Duct leaks are a fairly common way to lose conditioned air and suck outdoor air into your home. I once worked on a new 4,000 square foot upscale suburban home that smelled like a sewer. The owners had been living in a downtown hotel for three months to avoid the smell.
My testing showed that the ducts that carry air from the house back to the air conditioner, which was located under the house, were leaking. These leaks were creating a negative pressure and sucking in methane from the drain field of the septic system. That septic field was more than 40 feet from the basement!
We sealed the duct leaks and the smell went away. (And we didn’t use duct tape, which does not effectively seal ducts. The product to use for sealing ducts is latex, water-based mastic that meets UL 181 specifications.) No leaks, no suction, no smell.
In the past the solution to duct leakage was to compensate by oversizing the system. The contractors didn’t realize that they were compensating for duct leakage. They just knew that if they didn’t put in a system larger than the sizing calculations recommended, they would get callbacks from homeowners complaining that the system wasn't cooling the house sufficiently. This is like buying a bigger gas tank to compensate for the fact that it leaks a fourth of the gas you put in it. It might work, but it makes no sense, and it is an expensive solution. These leaks can jack up cooling energy bills by 20% to 40%.
As the suburban homeowner I was working with found out, these leaks also cause makeup air to flow into your home and can bring in gases like radon and even pesticides from below the soil. One interesting experiment showed that depressurized homes can draw in soil gases from quite a distance. As an agronomist told me after attending one of my trainings, People don’t realize that the air moves through the soil like wind blows through the trees. If it didn’t, all plant roots would rot and the plants die.
The house is a system, and that system extends out into your yard. Outdoors, what you put on your lawn and under the house for pests and termites can affect the quality of the air you breathe inside the house. Indoors, how well your air conditioning and heating system works can affect much more than your energy bill. Ignoring or not understanding the interactions among a home's components can lead to some surprising, and often nasty, home performance problems.