Seattle Home Inspector's Blog


Air-bypasses---got you coming and going


If you are not already familiar with the term “air-bypass,” I can almost guarantee that you will become more and more familiar with as time goes by. 

Air-bypass is a term that gets discussed in conjunction with the proper air sealing of buildings—or more likely, the lack thereof. Arguably it is “air-sealing” that is perhaps one of the most important considerations to take care of in any new home or in the weatherization of an older home.  

It is air-sealing that eliminates air-bypasses.  

The amount of insulation one installs is only relevant to the extent that the insulation itself is a good air barrier or that locations of air leakages have been properly sealed.  There is almost no end to the pathways for air to move in and out of buildings and this post is not here to discuss all of them.

This post is about one particular house I inspected recently where bypasses were installed “intentionally.”  Coupled with “stack-effect” (the natural tendency of buildings to act like chimneys whether it has a chimneys or not) these air by-passes were likely very “successful.”

Successful might not be exactly the correct word except in terms of correct grammar and correct for the time the by-passes were installed--back in a time when they were still giving away “free” oil and coal to heat our homes. Remember back when gasoline was 25 cents a gallon?

This home had 5 bedrooms on the second floor level and inside every closet there was a vent like the one in the next picture. 

 closet vent

These vents, due to stack effect, continually put the interior of the home under negative pressure and thus keeping the closets ventilated.  The smell of dirty socks and teenager’s rooms were continually vented to the attic--not a bad idea, right?

But all this venting had a dark side. Cold air in the winter could drop into the closets from the attic, as well as moist heated air could find its way into the attic.  Unless you own an oil company this is probably not the best of plans in this day and age.  We want to have a lot more control over the movement of air in and out of the house.  This closet venting would be virtually uncontrollable as it worked against our credit cards and/or check books night and day.

Another thing that I noted was that all of the holes were large enough to allow for any vermin that got into the attic to also get in the house.  Yikes!  As you can see in this next picture the view of the vent from the attic did not provide much in the way of protection from insulation, vermin or anything else.

closet vent in the attic 

One could almost consider this a heated attic—albeit a VERY difficult and expensive attic to heat.


Charles Buell, Real Estate Inspections in Seattle

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Comment balloon 9 commentsCharles Buell • August 12 2014 06:13AM
Air-bypasses---got you coming and going
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