Seattle Home Inspector's Blog


Does your house need “bypass surgery?”


Many do.

Air bypass into the roof structure (attic) is one of the biggest challenges to the home’s roof structure---from the “inside” that is.  Everyone is aware of the damage that can occur from failed roof coverings and roof flashings, but most are not aware of air by-pass issues.  Certainly most home owners are not aware enough.

When we heat our homes, the warm air takes on moisture due to the simple fact that warm air can hold more water (as vapor) than cooler air can.  As an example the cooler you keep your home the more condensation will form on your windows.  When you warm the house up, the condensation magically disappears.  This of course assumes that there is not so much moisture being produced that no amount of warming can hold all the moisture---we call this rain.

Any place this warm moist air can find a way into the roof structure we call “air bypasses.”  Think of them as paths of moisture vapor transfer---a moisture vapor transfer system.  Obviously this is a waste of heat and is a huge problem in itself in terms of energy efficiency, but this air movement can also result in ice dams.  The movement of water vapor with this air is problematic because it can result in wood decay rot in the roof structure as well as mold growth in the attic.

For the most part, stopping the air movement will stop the vapor movement.  In really cold climates it is a good idea to have a vapor barrier on the insulated ceilings to prevent moisture vapor from migrating to the cold roof surfaces by means of pressure differentials.  But this could be the subject of another post.

Back to bypass surgery.

Where do these bypasses occur (and this is by no means a complete list)?

  1.     Improperly sealed attic accesses.

  2.     Openings around plumbing pipes, electrical wires and HVAC ductwork.

  3.     Inadequately sealed skylights.

  4.     The spaces or chases around chimneys and b-vents.

  5.     Ceiling electrical junction boxes.

  6.     Can-lights---especially when the lights are turned on.

  7.     Kitchen, dryer and bathroom exhaust fans---even if the units have dampers.

Think of each one of these paths of air movement as little exhaust vents---with no damper---essentially working 24/7---in an attempt to destroy your roof structure and empty your wallet.

Stopping all means of air movement is critical to solving a lot of the attic’s health problems---and can result in avoiding surgery altogether.  A little preventative medicine goes a long way.

Some types of insulation, like cellulose fiber, are more forgiving of some of these bypasses because they essentially do a very good job of stopping air movement.  If you have any type of fiberglass insulation you can pretty much include everything on the list as a functional air bypass.

I came across a great example of can-light bypass the other day.  Granted, this is considerably more egregious than most can-lights but it is a perfect example to illustrate what we have been talking about so far.

Exposed light bulbs in wet location 

First of all, note the location.  These two can-lights above the tub are in a location that likely requires sealed covers (and certainly would require fixtures rated for a damp location if not a wet location).  So, besides the air bypass issue these would be considered wrong regardless.  Of course the exposed bulbs being CFL’s adds another interesting dimension to the installation.

These can-lights are installed in what might be considered the area of highest moisture vapor in the home.  It is a good thing there is a vent fan right there between them to help with the moisture.  Too bad the fan vents directly into the attic!

Bathroom exhaust fan venting into attic

Here is a picture of one of the can-lights as seen from inside the attic.

can light

Can you see all those round holes in the fixture?  The circled hole, and others in the fixture, are what make this thing a bathroom vent as much as a light fixture.

This next picture is what the fixture looks like if you were a mouse in the attic and the lights were turned off.

In the attic with the lights off

Can you see the cute curls of the CFL through the holes?  The mouse can.

So what kinds of “obvious” air bypasses do you have into your attic?


Charles Buell, Real Estate Inspections in Seattle


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Comment balloon 11 commentsCharles Buell • November 28 2012 06:56AM
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