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


Is your insulation doing what you think it is doing?

I do not want to tackle the huge ugly topic of whether fiberglass insulation should be used at all–as it frequently is in attics.

For today, I just want to talk about one obvious issue with fiberglass insulation that prevents it from performing as expected.

The short version of what is wrong with fiberglass insulation is that it is not an air barrier, therefor, if it is not encapsulated and air sealed on all six sides its performance suffers. In an attic, at most, only five sides is likely to be sealed leaving the entire top not sealed.

This post is about the sides, which can and must be air sealed. The vented sides are typically not adequately air sealed, especially along the eaves. Insulation baffles, designed to keep insulation out of the lower roof venting and to allow for air flow into the attic, rarely gets adequately sealed.

Soffit vent that allows air flow into the attic

Because the baffle/top plate connection is not air sealed, and because fiberglass insulation represents very little resistance to air flow, air pushes its way into the attic right through the insulation. As it does this, it either cools the ceiling in the area, or warms the ceiling in the area depending on the season and/or side of the house.

In the winter and/or the north side of the home, the air will tend to make the ceiling cooler in the area of the vent. In the summer, especially on the sun side, the air will tend to warm the ceiling in the area of the vent. This will increase both heating and cooling loads of the home.

This next picture shows what that area looks like at the interior ceiling with Infrared camera on the South side of the home. Warm air is moving through the insulation and warming the ceiling.

In the same house on the North side we can see how the ceiling area near the vent, as indicated by infrared camera, is “cooler.” In the actual picture we can see the fungal growth present because this vent happens to be in the area of the bathroom. The moisture in the bathroom condenses on the cooler surface creating a perfect environment for mold growth.

The only real repair for this condition is to pull back the insulation and properly air seal the gaps where the insulation baffle and the house framing meet.

Spray foams are good for sealing the areas where the baffle makes contact with the framing as indicated in the circle in the picture above. Of course, in a perfect world, we would not use fiberglass insulation at all, and instead use types of insulation that are much better at stopping the flow of air. Cellulose fiber insulation can do a much better job at this.


Charles Buell, Real Estate Inspections in Seattle

More attic ventilation is not going to help

Many people seem to think that ventilation is the answer to all moisture issues in the attic. The reality is that adequate ventilation–proper ventilation–will only deal with “normal” moisture conditions that the attic space is subjected to (in most cases).

Another way of stating this is that the best ventilation in the world will not deal with moisture condensing out of an inordinate amount of warm moist air finding its way into the attic. A while back, I inspected a home with ideal ventilation and yet it still had one of the worst condensation issues I have seen for some time.


For our discussion here, we are going to assume the roof does not leak.

At least as important as ventilation, if not more important, is having a proper air barrier between the living space and the attic/roof structure. If this air barrier is not continuous, moisture that is in the warm air that finds its way into the attic space, can, and will, condense on the cold surfaces it contacts–if that surface is cold enough. If these surfaces are below freezing then it will show up as frost.

There are an almost endless number of places that might not be properly sealed that will amount to breeches in the air barrier. Here is a partial list:

Openings around B-vents and chimneys,

Wiring holes,

Pipe penetrations,

Ventilation fans terminating in the attic,

Dryer vents terminating in the attic,

Heating ducts not sealed adequately,

Unsealed can lights,

Skylight penetrations,

Poor framing techniques,

Missing vapor barriers in some climates,

Attic access covers with no weather-stripping,

Ventilation fan housings,

And, Electrical junction boxes.

Obviously many of these cannot be determined in the course of a standard home inspection because of access and/or insulation. Repairs can be easy or difficult depending on how obvious the by-passes are.

In the case of this house there were a few obvious things that should be addressed prior to removal of all the insulation.

From the roof, thanks to the frost, one can see where there is less frost in one area and no snow in a large area that corresponds to the shape of the chimney chase at the interior of the home.


It does not take a rocket scientist to guess that the connection of the chase with the attic is not adequately sealed.

There were also exhaust vents terminating in the attic and there was no weather-stripping on the access cover. Addressing these three issues would be the first order of business. If that does not fix the issue then, the investigation would need to become more “aggressive.”

But that is not the whole story with this house. Another question that had to be asked and answered was why were moisture levels within the home so high? This was evidenced by condensation on windows that didn’t have any curtains or blinds. (Even under normal humidity levels in the home–when it is really cold outside–some amount of condensation will occur on windows that have curtains or blinds that limit air circulation.)

Of course lifestyle can be a factor. If the occupants don’t use exhaust fans when showering or the fans are not functional we can expect to have higher humidity inside the home–which can then find its way into the roof structure. But there was another big hint as to another possible source of moisture to the indoor environment–the crawl space. At many locations around the home there was evidence of poor air sealing at wall floor connections consistent with air infiltrating/exfiltrating from the crawl space. It was particularly evident at the steps from the entryway up to the main floor level as can be seen with the “ghosting” in the following picture.


A LOT of air is coming and going at these black areas and the carpet is a pretty good filter.

If the crawl space is “cold” and at high humidity and that air is drawn indoors where it is then warmed by air at high humidity the indoor air becomes even more humid. Stack effect works to continually pull air from the crawl space through the living space and on into the attic space.  The crawl space was flooded in some areas and showed a history of being flooded–worse than when I was there.


So now we have an attic moisture issue that is only likely to be fixed when the house drainage system is fixed–along with all the other by-pass issues we discussed.

Some people think that if they add a POWER vent they will surely move enough air out of the attic. But really this only serves to put the attic under even greater negative pressure thereby increasing the draw of moist warm air from the home.

More attic ventilation is not going to help.

By Charles Buell, Real Estate Inspections in Seattle

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