Seattle Home Inspector's Blog

     Older homes typically had gable vents if that was the type of roof that was installed.  As we came to understand how venting works, and came to understand the problems associated with inadequately vented attics and roof structures, we moved away from the use of gable vents.  Gable venting relies on movement through the vents from one end to the other to change the air in the attic and hopefully to carry any moisture that has entered the attic with it.  While in some cases this can be adequate it often left areas of the attic "under-ventilated" due to poor circulation.  The system was also subject to poor venting depending on the house orientation and prevailing wind direction.

     The next approach was to have soffit vents that circulated air up to roof vents located near the ridge of the roof.  This was better but could still be affected by wind direction and in some cases the vents could be buried in snow drifting across the ridge.

     By far the best method is to have the soffit vents in conjunction with continuous type ridge vents.  As wind moves across the ridge from any direction it acts to pull air out of the attic and would create a negative pressure on the attic if it were not for the soffit vents that allow for air to enter the attic----thus creating good air circulation.  I discussed these principles in my blogs, "What is Giovanni Venturi doing in my attic" and in , "Not all short-circuits are electrical."

     So how does this happen in New Construction?

     It is easier to see how it happens in old.  For example:  the gable vents are already there----and no one thinks to cover them up.  The roof vents are already there----and it is easier to put another roof vent over the hole than it is to patch the hole and roof over it.  In new construction we are sort of back to, "What were they thinking?"  Maybe the framers were just used to cutting the holes but the roofers where trying to do better?  Maybe the plans called for both?  Lack of communication?  Whatever reason it shows a serious flaw in someone's understanding of basic building science.  The following picture, from inside the attic, shows how the air is likely to flow to the continuous-type roof vent----pulling little air from the soffit vents.