Seattle Home Inspector's Blog


More attic ventilation is not going to help


The other day Jim Quarrello started a conversation on his blog about attic ventilation.  This post will attempt to agree with his post.

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.

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.  The other day I inspected a home with ideal ventilation and yet it still had one of the worst condensation issues I have seen for some time.

Frost covered roof sheathing

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 complete, moisture that is in the warm air 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,

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 no 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. 

 missing frost

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.

Ghosting from air infiltration/exfiltration

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.  The crawl space was flooded in some areas and showed a history of being flooded---worse than when I was there.

Dark areas are where there is standing water

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 negative pressure increasing the draw of moist warm air from the home.

More attic ventilation is not going to help.


Charles Buell, Real Estate Inspections in Seattle


* * * * * * * * * * * * * * * * * * * * * * * * *

Seattle Home Inspector


The Human Rights Campaign   QR code for Charles Buell Inspections Inc


WA State, Home Inspector Advisory Licensing Board

Comment balloon 11 commentsCharles Buell • January 15 2013 06:22AM


As with many house issues, you wisely point out that just one source may nit solve the problem.
Posted by Sally K. & David L. Hanson, WI Realtors - Luxury - Divorce (EXP Realty 414-525-0563) over 6 years ago

I have seen dark edges on carpet before but I never really knew what could be the culprit. I thought people just did not use their crevice tool when vacuuming. I learned something from you...again!

Posted by Kathryn Maguire, Serving Chesapeake, Norfolk, VA Beach ( (757) 560-0881) over 6 years ago

I am continually hearing the answer to this problem is to vent the attic more. This logic is no different than addressing mold as the problem, when in fact it is a symptom. Often what we see is the manifestation of something hidden. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 6 years ago

Charles, i always enjoy your posts...they are so thorough and informative....and, did I mention interesting?

Posted by Kristin Johnston - REALTOR®, Giving Back With Each Home Sold! (RE/MAX Realty Center ) over 6 years ago

So, when you were up there on the sunny side of that roof, did you turn my direction and wave?  Tell me before you do it next time so I can look, and wave back.

Oh, excellent post by the way!  Suggested as usual, as was Jim's yesterday.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 6 years ago

Sally & David, that is the way with so many things that need to be fixed on a home.

Kathryn, well sometimes it is just lack of cleaning J

Jim, yes---and putting the giant megawatt super sucker on the roof is only going to make things worse

Kristin, well “interesting” is the most important isn’t i? J

Jay, I pretty much have to wave “east” from here---no wonder you missed me J


Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

Charles -- thank you for bringing this information out as to how various parts of the house "work together" if they are not actually prevented from doing so.

Posted by Steven Cook (No Longer Processing Mortgages.) over 6 years ago

Great post Charles, and of course, I completely agree with you.  Funny timing with this post, as well as the one you wrote last week about CO alarms vs. CO detectors.  

I swear, I'm not trying to rip off your ideas :-).

One other important component to consider in these cases is the ductwork for the furnace.  I'm at IW2013 right now, and I sat through a great class yesterday put on by Jim Nemastil and Marko Vovk, where they made a big deal about leaky furnace return ducts in basements being a major contributor to attic moisture  problems.  I've done a ton of attic troubleshooting inspections, but I've never really considered what an impact leaky basement return ducts could make in an attic.

As soon as I get home, I plan to do some experiments on my own house.  Fun stuff.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 6 years ago

Steven, it is an often missed concept :)

Reuben, me thinks home inspectors like you and me probably should not be allowed to own houses :)

Posted by Charles Buell, Seattle Home Inspector (Charles Buell Inspections Inc.) over 6 years ago

There is a volume calculation to provide a square foot measurment value for the right surface area for the upper (ridge) and lower (sofit) vents. This is so there will be the correct range of air flow speeds entering tha attic space. 

Have you seen what happens when it is too high? It snows in the attic.! Air flow at the sofits can become fast enough to pull in falling snow from the exterior. When that air enters the larger volume space of the attic, it slows down and the snow falls out.

Guess what happens when it warms up.

Posted by Robert Butler, Montreal Home Inspector | Aspect Inspection (Aspect Inspection) over 6 years ago

Charlie, Great post as always. We really need to be thinking about how home perform as a system. All too often they re just an assembly of parts.

Also Robert I have seen snow through vent every ones in a while

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 6 years ago