Attic Mold – A Common Problem in Seattle area homes.

Attic mold is very common in Washington and throughout the Pacific Northwest. The problem occurs when warm humid air from the interior of the home travels into the attic and reaches the cold surface of the roof decking.  This rapid temperature drop leads to condensation, and in many cases, mold growth.  The problem is most often detected in newer homes, as these homes are built with materials such as OSB and plywood that are much more conducive to mold growth.

Common Causes

The #1 cause of attic mold is condensation.  This occurs when warm, damp air from inside your home migrates upward into the attic space and hits the cool roof sheathing.  Because cool air cannot hold as much moisture as warm air, the moisture in the air condenses out on roof sheathing.  This condensation, if not properly removed via ventilation, leads to mold growth.

The actual source of the warm damp air can change from house to house.  In some buildings it is due to disconnected or nonexistent exhaust fan ducts.  In other homes the problem arises from blocked soffit or ridge vents.  Additionally, several common types of ridge vents provide dramatically less effective air flow than advertised, leading to major condensation.

Occasionally, attic mold growth is due to liquid water intrusions such as roof leaks or rain water saturation during new construction.

Learn more about attic mold cleanup techniques > Comparison of Attic Mold Remediation Techniques

Attic insulation – should I replace it?

One of the biggest misconceptions in the industry today revolves around insulation removal.  Many contractors maintain that attic insulation must always be replaced when mold is present on the sheathing. Others maintain that replacing the insulation is unnecessary and a waste of money.  Let’s explore the underlying science a bit.

Stack Effect.

Due to the stack effect, attic pollutants, including mold spores, have very little opportunity to cross contaminate the conditioned air space below.  We’ve personally collected air samples in the top floor of hundreds of homes with severe attic mold problems and the results always came back negative (unless a compounding issue was present).  The only possible exception to this is if the cold air return is leaky and therefore, drawing mold spores into the HVAC. However, this is red herring, because this occurs before the furnace filter.

Available Moisture

Mold growth requires elevated moisture to grow.  So, for mold growth to occur in an attic, we need either a roof look or condensation.  Roof leaks are typically not a major issue, because the leak is quickly detected by the occupants and the problem is addressed.  Condensation is the far bigger culprit.  But remember, condensation only occurs on the first condensing surface.  In a ceiling/attic assembly, the first condensing surface is the heated(room) side of the sheetrock attached to the ceiling.  This is where the dew point occurs.  It is absolutely impossible for this condensation to occur on the attic side of the sheetrock or the insulation.

Simply put, mold growth won’t occur on the attic insulation in a cold climate.   Now, there is a caveat.  Mold growth can occur on the insulation when the condensation is so severe that the moisture drips back down onto the insulation.  This is rare, but it certainly does happen in extreme cases.

Is It Really Mold?

Often people mistake settled dust for mold spores. Attics with healthy ventilation will often develop a thin layer of grey settled dust on the top layer of insulation. Unscrupulous contractors often mistake this for mold.

Conclusion

Our job in the mold industry is to protect people and protect buildings. In the vast majority of cases, replacing the insulation in an attic suffering from mold growth does nothing to further either of those goals.