Understanding Air Flow in Homes for Energy Efficiency

Home Energy November 12, 2013|Print

Reviewed and Revised on 11/12/2013

Air tries to equalize between higher and lower air pressure areas. If there is a pathway (a gap) and a pressure difference, it will move through that pathway whether we want it or not. 

To be an effective air barrier or air flow retarder, a material must not only block airflow through it, it must also be installed in a way that eliminates even small gaps, be continuous all around the conditional space and be durable.

If the amount of air removed from an enclosed space (exhaust systems or air return to the air conditioner) does not equal the amount of air supplied to that space, then a pressure imbalance is created.

A negative pressure happens when more air is removed than added, causing the building to draw in make-up air (air infiltration).

In all climates, a balanced air pressure is ideal, but difficult to maintain. In warm climates, a slight positive pressure is advantageous; in cold climates, a slight negative pressure is preferable to prevent hidden moisture problems in building cavities.

  • Leaky ducts in attics cause negative pressure inside homes and increase infiltration of air through walls or crawl spaces and floors. In summer, that means hot, humid air infiltration. In winter, that means cold drafts.
  • In all climates, a balanced air pressure is ideal, but difficult to maintain. In warm climates, a slight negative pressure is preferable to prevent hidden moisture problems in building cavities.
  • A large enough negative pressure can lead to dangerous backdrafting of the chimneys and flues of combustion appliances.

 

Illustration of negative pressure caused by leaky ducts


A positive pressure happens when more air is supplied than removed from a space.

  • A positive pressure can be created and controlled with a fresh air intake.
  • A strong wind (hurricane) entering a home through a broken window can pressurize the house enough to damage it.

Forces can create simultaneous positive and negative pressures in different parts of a building. An example is closing a door to a room that has supply registers delivering air, but no return grille making the room positive and the area with the return grille negative.

Indoor air typically contains more types and higher concentrations of pollutants than outdoor air, even in industrialized areas. Some common home indoor air pollutants include:

  • Biological pollutants (mold spores, dust mites, bacteria, viruses, pollen, animal dander).
  • Combustion pollutants (including carbon monoxide), lead in dust (from old paint or lead-tainted soil).
  • Volatile organic compounds (VOCs) from off-gassing (of building materials, adhesives, paints, finishes, pesticides and some household cleaning products) and sometimes asbestos.
  • Radon, a radioactive soil gas, poses a serious hazard in many areas.

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