Attic Mold? – Have No Fear!

Mold Assessment

Attic Mold? – Have No Fear!

You’ve seen the report: “Suspected microbial growth.  Recommend evaluation by licensed mold remediator.”

You’ve heard the sentence: “There are black stains on the sheathing in the attic.”

The sellers are saying, “It’s been that way for years.”

The buyers are freaking out at the word “mold.”  They’re worried it’s throughout the whole house…in the walls and lying in wait.

The fact is, mold affects everyone differently, but it is less of a concern when it is outside the living space (like an attic). Testing the air in the living space could tell you if it is affecting the air quality in the living space of the home. While every situation is unique, air testing often shows that the indoor air quality is not usually being affected by what is in the attic. However, the presence of ductwork or an HVAC system in the attic does increase the risk of affecting the air quality in the living space.

With mold in attics, we sometimes see three-dimensional growth, fuzzy growth of all different shades of white, yellow, blue, and green. But most of the time we see black staining on the sheathing. I say staining because it is in the wood, not on the wood.

Is it “BLACK MOLD?”  We cannot identify mold by color. Let me say that again: we cannot identify mold by color. Of the hundreds of thousands of species of mold, as many as 20,000 are black in color. We should only be identifying mold through a lab analysis. If there is visible mold, testing is valuable to determine the extent of any concern and for pre-and post-remediation verification (to ensure that it is completely removed during remediation).

Getting it Clean

Can’t I just spray it with something?

The national standard for mold remediation is removal because our bodies don’t know the difference between live and dead mold. If someone has sensitivities, killing it and leaving it in place won’t be enough. Mold doesn’t affect everyone the same, so removal is truly the best option. Mold remediation is simply a very detailed cleaning and mold removal. In attics, this often includes HEPA vacuuming the rafters, roof sheathing, ceiling joists, decking and flat surfaces within the attic, and wire-brushing areas of intense or elevated growth.  Remediation also includes thoroughly cleaning the rafters, roof sheathing, ceiling joists, decking and flat surfaces on all exposed sides with an anti-microbial disinfectant solution. Additionally, any and all exposed piping, wiring, and the exterior of HVAC ductwork and air handler cabinetry should be damp-wiped with an anti-microbial solution.  In some cases, a microbial stain-lightener is applied to areas to lessen residual staining; this is a cosmetic application but helps to ensure the remediation has been done thoroughly.

Identify the Source

If you don’t identify the source of moisture that allowed mold growth and fix it, the mold will simply come back.

We see this type of staining in attics frequently and it is usually not due to outside water intrusion or ice damming. In our experience, the staining (black streaks on attic sheathing) is likely from warm air escaping into the attic during cold winter months. Some clues that this is the cause: nails are rusted and the staining is worst on the north-facing side. This supports the idea that the discoloration is coming from condensation collecting on the cold surfaces during the winter months. Nails are good conductors of heat and will be colder than the wood around them, so will collect the condensation first. In addition, the north-facing roof slopes will be colder as they get little direct sunlight. We’ve seen frost on nails in the winter months and not just frost but as much as ¼ inch of ice on sheathing during extreme cold stretches! What’s happening is the warm air from the living area is cooling in the attic and reaching dew point. At this temperature, water vapor in the warm air condenses out on the colder surfaces, wetting them and encouraging mold growth. It’s just like condensation on a glass of iced tea in the summer.

Whether it’s three-dimensional growth or the more common black staining, the three most common causes are some combination of these:

  1. Improperly vented bathroom fans (vented directly to the attic or to a soffit)
  2. Inadequate insulation and gaps in insulation
  3. Insufficient ventilation.

Specific Items to Address

Bathroom Vents:

  • Is the exhaust vented directly to the attic?
  • Is the exhaust vented to a soffit?
  • Is it vented through the roof or a gable end?

Inadequate Insulation / Gaps in Insulation:

  • There’s no insulation on the attic access cover or pull-down ladder.
  • Recessed lighting or utilities without insulation
  • Not air-sealed around utilities in the ceiling (ceiling fans, lights, etc.)
  • Insulation is rated at R-38 or better
  • The attic stays cold in the winter. The heat stays in the living space.

Note: Wood stove and pellet stove users may keep the indoor temperature higher than those with fossil fuel or electric.  They may need MORE insulation to keep the excess heat from escaping into the attic and causing condensation problems. (Ask us how we know!)


  • Insulation blocking soffit vents
  • Ridge vent not cut to the right size or shingles crushing the ridge vent
  • Mixed ventilation types (gable vents combined with soffit and ridge vents)
  • Daylight in the soffits showing good air movement
  • No contents or possessions are stored in the attic.

Like any other situation concerning mold, there is always an underlying moisture problem.

Mold is ALWAYS a moisture problem – 100% of the time.  Moisture is the problem. Mold is the symptom. Control the moisture, control the mold.

Mold is not the terrible problem some people think it is, and it can always be gotten rid of. Remove the mold; control the moisture. It’s that simple.

 Work with a professional company that can clean the area in accord with the national standard for remediation AND helps you identify the source of the problem, so it never comes back.

By Russ Harlow

Certified Mold Assessor and Remediator – (NORMI) #13587

National Organization of Remediators and Mold Inspectors

Institute of Inspection, Cleaning, and Restoration Certification – (IICRC) #249880


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