Black  moldy attic (C) D Friedman D GrudzinskiCauses of Mold in Attics

  • ATTIC MOLD CAUSES - CONTENTS: How do we diagnose the cause of moldy roof decks? How do we get rid of and prevent roof mold or attic mold? The relationship between wet basements, wet crawl spaces, and attic mold. Where to look for mold in a building attic
  • POST a QUESTION or READ FAQs about the underlying causes of mold in building attics
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What causes mold contamination or growth in attics and cathedral ceilings?

Here we explain the causes of moldy attics or roof mold in buildings, and we discuss the relationship between other building moisture sources (such as a wet basement) and attic and roof mold contamination.

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This document gives advice on how to find and deal with mold in building attics and roof cavities; We discuss when and how to clean up attic mold - how to get rid of attic or roof mold.

The page top photograph of dark mold found on the attic-side of plywood roof decking was contributed by David Grudzinski, a Cranston RI professional home inspector and member of ASHI and NACHI.


Black  moldy attic (C) D Friedman D GrudzinskiWhat causes mold growth in attics? Understanding why we get mold contamination in roof spaces helps define the steps necessary to prevent attic mold, cathedral ceiling mold and similar problems.

As this moldy attic photo (courtesy of David Grudzinski) also shows, mold on roof decking or roof framing may be quite extensive.

If you look closely at the buckling plywood in the upper right of this photograph, you may notice some cracking that might indicate that the plywood roof decking is actually delaminating and badly damaged.

If that proves to be the case, this mold cleanup job will be simplified, but more costly as roof decking may need to be removed and replaced.

When we see an attic with extensive visible mold on wood surfaces, we also suspect that the fiberglass insulation may also be mold contaminated, or may become so during any mold cleanup job in the area.


What Causes Attic Mold?

Cleaning up moldy wood surfaces, removing moldy attic insulation, will be a costly but wasted step if we don't understand and correct what caused these conditions in the first place. Below we describe the combination of two critical factors that make for a wet, moldy attic: inadequate roof ventilation and an indoor source of un-wanted or excessive building moisture. A third source, roof leaks, is more obvious and should also be considered in any building inspection for leaks, moisture, or mold.

Inadequate Roof Ventilation

David pointed out in his emailed comments that the attic of this home was not adequately vented. We don't see good, continuous intake ventilation at the house eaves or soffits (the lower roof edges.) We also did not see good roof exit venting along the ridge of the building.

Our example photos (below) show characteristic rust stains around roofing nails in a poorly-vented attic (below left), and the absence of those stains around roofing nails in a well-ventilated attic (below right). Even our poorly-vented attic (below left) was not as severely wet and moldy as the attic in Grudzinski's photos (above).

Attic roof stains around roofing nails (C) Daniel Friedman Attic roof stains around roofing nails (C) Daniel Friedman

Frost on attic roof decking (C) Daniel Friedman

Our photo of frost on the under-side of an attic roof deck (left) shows another clue of inadequate under-roof ventilation that may be visible if the building is located in a freezing climate.

Regardless of its source, moisture entering the attic from anywhere is trapped in this space - contributing to mold growth and longer term to building rot or perhaps even inviting insect attack on the structure.

If the moisture levels were low, no mold problem may have occurred. But if we combine poor roof ventilation with high moisture levels, we've asked for trouble with mold and rot.

Insulation improvements as well as attic ventilation improvements are discussed at INSULATION INSPECTION & IMPROVEMENT, and

at ROOF VENTILATION SPECIFICATIONS we give specific details on good attic ventilation.

Indoor Moisture Sources Causing Attic Mold

Bath venting into attic (C) DanieL FriedmanPoor insulation & bathroom, dryer, or kitchen fans not vented outside:

Mr. Grudzinski also observed that the moldy attic in this home was fed moisture from a combination of poor insulation, and two bath fans that were venting directly into the attic rather than being directed outside. "Poor insulation" means increased heat flow into the attic in cold weather.

Our photo (left, from a different home) shows a rats nest of bath vent fan ducts that the building owner emptied into a building attic. These vents should have been conducted to outside the building.

"Heat flow" into a cold attic means warm moisture-laden air flow from the occupied spaces in the home below is moving into the chilly or cold attic space. This airflow not only increases the cost of heating the home, but it brings up moisture from below into the attic area where water condenses out of the warm air onto cold attic surfaces.

Wet basements or crawl spaces mean wet moldy attics:

At BASEMENT MOLD WATER IMPACT another series of photographs by Mr. Grudzinski demonstrates how a wet (rotting, moldy) sub-basement served as a moisture source that was almost certainly a major contributor to if not the main cause of the moldy attic shown in our page top photographs here.


at ROOF ICE DAM LEAKS we describe problems caused by inadequate roof ventilation that include not just attic moisture and mold, but ice damming and even leaks into building ceilings and walls.

Also see our photo guide to finding attic moisture and mold at How to Inspect Attics for Condensation & Moisture - to Detect & Correct Attic Condensation, Frost, Ice Dam Leaks & Attic Mold

Photograph: toxic mold on pine tongue and groove roof sheathing - © Daniel FriedmanToxic attic mold: The photo at the left was identified as a toxic mold that probably should be removed. Lots of protruding nails through a roof deck preclude wiping or scrubbing.

Mold growth, provided it is not just cosmetic mold which can simply be left alone, (see Cosmetic Mold) can be cleaned from wood surfaces by blasting, scrubbing, or even simple surface wiping, depending on the surface accessibility and smoothness.

Sanding wood building surfaces such as plywood or tongue and groove roof sheathing, wall sheathing, or wood framing is usually unnecessary and inappropriate.

If you must return wood surfaces to immaculate, pristine looking condition, perhaps for cosmetic reasons where wood surfaces are left exposed to view in a building interior,

Procedures for total cleaning / restoration of mold-contaminated wood surfaces

Attic Mold Contamination Articles


Continue reading at ATTIC MOLD IS A PROBLEM ? or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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