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

Confused About Attic Ventilation?

Builders, roofers, insulation contractors, homeowners and home inspectors are seemingly dazed-n-confused about attic ventilation.

Lets first discuss what the purpose of ventilating the roof is.  The primary purpose is heat reduction.  With the ventilation necessary to accomplish this it will also remove very minor amounts of moisture that may find its way into the space as well.  Its overall purpose is NOT TO REMOVE MOISTURE.  We want to reduce heat to reduce cooling loads and improve indoor comfort.

One can find an endless number of articles on the web about the solution to moisture issues in attics being to add more ventilation.

While I think a lot of these answers are only accidentally correct, when someone says moisture issues in the attic are a “ventilation issue,” I think they are missing the big picture and may result in the wrong solution to the problem.  These solutions too often make things worse.

Sure the ventilation can be wrong in terms of “amounts,” or it can even be completely missing.  It is interesting, because attic ventilation can actually be completely missing and the issue might still not happen at all.

I think it is safe to say, having too much upper ventilation would be worse than none at all, because it will increase depressurization of the lid.  This will lead to even more house air (which is at higher pressure) pushing its way into the attic.

So ventilation may need more balancing of intake and exhaust, but that alone will not correct the issue.

What the house needs is proper air sealing more than ventilation correction. 

Of course in the context of any air sealing the attic, it would be a good idea to adjust the ventilation to industry standards.  This is typically 50% upper  and 50% lower spread around the sides.  As long as that upper level does not go above 50% it is fine.  40% upper and 60% lower is fine as well.

You can get ventilation horribly wrong per code recommendations, and if you get air sealing correct, the attic will behave itself.

This attic had 15% upper ventilation and 85% at the eaves and this is what it looked like 33 years later.  (R-60 cellulose, raised heel trusses, and who even knew about spray foam in a can back then?) (As a side note:  the shingles on this roof were white 3-tab, over no underlayment and lasted 30 years with no sheathing damage—even at the eaves.) 

You can get ventilation perfect per code recommendations, and if you get air sealing wrong, the attic can go horribly wrong.

This is a 15 year old attic with perfect 50/50 ventilation.  (R-38 white fluffy, conventional framing)

It is more accurate to say this is an air bypass issue than a ventilation issue because the issue cannot be corrected by just focusing on ventilation.

This is where power vents become “snake oil” and can make the condition radically worse.

To illustrate the problem, I pose a question?  How much do I need to increase attic ventilation to fix a roof leak?

I think most would agree you fix a roof leak by fixing the roof.

The same is true for too much moisture in the attic from indoor sources–you exclude the moisture as necessary–you fix the moisture bypasses.  This is accomplished with proper, adequate air sealing and in some areas this will include vapor barriers.  The vast majority of moisture finding its way into attics is from air bypasses—not vapor diffusion.

This air sealing can be difficult in older homes and should be automatic in newer homes.

One of the biggest hurdles to overcome is that roofers are not air sealing experts and many insulation contractors are also not on board with the science of it all.  As a homeowner, you need to make sure the roofer and builder and insulator you hire knows how these systems play together.  You will also need to find a home inspector or building performance professional to help you sort it all out.

Charles Buell

Real Estate Inspections in Seattle

 

Do your exhaust vents terminate at your soffits?

The building codes specify that exhaust fans shall not terminate in the attic or soffits. While there are those that argue “at the soffits” is not the same as “through the soffits,” I think it pretty much amounts to the same thing.

The reasons we do not want to vent warm moist air into attics is well known and documented. It can lead to mold growth and other four letter words.

So what exactly does the code say:

M1501.1 Outdoor discharge. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.

That seems straightforward enough, and since “aimed at the soffit” is still in the attic, my personal opinion is, when the code says “Shall not be exhausted into a …..soffit,” they are meaning “through the soffit” as well.

So let’s forget about the code, and let’s see if my opinion can be supported by building science.

Wow, do we really have to go to “science?”

What are the building conditions that would come into play to sort this all out? Typically, or at least most of the time, in a properly vented attic, the attic space is under negative pressure relative to the higher pressures at the soffits and at the ridge. Because of this, air is attempting to push its way into the negative air space to make balance–24/7. All air in the vicinity of the soffit vents is forcing its way into the vents.

Now lets place a bathroom exhaust fan vent right at the soffit vents.

We are exhausting warm, wet, buoyant air that it is already moving upward and increasing the pressure in the area of the soffits. This increase in pressure difference between the attic space and the soffits makes that warm wet air work even harder to get into the attic.

In this picture, you can see evidence of where corrections have been made of the four vents that terminated too close to the soffit. The opening have been covered over, and hopefully they now terminate properly through the roof.

The staining on the siding above the lower vents is consistent with the buoyancy of the air from the vents.

Staining above the soffit vents on the underside of the roof sheathing is consistent with the upper vent’s previous termination at the soffits.

I think the codes need to clean this up a bit and require minimum distances to vented soffits.  Any current guidelines are at best “vague.”

Until then we should resort to good sense.

Charles Buell, Real Estate Inspections in Seattle