1. Plumbing Vents Not Run Through the Roof
Here a plumbing vent pipe is terminated in the attic– it never was run though the roof at time of construction.
Lots of warm moist air can enter the attic from a plumbing vent stack.
Plumbing Vents Not Run Through the Roof
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

2. Air intrusion from the electrical mast
Here is another air by-pass that often gets missed. No seal around where the service entrance wires enter the box.
This coupled with stack effect can introduce lots of outdoor air into the building--cold or hot.
Did you know these conduit openings are supposed to be sealed?
Air intrusion from the electrical mast
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

3. Plugging this opening prevents cold air from entering the panel that could cool the metal panel such that condensation would occur resulting in corrosion of the panel components.
This should not be a source of outside air to the home.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

4. What drives bypasses?
What Drives Air Bypasses?
If there were openings, and air did not move through them, there would not be a problem, would there?
There are forces that support, allow and/or “drive” air movement.
Is there anyone here that does not know what Stack Effect is?
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.
Used with permission, copyright 2017, Building Science Corporation
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

5. Stack Effect
Moisture moves from wetter to drier
Heat moves from warmer to cooler
Air moves from higher pressure to lower pressure
Mechanical Exhaust equipment (both heating equipment and bath/kitchen/laundry)
Wind
All buildings experience some amount of stack effect and it can be quite dramatic in a four story town house.
I inspected a townhouse where the roof deck door was propped open, the street level door yanked out of my grasp and “whistled” when I closed it.
If it can do this to the front door, what do you think it might do to upper level exhaust vent flaps?
This is a good reason why there needs to be flaps in the units themselves as well as at the exterior.
I had a house once where on the downwind side of the house all the vent flaps were tick-tick-ticking as they opened a bit and closed again. When the exhaust fans had been installed, the installer had removed all the flaps from the fan units themselves.
Sometimes this same principle can happen if the outside flap is stuck open or missing. In this case it is the interior flap that might be tick-tick-ticking.
We need to be thinking about making these flaps better sealing---not eliminating them.
The “pressure zone” created between the two flaps, helps to keep both flaps closed when either flap is placed under slight negative or positive pressure.
This force can be working 24/7/365 on a house, and can cause all kinds of issues.
It is why air sealing is so critical. If openings in walls, around windows and doors, are not properly air sealed guess what happens? Depressurization tugs that outdoor air right into the building at the lower level and pushes it out the same kinds of openings at the upper level.
The way this all works, makes it necessary for us as inspectors to understand a whole lot more about how our houses work so that we can convey that information in an intelligent manner to our clients.
This of course, assumes we care. Some inspectors see all of this as being well outside our SOP’s, but for me it is all about health and safety---as well as energy efficiency.
So exhaust fans work just like stack-effect--it is just man-made, artificially induced stack effect.
Wind blowing by the house increases stack-effect as well, and even if there is no, or minimal, stack effect the wind creates pressure differences that pull air out of the house on one side and pushes into the house on the other.
Wind can be natures exhaust fan.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

6. The best of intentions……..
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

7. Myth #10:
Bringing High Humidity air into a well ventilated NW crawl space in the winter will raise overall crawl space moisture levels.
Myth #10
Bringing High Humidity air into a well ventilated NW crawl space in the winter will raise overall crawl space moisture levels.
Lets explore what actually happens.

8. Myth #10 Brain Teaser
(Not an attic question, but helpful to get us where we are going)
The temperature outside is 35 degrees Fahrenheit and raining hard--probably close to 100% humidity. What will happen in an adequately ventilated crawl space that is 50 degrees Fahrenheit with a relative humidity of 50%? (This is a “typical” wintertime condition for a Northwest crawl space.)
I know this talk is about attics, but it is helpful to slither into the crawl space a bit on our way to understanding what is going on in the attic. This is also something of utmost importance to inspectors in the Maritime Northwest.
The temperature outside is 35 degrees Fahrenheit and raining hard--probably close to 100% humidity. What will happen in an adequately ventilated crawl space that is 50 degrees Fahrenheit with a relative humidity of 50%? (This is a “typical” wintertime condition for a Northwest crawl space.)
When that cold wet air is heated by the warmer air in the crawl space, not only can that crawl space air easily hold the moisture of the incoming air (and not rain in your crawl space) but it will mix with the warm humid air of the crawl space resulting in a net lower humidity of all the air in the crawl space.  As that air then moves out of the crawl space the humidity of the crawl space will end up being lowered.
Temperature in the winter in the NW is generally favorable for this to happen and reverses slightly in the summer.  In this sense, general humidity and moisture levels can be expected to rise in the summer and then drop back down in the winter. 
The common practice in some parts of the country of blocking crawl space vents in the summer would prove disastrous in the Maritime Northwest.
Of course without proper venting the moisture levels can rise above acceptable levels, resulting in wood decay/rot and infestations of Anobiid Beetles etc.
I like to use this example of how different it can be in different climates because you certainly would not want to try out this arrangement in South Carolina or Mississippi or even the Northeast.
This is a Northwest, maritime climate, solution. You have to understand the science of all this as applicable to your own area. The science is the same, the results are just different.
Of course, none of this touches on whether crawl spaces should be vented period—anywhere.
Whatever you may be thinking, this is not reducing moisture levels with ventilation, it is reducing moisture levels with temperature differences. In attics we do not have those temperature/humidity differences.
Ventilation is merely moving the air from one place to another.
In hot and humid climates, what happens in crawl spaces with ground temperatures at a constant 55 degrees F? It rains. In an attic, in the same hot humid climate, if we artificially bring that cooler ground temp into the attic in the form of cooling ducts and cold ceiling below, once again---it rains.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

9. Window air inlet
How many of you are seeing these window inlets in your areas?
These air intakes at the windows are one of the ways to meet modern energy code requirements to bring fresh air into the home. 
They can contribute to loss of control of house humidity levels due to stack effect. 
These and other openings anywhere in the building envelope can result in air moving in and out of the building where we don’t want it to---and when we don’t want it to. 
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

10. X 2 X X X 1 X
The problem with these window air intakes is that in multi-story houses, they do not just bring fresh air into the home whenever an exhaust fan is run.
They continually vent outward at the top and inward at the bottom (due to stack effect) when fans are not running.
Simplified in the drawing, air goes in opening 1 and out opening 2.
This creates WAY more air changes in the home than necessary, wasting considerable energy, and it can lower the humidity in the home by the same principles that reduces humidity in the crawl space.
In the drawing we can see that penetrations like plumbing pipes (marked “X”) are sources of such leaks—but “X" should also symbolize b-vents, chimneys, wiring holes etc. 
Inadequately sealed attic hatches and crawl space hatches can represent very large breaches in the building envelope.
Obviously exhaust fans can represent breaches—but they are designed to be breaches.  Unfortunately, even when the fans are not running they can still have inadequate dampers and leak around the fan housings themselves.  Of course running them 24/7 would result in pulling all that cold/wet air into the home around the clock.
Can-lights or any electrical junction boxes in the building envelope—especially at the attic level—can represent significant breaches of the building envelope as well. 
While right now we are primarily discussing the effect of bringing cold/wet air indoors, we can obviously create problems for the home if this heated/moist air can find its way, or is vented directly into the cold attic space. X X
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc

11. Two story house venting to the exterior due to stack effect, 24/7/365.
The discoloration on the siding is consistent with ongoing moist air movement up the siding.
Myths of Attic Ventilation © Charles Buell, Charles Buell Inspections Inc