Modern tight houses can easily become depressurized when exhaust fans are turned on. What this means is there is no place for the air to come into the home to replace the air that is trying to leave.
If there are gaps around door weather-stripping, or gaps around window sashes or similar locations, the air will come into the home at these locations. Sometimes even chimneys might be the path for this air. If we operate and exhaust fan in a bathroom the house becomes depressurized, or an area of “lower pressure.” Areas of higher pressure will tend to make balance with areas of lower pressure, so the air outside the building literally “pushes” its way into the area of lower pressure.
Most houses are not tight enough for the air to not find its way in somewhere, and general infiltration was once allowed to be the source of this air replacement.
This small condo unit was too tight for general infiltration to be the source of make-up air, as was evidenced by its finding a path through the range hood exhaust.
Most range hoods have a back-draft damper in them, but there should also be one in the cap at the exterior of the building as well. You can see in this picture there is no damper—but there is a screen.
Exhaust fan vent termination with no back-draft damper
With two bathroom exhaust fans and the laundry exhaust fan running, the purple/violet colors of the thermal image of the chase and microwave/hood shows cold air cooling the chase and the area around the microwave.
The screen at the exterior cap location did hold a tissue paper to show that indeed air was pushing its way through the microwave/hood.
So, let’s say we “fix” the cap at the exterior with a proper back-draft damper. Where will replacement air come from? General infiltration may still be adequate, it is just easier coming from where it is now. If it is not adequate, the functionality of the exhaust fans will be reduced. In other words, they will make noise but not exhaust enough air from the room. It is like turning a 100-cfm fan into a 50-cfm fan.
For exhaust fans to do their job, replacement air is necessary and is required by modern codes when houses get to a certain point of air-tightness. This one may be at that point, even though it is an older home in that respect.
Some “positive” means of allowing exterior air to enter the home may be indicated if exhaust fans do not function properly after the exterior cap is repaired and its back-draft damper installed.
By Charles Buell, Real Estate Inspections in Seattle