding problems you may be currently facing in your home. In this article, you’ll learn:

• How the stack effect works
• Problems that stack effect ventilation can cause
• Ways to reduce stack effect in your home
• Why ventilation is important for maintaining a healthy, comfortable home
• Strategies for optimizing your home’s ventilation and insulation

What Is The Stack Effect?

At its most basic, the stack effect is all about how temperature differences create air movement in and outside of a building. If you look up the equation for calculating stack effect, you’ll be presented with confusing symbols and fractions. Though it looks complicated, the concept is rather simple. 

There are a few terms that must be defined to understand stack effect ventilation: 

• Air density
• Thermal buoyancy
• Positive and negative air pressure

When we talk about density, we’re referring to the mass according to volume. Imagine that you have a couple of one-gallon jugs. You fill both up to the top with loose Styrofoam filler. Then, you pack down the Styrofoam in the second jug. Once it’s packed down, you have more space, so you fill it with more Styrofoam filler and pack this down, too. Once you can no longer pack it down and add filler, you’ll find yourself with two jugs filled up to the top. Both jugs still have an equal volume—one gallon—but the second jug has a higher mass of Styrofoam and therefore a higher density. 

Air changes in density according to elevation, humidity and temperature. Cold, dry drafts have a lower density than hot, humid drafts. Hot drafts are therefore more ‘buoyant’ than cold drafts. Returning to grade school science class, this is what causes hot air to rise and cold air to sink. The force that drives drafts to move is a difference in temperature. During cold weather—such as in the depths of a Kansas City winter—outdoor temperatures are much cooler than the temperature inside your home. Warm drafts will rise from the ground floor of your home, up through any cracks and crevices to the upper floors, and finally to your attic. Depending on how insulated your attic is, lots or very little hot air will escape through your roof. 

The flow of drafts caused by interior and exterior thermal differences result in pressure differences. The bottom levels of your home will experience negative (or low) pressure, while the top floors experience positive (or high) pressure. When high and low pressure meet, they attempt to stabilize, or reach equilibrium. Thus, any gaps on lower levels, such as doors, windows and ducts, will provide a channel for outside drafts to infiltrate your home's interior. Likewise, ventilation openings in your attic will create a pathway for high-pressure warm interior drafts to exit, and low-pressure cold exterior drafts to enter. 

The stack effect has a differential effect according to building type. In very tall buildings, because the height difference is very large, the natural flow of cold and warm air happens on a larger scale, causing a more pronounced effect. 

Problems Caused By The Stack Effect

Winters in Kansas City can get very cold. Though we don’t get the extreme cold temperatures that our neighbors up in Minnesota experience, the temperature can still fall below freezing. Heating your home in the winter with mechanical systems, such as a furnace, is therefore non-negotiable.

Unfortunately, if you have too much air movement and thermal exchange, your heating system will have to work harder to make up for the energy loss. This can drive up your energy costs considerably. If you feel like you’re always bumping up the temperature on your thermostat, or you’re bundling yourself up in blankets and several layers of socks to prevent spending more on your utility bills, the stack effect is working against you.

During the summer, a similar process called the ‘reverse stack effect’ can occur in homes with an air conditioner. With warmer air outside and cooler air inside, positive and negative pressure are flipped, but an exchange still occurs. Hot drafts rush inside and cool drafts are sucked outside, forcing your AC unit to work harder and therefore, use more electricity. 

In high-rise buildings, stack effect ventilation can be downright dangerous. Lethal fires have ripped through office buildings and even the London Underground as a result of this process. The higher the pressure difference, the more ferociously a fire will spread. The stack effect must therefore be controlled not only to improve comfort and reduce energy bills but to ensure fire safety as well. 

How To Stop The Stack Effect

The key to stopping the stack effect is to create a neutral pressure situation. By eliminating unwanted building heat loss, no major temperature differences and exchanges will take place. Preventing the stack effect from occurring is the main reason why you need air sealing and adequate insulation. 

There are two main things you’ll need to minimize to stop this phenomenon from occurring: attic air leaks and lower-level leaks. Minimizing these two types of leaks prevents hot air from escaping through your home’s roof and cool air from infiltrating your lower level through pressure equalization. 

Optimizing Attic Insulation

When was the last time you had your insulation installed or retrofitted? If you can’t remember, that’s a pretty good indication that it’s time to supplement it with blown-in insulation or batt insulation. Insulation is one of the last things homeowners think about, as most assume that it only needs to be installed once and remains effective for decades. In reality, this couldn’t be further from the case.

