Dodge The Draft
Understanding the stack effect towards a cozier home
Ever wondered how air moves within buildings and how it affects you? If you are freezing or stifling in some rooms, have a lot of dust or allergens, the stack effect may be causing many of these issues.
When we were trying to size new top floor windows, we removed a small portion of the wall below the window so we could see how far down the actual opening went. As soon as we made a hole, we felt a gust of air from below. I knew excessive airflow was problematic and indicated a stack effect. Even though the stack effect is a natural phenomenon, if left unchecked, it can result in excessive energy expenses, uncomfortable temperature swings, and cause poor air quality.
What is the stack effect
The stack effect is the tendency for air to move vertically through buildings (and chimney stacks) due to buoyancy. The air movement is caused by the temperature and pressure differences within the home and outside. There are two basic concepts you’ll want to understand:
1. Air always flows from areas of high pressure to areas of low pressure. This is due to the natural tendency of particles to move from areas of higher concentration to areas of lower concentration. In other words, air moves from where there's more air (high pressure) to where there's less air (low pressure) because it seeks to balance them out and reach equilibrium.
2. Similarly, heat will always go from the hotter object to the cooler one to seek equilibrium. In winter, there is a continuous flow of heat from inside to outside. In summer, it is the opposite. Heat will flow irrespective of pressure differentials between inside and outside.
With the stack effect, two things are at play:
Temperature: Because heat always moves from areas of higher temperature to lower temperatures… in winter, the warm air in your house tries to escape to the outside. Throughout winter, air constantly escapes through the walls and the roof, as well as through gaps and openings like vents and chimneys.
Air pressure: Every building has a neutral pressure level where the pressure difference between the building and its environment is the same. When your heating system warms the air, its molecules gain energy and move faster, causing them to spread out and become less dense. Less density means warm air is lighter, and as a result, it rises. As the warm air moves upward and accumulates towards the upper levels of the house, it will start to escape through openings.
When the air moves up, there is less air at the lower level, which means the pressure at the bottom of the home decreases. This creates negative air pressure at the lower levels. To fix the pressure imbalance, colder air is drawn inside at the base of your home to fill the void. This balances the air pressure between inside and outside.
The cooling season
In summer, the stack effect is reversed. The hot air from outside enters the upper portion of the cooler building. Air-conditioners cool hot air, which sinks because it is heavier. This causes air to enter the upper levels due to the void left by the air moving down. Then, as air builds up on the lower floor, a pressure imbalance with the outside causes it to exit.
Understanding its impact
During winter, the warm air rises and escapes through openings in the upper parts of the house. As a result, the warm air that you've paid to heat is lost, and your heating system has to work harder to maintain a comfortable temperature, leading to increased energy consumption and higher utility bills.
During the summer, you are wasting energy cooling air, which is lost to the outdoors. The temperature difference between indoors and outdoors may only be 10-30°F, so the stack effect and the airflow through the building aren’t as great as in winter. But the hot air continuously drawn in through the top of the house can result in an uncomfortably hot top floor.
It is cyclical and never ending
In winter, cold air getting pulled in makes the the lower floors cold, so people turn up the thermostat. As the upstairs gets hotter and hotter due to the rising warm air, people open windows to cool off. This increases the air leaving the building, which then increases the flow of air coming up. Then the people downstairs start to use space heaters. It starts a cycle where air is drawn in, heated, and then expelled at the top.
Even in one- or two-story homes, it can have a big influence on energy loss during the winter. The greater the difference between the indoor and outdoor temperatures or taller the building, the worse the airflow becomes.
Recognizing the signs
Here are a list of things to look for that indicate a stack effect:
Temperature imbalances: Noticeable temperature differences between floors, with upper floors being significantly warmer in winter and cooler in summer than lower floors.
Drafts: Feeling drafts near windows, doors, or other openings, especially on upper floors. Drafts at the perimeter of floors and in wall cavities.
High energy bills: Increased heating and cooling costs due to the HVAC system having to work harder to compensate for temperature imbalances. The heating and cooling systems may turn off and on constantly.
Condensation: Moisture buildup on windows or walls, particularly on upper floors, indicating that warm, moist air is rising and condensing on cooler surfaces.
Inconsistent temperatures: Difficulty in maintaining consistent temperatures throughout the house, with some rooms feeling excessively hot or cold.
Dust: Excessive dust buildup on surfaces; air movement caused by the stack effect can carry dust particles throughout the house.
Odors: Odors traveling from one floor to another through the ventilation system, indicating that air is moving between floors due to pressure differences.
Water stains: Water stains or damage on ceilings or walls, particularly in upper floors, which could indicate that moisture is infiltrating the building due to pressure differentials.
Airborne allergens: Increased presence of allergens such as pollen or pet dander, as the stack effect can draw outdoor air into the home through leaks and especially bad air from crawl spaces and basements.
Inefficient fireplaces or chimneys: Difficulty in getting smoke to properly vent from the fireplace or chimney, as the stack effect may interfere with proper airflow.
How to reduce it
Effective ventilation, air sealing, and insulation techniques can lessen energy loss and the stack effect. Since households will spend two to three times as much on heating as on cooling, it makes the most sense to begin air sealing efforts at the top. Next, take care of the house's lower levels before focusing on any leaks that may exist in between.
On the interior, air can leak through various parts of the home, such as cracked walls, floors, vents, and especially the attic. However, you can start to seal the air leaks. Utilize air-sealing techniques like caulking, expanding foam, and weatherstripping. Caulk and foam should be applied to cracks and openings around pipe holes or recessed lighting, as well as door and window frames. Caulk anything thinner than your pinky, and use foam or putty for everything else. Weatherstripping should be used for sealing movable or opening components such as doors and windows.
Also inspect the home's exterior to find air leaks. Pay close attention to areas where different building materials meet, as well as gaps around utility penetrations such as electrical boxes, vents, pipes, etc.
The efficiency of your sealing efforts can be increased by properly mitigating heat loss through insulation. Make sure to concentrate on the attic, walls, and floors while building a more complete thermal barrier in your house. You may drastically cut down on energy waste and improve the year-round comfort and effectiveness of your house by combining air sealing, insulation, and thorough external inspections.
Related content:
The stack effect isn’t just about energy loss, it can be dangerous in some circumstances. For example it contributed to the Grenfell fire. While the primary cause of the fire was the cladding, the stack effect likely played a role in the rapid vertical spread of the fire once it started. As the fire burned, hot air and smoke rose through the building's internal shafts and voids, creating a chimney effect that drew fresh air into the fire from lower levels which accelerated its spread upward. This stack effect exacerbated the fire's intensity and speed, making it difficult for residents to escape and for firefighters to contain.
Queen’s Repository
Both videos are helpful for identifying the air flow as well as the impact:
Such an insightful read 🤎
The stack effect is so interesting!
I did not know this at all and I think it just helped explain why the apartment I rent stays so hot in the summer — there isn’t enough ventilation for the hot air to escape!
It’s so interesting how your nightmare reno meant you learnt all this stuff you probably wouldn’t have otherwise known!