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Seal Your Attic and Basement to Prevent the Stack Effect

by vscontent on April 27, 2018

Seal Your Attic and Basement to Reduce the Stack Effect

When looking for ways to improve your home’s energy efficiency, you may have run across the term “stack effect.” The stack effect is a physics phenomenon that occurs in every single building. It refers to the vertical movement of air from areas of high pressure to low pressure.

In the winter, the warm air generated by your furnace rises since it is less dense and creates less air pressure. The warm air will try to escape out of any gaps, cracks, and holes in the top portion of your home.

In the summer, the effect is reversed with cool, conditioned air escaping through the lower portion of your home, pulling in warm air from the attic and upper levels.

The greater the temperature/pressure difference between the indoors and outdoors, the greater the stack effect. This is why the stack effect tends to be stronger in the winter when the temperature difference is greatest.

Fun Fact: The reason why revolving doors were invented was to prevent the strong rush of cold air through a traditional door. This pressure from the stack effect made opening exterior doors extremely difficult. Revolving doors are mainly used in skyscrapers and tall buildings because the stack effect increases with height.

What is the Stack Effect?

Warmer air has a lower density and thus rises. The same thing happens with fluids. The lower density object or fluid rises to the top.

When the warmer, less dense air rises to the higher levels, positive pressure is created in the higher levels and negative pressure is created at the lower levels of the building. The positive pressure in the attic causes air leaks out of the attic hatch, chimney chase, and other holes and gaps near the top of the house.

When air leaves the home through the top of the building, a suction effect is created near the bottom of the home to replace the air lost through the top. Like wind, the air rises up and out of the home, drawing in new air from the air leaks in the bottom.

What Contributes to the Stack Effect?

The three main factors that contribute to the stack effect are:

  • Air leaks
  • Height of the building
  • Temperature/pressure difference between the inside and outside

Warm air rises and cold air falls. Large pressure differences between the outside air and the inside air exacerbate the stack effect.

The pressure differences can be affected by wind, exhaust fans, ventilation fans, chimneys and flues.

How Sealing Air Leaks in Attic & Basement Helps

Sealing air leaks in your building envelope is the best thing you can do to prevent the stack effect all year round. Speak with a professional HVAC technician about conducting a “blower door” test to pinpoint all of the energy leaks in your home. They can also help you select the appropriate materials, such as caulks, foams, weatherstripping, and door sweeps.

Don’t send your money and conditioned air through the roof or basement. By lessening the suction of outdoor air, you will experience lower energy bills, less moisture problems, and improved comfort and indoor air quality.

You’ll also want to check your attic and crawl space insulation levels for the proper R-value.

Common Attic Air Leaks

Check the attic for the following air leaks:

  • Attic hatch/door
  • Recessed lights
  • Chimney
  • Flue
  • Plumbing
  • Electrical Wiring
  • Exhaust vents

Common Basement Air Leaks

Check the basement for the following air leaks:

  • Rim joist (area where the first floor rests on the foundation)
  • Sill plate (just beneath the rim joist)
  • Windows and doors
  • Gaps and cracks around floors, ductwork, and where utilities enter the building.

Use weatherproof caulk for cracks less than a quarter inch. Use expandable foam spray for any gaps larger than that.

By making these air sealing and insulation improvements, you can save up to 50% on your energy bills.

Now that you know about the basic air movement principles of the stack effect, you can address the problem by hiring a HVAC professional to take a look.

After making any significant insulation improvements, speak with your HVAC technician about proper ventilation levels.

For more information on reducing the stack effect in your home, contact Pacific Heating & Cooling.

Contact Pacific Air Systems Heating & Cooling for professional home energy evaluations and indoor air quality solutions. Reach us 24/7 at (253) 292-3995!

Since 1984, we’ve been proudly serving our communities in Federal Way, Graham, Spanaway, University Place, Steilacoom, Sumner, Lakewood, Puyallup, Tacoma and Gig Harbor.

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