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Attic Mold Growth - How to Avoid the Scams and Find Permanent Solutions

Mold growth in the attic is a relatively common occurrence in climates with cool, damp winters, such as the Pacific Northwest.

First, the good news.  Mold growth in an attic rarely impacts the indoor air quality.

The bad news…  Mold growth in an attic can be a difficult problem to solve.  We regularly encounter attics with ventilation far in excess of code, and yet they are covered in mold.  And yet other attics we inspect have terrible ventilation and remain completely mold free.  Confusing.  Let’s examine the factors that cause mold growth to occur in an attic.

Diagnosing attic issues.  Is it really mold?

Attic mold growth comes in a wide variety of colors and growth patterns, some of which are often misdiagnosed.  Fuzzy / 3 dimensional mold growth is nearly always identified correctly.  When the growth pattern is very flat and appears like staining, it can cause confusion.

Low profile mold staining

Uniform black mold growth on attic sheathing

Uniform black mold growth on attic sheathing

Dark staining from mold growth can almost look like soot

3 dimensional spotting

White and green fuzzy growth

White and green fuzzy growth on OSB sheathing

Determining the cause of attic mold growth

While several moisture sources are technically possible, 99% of mold growth in an attic is due to condensation.  Despite what your roofer would like you to believe, roof leaks rarely cause significant mold problems.  Why?  Roof leaks are obvious.  It’s pretty hard to miss water dripping through your ceiling.  Because of this, people usually address their roof leaks quickly – long before mold growth can occur.   Also, roof leaks occur in one area (again, despite what your roofer will tell you).  They do not leak en masse.   Even if a roof leak goes undetected long enough to cause mold growth, the problem will be localized to a small area directly around the leak.

Heavy condensation can look like a roof leak

During cold periods, condensation can become so severe that moisture will drip from the sheathing.  This can often fool home inspectors and homeowners into believing they have a roof leak.  Unfortunately it is a costly error.  Once a roofer is called to investigate, you can rest assured they will determine you need a new roof…

Wet attic with mold growth

Mold growth in attic valleys

Below is an image of heavy mold growth in a valley (where two roof sections meet).  This often causes confusion, as these sections of the roof do commonly leak.  However, they are also areas of poor circulation and therefore, condensation based mold.  The best way to determine the source is to examine the attic during a period of cool, dry weather.  A roof leak will stop but condensation will get worse.

Attic mold growth in valley

Mold growth in roof valleys is often misdiagnosed as a roof leak.

Condensation occurs when the temperature of a surface of a material reaches the dew point.  This means the air is 100% saturated and cannot hold any more water molecules.  Once this occurs, the water molecules begin to condense on anything that is less saturated.  In an attic, this is typically your roof sheathing or framing.  Usually you cannot see the individual droplets unless there is an exposed area of metal or the outside temperature is especially cold.

Attic Mold Growth - condensation

If the condensation is really severe, you’ll see drip marks on the surfaces below.  In this case the attic was used for storage (never a good idea) which provided dramatic evidence of the condensation and dripping.

Mold in attic - dripping

If condensation lingers long enough, the mold spores will begin to grow.  Many people think mold growth is prevented by eliminating the mold spores themselves.  This would be great if it were true. But mold spores are ubiquitous.  Even if you managed to kill all the mold spores in an attic, ten thousand new spores would drift in within hours and settle on top of your nice clean surface.

Determine the solution

This is where it gets tricky.  Because the majority of attic mold cases are caused by condensation, it means you’ll rarely find a smoking gun.  Typically the condensation is caused by several compounding factors.  There are two main factors that cause condensation.

  1. Excessive moisture entering the attic from the attic space below.
  2. Insufficient ventilation in that attic.

If one of these factors is operating poorly, an attic might avoid mold growth by succeeding well in the other factor.  For example, if a house has poor attic ventilation, but the ceiling is well air sealed and the interior of the home has excellent ventilation, mold growth will not likely occur.  The opposite is true, but too a lesser extent.  Homes with excellent roof venting can overcome poor air sealing but if the interior RH is high, mold growth will override the venting and mold growth will occur.

PROBLEM #1.) Excess moisture transport from the home into the attic

In most cases, the moisture that causes the condensation on the roof sheathing is from occupant activity in the home below.  Showering, cooking, laundry, even just breathing adds water vapor into the air.  Due to the stack effect (warm air rising), this moisture slowly migrates toward the attic.  This is why you rarely find mold growth in the attic above a garage or a rarely used portion of the home.

Some of the moisture movement occurs directly through the building materials, such as sheet rock.  This is why building codes often require a vapor retarder on the ceiling.  In the past this was accomplished via tar facing on the the fiberglass batts.  Many builders now simply use a paint/primer with a low perm rating to act as the vapor retarder. While this is a nice touch, the vast majority of moisture transport comes through air movement.   This occurs when air flows through penetrations in your ceiling (can lights, fans, etc.) and top plates (holes for electrical wires).  As the air flows through these holes into the attic, it pulls the moisture as well.

The result?  The vast majority of condensation is due to air movement transporting the water vapor rather than the water vapor directly moving through the sheet rock ceiling. Below is a dramatic shot of this principle in action.  The blue piping is unsealed in the house, allowing warm, moist air to move into the attic space.  Once it hit the cold gable wall of the attic, the moisture immediately condensed on the surface, leading to mold growth.  While it is rarely this visibly dramatic, this same principle is behind the majority of attic mold problems.

