Renovation & Design

ASK THE INSPECTOR: Home's air pressure a delicate matter

QUESTION: I am the owner of a home with a cold, damp basement with no cold air returns. In your Aug 2010 Free Press article you advised that return air ducts will help to reduce the cold and dampness of a basement living area. You did not mention any concerns about creating a negative air pressure that might introduce CO2 into a house. Is this issue real?

Thanks, Garth Drake

ANSWER: Negative air pressure in a home can be quite a complex issue, and have one or more of several causes, but return air ducting in a basement is not likely one of them. We will explore the implications of this phenomenon, discuss some possible reasons it could happen and the correlation to your question about CO2 buildup.

While it may seem like a very technical scientific thing to understand, negative pressure in a home results simply when the air pressure inside the home is lower than that outside the building. This normally occurs when more air is leaving the interior of the living space than is being replenished from outside the building.

While this may happen on occasion due to unusual weather conditions, such as very high winds or severe storms, normally it is caused by mechanical means. Drawing too much air out of the home by use of exhaust fans, dryers, or through the chimneys of wood burning appliances can lead to lower air pressure inside, especially in well sealed homes during the heating season.

The reason that air-pressure differentials are so important in buildings is that severe problems can occur in a few areas if a reasonable balance is not achieved. If there is too high a positive pressure within a structure, a large amount of air leakage can occur into the exterior walls, attic and other components of the building envelope. Within this air is dissolved water vapour, which can wreak havoc if it becomes trapped in these areas. Rot, mould, and major structural damage can occur if this phenomenon goes unchecked for a long period of time.

Conversely, negative air pressure can cause uncomfortable drafts around windows and doors, back-drafting of fireplaces and gas or oil-fired furnaces and water heaters, and poor indoor air quality. This can have major health effects on the occupants of the home, especially if it is strong enough to cause products of combustion to leak into the interior air rather than harmlessly venting outside.

One solution to a negative-pressure situation is to bring in additional air from outside to compensate for the larger volume exiting the living space, no matter the cause. This can be done by installing a fresh-air intake duct into the furnace area or return-air ducting or by installation of a mechanical ventilation system such as an HRV.

Care must be taken to properly install either a passive or mechanical fix, to prevent frost, condensation or a large rise in energy consumption in the heating system. Use of insulated ducting, mechanical dampers, or a properly balanced heat recovery ventilation systems should help prevent this issue.

The largest concern with a building under a negative air pressure condition, is the quality of this air in the indoor environment. To maintain a healthy living space for occupants of any building, the stale indoor air must be exhausted and replaced with fresh air many times a day. This is necessary to reduce odours, excessive moisture, products from heating and cooking appliances, and especially products of human metabolism.

Chief among these indoor pollutants may be carbon dioxide (CO2). As humans and animals breathe, we absorb oxygen from the air and expel CO2 from our lungs. In a primarily closed system, such as the inside of a Canadian home during the heating season, we must actively remove CO2 to prevent poisoning ourselves. While this may be achieved by active use of bathroom and kitchen exhaust fans, too much ventilation can cause the negative pressure issue we are addressing. This is a common problem in some homes with countertop ranges that have powerful integral downdraft kitchen fans.

he good news is that the same cure for equalizing a differential in indoor and outdoor air pressures addresses the air quality issue beautifully. By bringing in additional fresh outside air, not only is the pressure equalized, but the problem of CO2 buildup is eliminated. Whether an HRV is incorporated into the HVAC components of your home, or a simple fresh air intake duct, ensuring an adequate number of "air changes" per hour is the key.

Adding return air ducting and registers to your basement should improve air movement in this area, and throughout the entire home, which can only help increase the warmth and dryness you desire. Anything that moves the air through the home more efficiently will also benefit attempts to maximize air changes, further improving indoor air quality.

This may have minimal affect on pressure differentials or CO2 issues, but adding a fresh air intake into the return air ducting at the same time can dramatically reduce chances that neither will be a future concern.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba ( Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at



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