There are two main types of insulation used in Kansas City buildings: fiberglass and cellulose. Generally, blown-in insulation (loose insulation that is installed using a blower machine as opposed to using preformed strips or rolls) can last upwards of about 15 to 20 years. However, building codes change over time. Even if enough insulation was put in your home when it was built ten years ago, it may benefit from a top-up today. 

Insulation experts will talk about R-values. An R-value is a measurement used to describe how well insulation blocks heat from passing through it. The higher the R-value, the more efficient it is at blocking heat. Generally, every square inch of insulation provides an R-value of between 2.2 and 3.8. Its exact value depends on the specific insulation that is used.

In Kansas City, homes can benefit from an R-value of 38 to 60 in their attics. To achieve an R-value of 60, you would therefore need between 16 and 26 inches of insulation (as calculated with the values above). Over time, insulation can become compact, thereby reducing the number of square inches it covers. It may also become damaged by pests, moisture and other issues. To prevent as much heat from leaving your attic as possible and therefore prevent the stack effect from occurring, you need an optimal amount of insulation to provide that barrier. 

Air Sealing

With your attic optimally insulated, you’ll need to tackle another cause of stack effect ventilation: air leaks on your lower floors. The tricky part of solving this problem lies in the difficulty of identifying air leaks. With the help of a professional air sealing service, a ventilation expert will identify any problematic gaps and drafts. Some locations in which leaks may exist include:

• Exterior doors and window frames
• Chimneys and fireplace dampers
• Electrical and plumbing infrastructures, such as switch plates, electrical outlets, recessed lighting, holes for wiring and plumbing vents
• Dryer vents
• Crawl spaces
• Wall and window AC units

To identify these areas, professionals may use something called a ‘blower door test.’ This test essentially uses principles of the stack effect by fitting a powerful blower into the frame of your exterior door to suck out air. Your ventilation specialist will measure the changes in pressure to help identify the areas where these pressure changes are being facilitated.

Now that the gaps have been found, they can be sealed. Several types of products may be used to close these gaps, depending on the nature of the gap. Because windows and doors are frequently opened and closed, adding weatherstripping is usually the best solution. Caulking and spray foam may both be used for more stationary areas, such as joints between window sills and walls, or even in the joists below the floor and in between an exterior wall. Once these areas are filled, stack effect ventilation will be significantly reduced.

Why Eliminating Ventilation Is Not The Answer

At this point, you’re probably thinking that the stack effect is very bad and should be eliminated at all costs. We can see why you might think this, but having adequate ventilation rates in your home is very important. 

Without any ventilation, the air in your home would be stuffy and stagnant. Humidity would build up without anywhere to go, and odors would remain trapped. The intake of fresh air is, therefore, key to facilitating good air quality in your home, and therefore ensuring that the air you breathe is not detrimental to your health. The key is therefore to ensure that you have an adequate amount of ventilation, including stack effect ventilation, but not too much. Striking this balance can be a challenge. 

Making Ventilation Work For You

The trick to creating a home with optimal energy efficiency and high air quality comes down to careful design. The higher your energy efficiency, or lack of thermal exchange, the lower your operating costs will be. To get the perfect amount of ventilation in your home, you need to consider how your active and passive systems work together. There are three main types of passive, or natural ventilation:

• Wind-driven ventilation
• Night-cooling ventilation
• Passive stack ventilation

Similarly, there are three types of active, or mechanical ventilation: 

• Exhaust-only ventilation
• Supply-only ventilation
• Balanced ventilation

Mechanical ventilation uses fan-forced ventilation to push air through openings (exhaust) and into openings (supply) within your home. The advantage of stack effect ventilation is that it does this without the use of additional energy, making it an eco-friendly and energy-saving option. The problem is that achieving adequate stack ventilation without creating too much is challenging to achieve. It is therefore generally recommended that homeowners use a combination of mechanical and natural ventilation strategies. 

If you’re not keen on using electricity to power mechanical devices, you can use solar energy. There are both pros and cons of solar attic fans, but with the help of an insulation expert, you can determine whether this option may be beneficial to your home. 

Koala Insulation of South Kansas City Can Improve Your Home’s Insulation and Ventilation 

Concerned that your stack effect ventilation is out of hand and driving up your energy bills? At Koala Insulation of South Kansas City, we can inspect your home to determine whether changes need to be made to your insulation and ventilation. As a professional and experienced insulation company, our team has the knowledge, tools and expertise to find and seal leaks and assess and boost your attic insulation levels. Perhaps most importantly, we can determine exactly how much insulation and sealing is enough to prevent over-tightening. 

To find out more about how we can reduce stack effect ventilation in your home or arrange an on-site project estimate, feel free to reach out to us by phone at 816-929-8255, or book your free evaluation online.