Inadvertent Reduction in Attic Ventilation

Posted by in Apparent Mold/Biological Growth, Attic Ventilation, Roof, Water Sources, Winter Concerns, Wood | May 30, 2014

We are often called to evaluate the source of moisture causing mold growth, water damage or wetting of building components in an attic.  These investigations commonly conclude with pointing out a roof or siding leak from a breach in the roofing or siding system or pointing out condensation from an inadequate attic ventilation system.

When the cause is condensation from an inadequate attic ventilation system and the physical evidence indicates that there has been little to no moisture problem before, we not only evaluate the attic ventilation, but we also especially consider what change occurred in the building to cause the recent increase or extension of the moisture problem.  There are numerous items that could affect the accumulation of moisture in an attic, but today we consider only one: an inadvertent reduction in the free vent area of the attic vents.

We have noticed the following causes for reduction in free vent area during our evaluations:

  1. Roof vent (ridge, turtle, can, gable, etc.) crushed by foot traffic, thus reducing the free vent area.
  2. Birds’ nests in the vents.
  3. An accumulation of dust due to excessive air flow from a nearby power vent.
  4. Paint over and across the louvers of soffit vents (see photo above).
  5. Vents covered with plastic to reduce cold air entry in the winter.  Do not cover vents in the winter.  They are needed to vent airborne moisture in the winter months.
  6. Blockage or restriction of the rafter or truss space from the attic to the eaves (vents) from the addition of excessive amounts of insulation or lack of a baffle to hold the insulation clear of the underside of the roof deck.  Se photo below.
  7. Removal and replacement of the previous roof vents with less effective vents.

Dust accumulations on screen reduce free vent area.

Blue baffle in one rafter space. Note insulation tight to underside of deck.

Metal vent with wind deflector to promote Bernoulli Effect.

Low profile ridge vent without wind deflector to promote Bernoulli Effect.







These are some of the changes to look for and address during the annual spring and fall roof and attic inspections.