Stack Effect does not need help!

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. 

Air Intake

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.

Stack Effect

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


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Thanks, Charlie

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  1. Gerry VanSambeek says

    Hi Charles,
    Just watched your salt water GFCI trip video.
    Its Gerry from Carlton, Oregon. I was at a recent class of yours in Seattle.
    Since your class I have been using the amp meter and testing the voltage = Question – I’m getting lots of homes with over .1 of an amp on the grounds and gas pipe bonds & so far I have had little feed back on results – is this so very common to have so many homes with current on the grounds – 85%? In a subdivision with a larger group of apartments nearby I met the utility company out to one high reading property – they had the meter pulled and I had the electric panel off – still had a .22 reading – they did not care – stated it is just a mili-amp.
    The high reading at the same property with everything connected up was almost 2 amps – noted when the furnace was on – another home had a high reading of current on ground with the furnace on??
    ** Is there a rule change for duplex receptacles not needing to be GFCI protected – Yesterday: In a new construction foundation crawl space to a 24/7 fan for radon reduction – unprotected duplex receptacle, last week an electrician answered my attention comment that the duplex receptacle on the exterior of the new construction home did not need GFCI protection – it had a sprinkler timer control plugged into it???

    • Charles Buell says

      I would consider .1 to .2 amps on the ground “not normal.” .01 or .02 maybe—but when you get above a 1/10 of an amp it is concerning—but can be difficult to figure out where it is coming from. As to the receptacles outside, the only one that is not required to be GFCI protected is one for ice melting (up high typically). ALL receptacles and lights in crawl spaces required GFCI protection per un-amended NEC.

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