Why are soffit vents important?

There is perhaps no topic that gets many home inspectors more riled and confused than attic ventilation.  If your inspector tries to tell you that mold on your roof sheathing is due to lack of ventilation, it might be best to find a different home inspector.  As a consumer it is a question that could be used to vet home inspectors.

The primary function of attic ventilation is heat reduction.

And yes, incidental amounts of moisture will be vented out as well.  But, if you have enough moisture that you are getting mold growth, we have to look elsewhere to find out why.  There can be a plethora of sources of moisture.  Too much moisture in crawl spaces is a big one. Moisture sources, or any combination of a dozen or more air bypasses from the house into the attic, can lead to moisture in the attic.

In older houses it can be very difficult to seal all these bypasses, but if it is not accomplished to a high degree, you can get mold growth in your attic.  This can be exacerbated by climate as well.

Night-sky radiational cooling.

A more unusual source of moisture in attics shows up in houses that are VERY tight and super insulated.  In these structures, the high-humidity winter air in maritime climates is drawn into the attic where it condenses on the cold roof where the cold night sky has suppressed the roof temperature below the dew point.  Research is ongoing to determine how best to deal with this but reducing recommended amounts of ventilation at the ridge can help as less moisture is pulled into the attic.

The flow through the soffit vents can actually reverse as the cold attic air drops through the vents to the warmer outside ambient air.  Generally speaking, the soffit vents are almost always under positive pressure as air pushes its way into the attic.

Quite some time ago, I insulated my attic with 12-14 inches of cellulose fiber.  I installed soffit baffles all around the edge at that time assuming that one day I would install the vents.  Not wanting to pass up an opportunity to experiment, I deliberately never vented the closed soffits.  I wanted to see if having only ridge vents and a couple of gable vents would result in moisture issues in the attic or would that much cellulose prevent house humid air from finding its way into the attic.

It is almost embarrassing to confess it had been this way for 20 years.  Time flies when you are busy ignoring things. Regardless the attic suffered no adverse effects from my experiment.

Welcome to the Heat Dome of 2021.

I noticed that when it was very hot outside (for Seattle that is) the top corners of the outside walls of the rooms got VERY warm compared to the ceiling and walls generally.  Remember earlier I said the primary function of ventilation is heat reduction?  Well not venting the attic created a little “soffit oven” that heated up the corners of the rooms—all around the house.  This was exacerbated by some settlement of insulation and missing insulation above window headers.  As you can see in this first drawing, the red areas represent the closed soffit with no vents installed.

This next drawing shows how the heat builds up in the corners and moves to the cooler interior.

This picture shows the thermal image of the ceiling corner adjacent to the soffit and how much it heated up on a 90°-day midafternoon.

This next picture is the thermal image of the same area the next day after vents were added and the outdoor temps were slightly warmer midafternoon.

When I was drilling the holes, it was early in the morning when it was about 65 degrees ambient, and I noticed cool air was dumping out of the new drilled holes.  The air temp measured 58° F at the vents inside the soffit and the roof surface temp measured 51°F.  These two thermal images show the cool air pushing its way out of the attic and hitting the frieze board under the soffit–directed there by the louvers in the vents.

What is the takeaway?  We need good attic ventilation, neither too much nor too little and in correct proportions.  Code guidelines are for the most part adequate if we can get air sealing right.  Some adjustments to these recommendations may be warranted for night sky radiational cooling.

Seal that ceiling between conditioned space and the attic!!

Also, looks like I am going to have to get around to top off that insulation–hopefully not 20 years.

One possible solution to the inherently poor insulation in these areas would be to retrofit the interior ceiling all around the room with insulated soffits.  This soffit detail (uninsulated) was a common detail in homes in this area in the late 1950’s.

By Charles Buell, Real Estate Inspections in Seattle

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