Attic mold caused by air movement

 Air leakage around sky lights

This is another example of an area that commonly suffers from actual roof leaks (flashing around the skylight).  And yet because these skylights are often located up in a dead end air space, air leakage into the attic can be severe.  The issue is also exacerbated when the skylights are located in a bathroom, which suffers from dramatically higher moisture loads.

Looks like a roof leak, but this is actually due to air leakage

Looks like a roof leak, but this is actually due to air leakage

Ironically, if a home is well sealed everywhere (windows, siding, etc.) this can actually make problems worse, because the natural dissipation and dilution of excess moisture does not occur.  Even in a well sealed ceiling, the moisture, finding no other pathway, will eventually makes it way into the attic.  If you’re going to seal your home, ensure the ceiling / attic junction is sealed better than the walls and windows.  Most homes are poorly sealed and therefore require substantial ventilation.

Unsealed gaps between condo partition walls

Below is an image of mold growth (and condensation) from unsealed air gaps around the perimeter of the attic.  The air gaps were between adjacent condo units, which allowed large quantities of moist air to migrate up into the attic space.

Attic mold from moisture

Mold growth on attic partition walls in condo complex.


PROBLEM #2.)  Poor ventilation of the attic assembly

The need for ventilation in the attic is directly related to the amount of moisture coming from the occupied spaces below.  If the house has an extremely well sealed ceiling / attic floor junction, very little moisture will enter the attic from the home.  Minimal ventilation will be necessary to prevent condensation from occurring.  Alternatively, if the ceiling is leaky and full of unsealed can lights, penetrations, etc. the roof will need much more ventilation.

Common causes of poor ventilation:

Attic Mold soffit

Undersized bird block holes in soffit (blocked by insulation as well)

Mold in attic above soffit

Heavy mold growth on attic sheathing above undersized soffits

Attic mold - blocked ridge vent

Oops. A ridge vent doesn’t work to well if the sheathing isn’t cutback to allow air flow.

In some attics, traditional soffit venting is impossible
No soffit vent possible in attic mold project

Some older homes have zero soffits /eaves to add venting.

Blocked by insulation
Mold on attic blocked soffit

Soffit vent completely blocked by insulation.


Ridge / Upper Roof Ventilation Problems

Upper roof ventilation can be grouped into 3 types:  ridge vents, RVOs and powered/mechanical units.

Ridge Venting

These are long, continuous strips of ventilation that sit directly on the ridge of the roof.  They are available in two types, externally or internally baffled.  The externally baffled models can be highly effective.  Internally baffled models are often worthless.

Blocked Ridge Vent

Without upper roof ventilation, moisture laden air cannot escape

Internally Baffled Ridge Vents

These ridge vents basically look like multiple layers of a corrugated cardboard box (though they’re made out of plastic).   They have one thing going for them – they’re cheap.  On the negative side – they don’t seem to pass much air through them.  As you can see in the photo below, the channels are very small.  The air flow in an attic is very slow, so any impediment will drastically reduce the movement of air.

Internally baffled ridge vent.

Internally baffled ridge vent.

Condensation and mold in an attic due to ridge vent

Condensation directly on an internally baffled ridge vent.

Externally Baffled Ridge Venting

As the name implies, externally baffled ventilation relies on barriers on the outside to prevent water intrusion.  This allows for much larger openings in the actual ventilation portion of the ridge vent.  These ridge vents also sit significantly taller than a typical internally baffled model.  This also allows for great air flow.

Many externally baffled models are built with a plastic fin on the outside edge.  This is designed to create a low pressure zone over the vent as wind moves over the ridge.  In the Pacific NW our wind is fairly irregular, so this feature should not be relied upon.

Yes, they can become clogged with needles, but I’d still be on a clogged externally baffled ridge vent over an internally baffled model.

An example of an externally baffled ridge vent.

An example of an externally baffled ridge vent.

They often arrive with a loose mesh to prevent bugs from entering the attic.  They can become clogged with dust and debris and cut down on the air flow.  We typically recommend removing the fabric unless there is a history of pest issues.

Clogged filter fabric can greatly reduce air flow through the ridge vent

Clogged filter fabric can greatly reduce air flow through the ridge vent


How not to build an attic…

Attic mold - flat roof

Attic mold – flat roof

Notice anything unusual here?  Yep, that is a flat roof completely encompassed by a regular, pitched roof.  Why?  Our best guess is the builder was forced to chop the top of the roof off the comply with height regulations.  The home is located in a neighborhood with views.  Likely a neighbor complained of the violation, which resulted in this rather bizarre solution.  Surprisingly, the mold growth in the flat section was fairly minor (although there was heavier growth in other areas).  Flat roofs are notorious for mold growth.  The issue is less related to roof leaks and more due to the lack of air circulation that occurs in low angle or flat roofs.  Homes depend upon the stack effect (warm air rising) to push warm, humid air out of the attic space.  When a roof is flat, this effect is dramatically reduced and the air stagnates, causing condensation and mold growth.

Light mold growth in the flat roof section.

Light mold growth in the flat roof section.

Treating and venting the flat roof section would be impossible due to the limited access.  Therefore, our recommendation was to focus on lowering the interior humidity levels and wait until the roof is due for replacement.  At this time, the ventilation should be entirely removed and insulation installed directly on to the roof sheathing.  This is referred to as a conditioned attic space and will completely eliminate condensation and mold growth.

Heavier mold growth in pitched section of attic.

Heavier mold growth in pitched section of attic.

This project is particularly challenging because the area where you would normally install a ridge vent is actually designed like a shed roof.  A shed roof has a single sloping surface which ends in a vertical peak.