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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  1. T. O'Connor says

    Great post, as always Charles you have the answers many don’t understand completely. Thank you!

  2. Vicki Campbell Mensah says

    Thanks for a great post Charles. Always enjoy your posts.

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