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
