Atmospheric pressure, also known as air pressure or barometric pressure, is a subject that is not always considered when designing the mechanicals in a home. Yet, atmospheric pressure can have a huge effect on air quality and safety. Let’s take a simple look at understanding this.

When we turn on an exhaust fan in the home (over the stove or in our bathroom), we are sucking air out of the room creating a negative pressure within the home. The effects of this negative pressure is going to demand new air to enter the room to equalize the pressure. If you have a window open, the new air will enter to satisfy this equalization. One problem is most people never open their windows – especially in the winter. But the new air has to come from somewhere, and this can be problematic.

One of the laws of thermodynamics is that “air and gas move from high pressure to low pressure.” When you blow up a balloon, the air in the balloon is at a much higher pressure than the outside. When you release the high pressure in the balloon, it will move to low pressure to equalize. This concept plays out in our home every day.

So where does this ‘new air’ come from when the windows are closed? There are several answers to this. Unfortunately, the new air results are not good.

• New air can be pulled in through cracks at the walls, behind the base boards, and around windows. This air is not only dirty, but it may include mold spores because of lingering moisture in the walls. The new air can also cause dehumidification in the winter when we are trying to put humidity indoors. On the flipside, it can draw in moist air in the summer time when we are trying dehumidify the air inside the home.

• New air can be pulled in from the basement and crawl space. This dirty air can be distributed in the living space and can contribute to dust we see on the furniture.

• New air can be pulled in from a fire place chimney that will draw soot into the living space.

• New air can be pulled in from gas flues from mechanicals such as furnaces, boilers, and water heaters that introduce carbon monoxide into the home.

At one of my past assessments, I noticed a service manual that was taped to the top of the water heater. The homeowner said that the service man taped the manual there when the water heater was newly installed. This water heater was in the basement’s mechanical room. At the other end of the basement was an exhaust fan in the ceiling. When that exhaust fan was turned on, carbon monoxide and heat would be pulled down from the water heater flue that was starting to burn the manual. See photos below

Here, you can see paper and plastic on the water heater manual starting to burn due to air being pulled in from the flue by negative pressure when the fan was turned on.

Many people believe that we build homes too tight and that they cannot breathe. One view in building science says that a tightly built home will have better air quality with the proper ventilation. Build tight and ventilate right.

I believe this is partially true, and it’s not always so black and white. An exterior wall needs to breathe. That means that an exterior wall must have the ability to allow moisture to escape within the wall cavity due to the condensation created through thermo bridging, and moisture due to poor flashing areas around doors and windows.   

Designing your mechanicals to maintain a neutral pressure within your home plays an important part in having healthy indoor air quality. Also, keeping a negative pressure under concrete floor slabs will further complement good indoor air quality.

John