Mold on Firewood: What’s Normal, What’s Not, and How to Prevent It

If you burn firewood, you’ve probably noticed white, gray, or fuzzy patches on a stack at some point. Mold on firewood is common — and in many cases, it’s more of a storage-related issue than a serious problem. That said, not all mold is the same, and knowing what’s normal versus what signals a bigger issue can help you burn cleaner and store your wood more effectively.

Why Firewood Gets Moldy

Mold grows when three things are present:

• Moisture

• Limited airflow

• Organic material (wood is a perfect host)

Firewood can develop mold when it’s:

• Stacked too tightly

• Covered improperly (tarps trap moisture)

• Stored in humid conditions

• Exposed to rain without time to dry

• Moved from outdoor storage into a warmer, damp space

Even properly dried firewood — including wood that is fully seasoned (under 20% moisture content) or kiln dried — can develop surface mold.

In these cases, mold forms on the outside of the wood, usually due to brief exposure to humidity, condensation, or restricted airflow — not because the wood is wet internally. The interior moisture level remains low, and the wood is still structurally sound.

What Mold on Firewood Usually Looks Like

Surface Mold (Typically Not a Structural Issue)

This type of mold is common and usually not a major concern:

• White, light gray, or pale green patches

• Powdery or fuzzy appearance on the surface

• No soft or crumbling wood underneath

• Wood still feels solid and relatively dry

This mold forms on the surface of the log and does not mean the wood is rotting.

Mold That Signals a Problem

This type of mold suggests deeper moisture issues:

• Thick, dark mold (black or deep green)

• Strong musty or sour odor

• Wood feels damp, spongy, or unusually heavy

• Bark falling off easily

• Mold covering large portions of the stack

When mold looks like this, it often means the firewood hasn’t dried properly, lacked adequate airflow, or has been repeatedly exposed to moisture. In these cases, the wood may have elevated internal moisture, leading to poor burn quality and an increased risk of rot.

Is Moldy Firewood Safe to Burn?

For most people, small amounts of surface mold are not dangerous when firewood is burned in a properly functioning fireplace or wood stove. The heat of combustion destroys mold spores.

However:

• Mold can be irritating to handle, especially for people with allergies or asthma

• Moldy wood often burns less efficiently

• Excess moisture can lead to more smoke and chimney buildup

Firewood that is heavily moldy and damp may be better suited for outdoor fires or allowed more time to dry before being burned indoors.

How to Prevent Mold on Firewood

The good news is that mold is mostly preventable with proper storage.

Stack Smart

• Stack firewood off the ground

• Leave space between rows for airflow

• Avoid stacking directly against buildings

Cover Correctly

• Cover only the top of the stack

• Leave sides open for ventilation

• Avoid wrapping stacks tightly in tarps

Choose the Right Location

• Sunny, breezy areas are best

• Avoid low, shaded, damp spots

• Keep indoor storage limited to what you’ll burn soon

Firewood with very low internal moisture — especially kiln dried firewood — is far less likely to develop ongoing mold issues, even when stored indoors.

Rotate Your Wood

• Burn older wood first

• Avoid letting stacks sit untouched for long periods

Mold vs. Rot: Know the Difference

It’s important to separate mold from rot:

• Mold: surface-level, often manageable

• Rot: structural breakdown of the wood; soft, crumbly, and poor-burning

Rotting wood produces less heat, more smoke, and more residue and should not be burned indoors.

When Mold Becomes a Bigger Issue

Mold should raise concern when:

• Firewood consistently feels wet

• Mold returns even after improving storage

• Burning produces excessive smoke

• Chimney buildup increases noticeably

These signs usually point to moisture content, not just surface mold.

Why Large, Untouched Wood Piles Create Mold and Rot

Firewood dries best when air can move around it. When logs are left in large, tightly packed piles for long periods, drying slows — especially in the center and bottom of the pile.

In these conditions:

• Moisture becomes trapped

• Airflow is limited

• Bottom and center pieces stay damp much longer than outer layers

Over time, this can lead to heavy mold growth and even rot, even if the outside of the pile looks dry. Wood stored this way may be split months before delivery but left sitting in the same pile without being restacked or rotated. When that wood is eventually moved, pieces from the bottom or center are more likely to show mold, rot, and poor burn quality.

The same thing can happen at home if firewood is left in a single pile on the ground or stacked too tightly without airflow.

Properly dried firewood should be split, stacked, and stored to allow consistent airflow, kept off the ground, and arranged so air can move freely around the wood. Immediately after splitting, stacking firewood in raised crates or racks helps prevent ground moisture absorption and supports even drying, reducing the risk of mold and rot.

The Bottom Line

Mold on firewood is common and often harmless when it’s light and limited to the surface. In most cases, it reflects storage conditions, not poor-quality wood.

By managing moisture, improving airflow, and storing firewood properly, you can greatly reduce mold and enjoy cleaner, more efficient fires.

Stay warm,

Nick & Michelle Persolja

The Firewood Co.

Dry My Wood™

37859 Stevens Blvd

Willoughby, OH

FirewoodOrder.com

office@firewoodorder.com

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