Stack effect does its best to create a river of air through your house–it wants to flow in at the bottom and out the top.
Stack effect is the movement of air into and out of buildings, chimneys, flue-gas stacks, or other containers, resulting from air buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect helps drive natural ventilation, air infiltration, and fires.
Our air sealing efforts can mitigate stack effect but it is always ready flow as soon as there is an opening.
While houses that have less than 1ACH-50 (air changes per hour) come close to combating stack effect, current energy code requirements of 3ACH-50, does not. (Washington State Energy Code is still stuck on 5ACH-50–likely until the 2021 Code Cycle.)
For this post we will be talking about homes that meet current energy code requirements—or are worse than current code requirements.
In modern tight construction, for exhaust fans to function and to change air in the home, we must also provide a path for fresh air to enter the home when the fans are running. Sometimes the air intake locations are with vents built into the vinyl windows themselves. I do not want to discuss all the other means of providing fresh air to the home. This post will only focus on the window air intake type vents.
There are problems with these air intakes in multi-story homes. When the vents are open, they allow that river of air to flow 24/7. In my experience, these vents are either always open or always closed because the homeowner does not know what to do with them. Just as often they do not even know they are there. The result is WAY more air changes per hour than the house was constructed to meet.
In the following pictures we can see the result of stack effect on vinyl windows with the window vents open.
The windows are covered with plastic related to painting the building. Notice how the plastic puffs out at the top and sucks in at the bottom–clearly demonstrating the power of stack effect.
Solutions to this issue are illusive, but there certainly should not be any vents up high. If there are, they should be kept closed. At 3ACH-50 there will always be enough air leakage to change the air more-or-less continually at the upper level–but obviously this is not the desired way to do it. Leaving the vents open at the bottom level to allow for fresh air intake when exhaust fans or the whole house air exchange fans are running should be sufficient.
Abandonment of window intake type vents in favor of barometric type intakes would be a far better option.
We certainly cannot allow the river of air to flow wild.
A while back I did another post about how when these vents are left open at both levels it can result in too low of an indoor humidity. The window intakes can allow us to lose control of the indoor environment when some become outlets.
Charles Buell,
Real Estate Inspections in Seattle
