You need to Differentiate between Wood Rot & Mold and how they Propagate

Let’s compare and contrast mold and wood rot fungi. They both impact our lives, but from different facets of our homes and lives. The terms, rot and mold get tossed around a lot when discussing moisture and leaking issues in homes. Let’s briefly draw further distinction. We need to know them distinctly so that we understand how each propagates and how we can prevent their appearance or control and eliminate their blooms in our homes.

Mold and wet wood rot are similar. And you hear another term too. Mildew. Mildew is mold that grows on fabric.

Both mold and wood rot are fungi. They both require moisture. They both emit a musty odor and both damage surfaces.


But there are significant differences between mold and wood rot.

Wood rot fungi grows only on wood and digests wood. Wood rot fungi either attack sugars in wood fiber or attacks the lignin itself. The lignan is what gives wood it strength, and wood rot is  attacking the structural soundness of your wood. Molds will attack woods but not with the devastating effect of wood rot on natural wood structural members. Both mold and rot have an affinity for engineered woods.

Mold can damage wood but the biggest issues with mold are its opportunistic ability to grow about anywhere. Mold can grow on wood surfaces, but it can also grow on foods, plaster, soil, grout, tiles and carpeting. Mold’s digestive fluids decompose organic materials such as paper of sheet rock, making nutrients available.

Mold can also digest some synthetic materials used in building materials such as adhesives, pastes and paints. While mold cannot get nutrients from inorganic material such as concrete, glass and metal and tile grout but it can grow on the dirt present on these surfaces. Molds prefer damp or wet material.

With high humidity in the homes, and the conditions inside which can allow molds and mildews to perpetuate, the molds are the culprits which impact our health and vitality while the wet wood rot is more likely attacking our wood structure hidden away from air circulation behind a wall.

Mold spores are highly reactive in people’s bodies, depending on the sensitivity of the individual, and can make you sick or sicker. We discuss mold and the impact of mold in detail here.

There is also a difference between mold and rot relating to how they flourish at different moisture levels.

Some molds can get moisture from the air when the air is very damp, that is when the relative humidity is above 80%. The high humidity makes surfaces damp enough for mold to get a foothold.

When mold gets a start it quickly spreads out by extension of hyphae which are like tiny root hairs. In this way, a small colony of mold can expand to cover large areas. When conditions are not ideal- too dry, too hot, too cold- Mold spreads by tiny spores- like microscopic seeds. When spores are released they can be carried by air or water to new locations, include human noses. Molds are a big culprit in unhealthy houses.  

Wood rot does not appear until humidity is quite near 100% which can bring up the moisture content to over 19% in the impacted wood product. High humidity seldom causes rot until it is condensing at dew point on a regular basis so that the moisture content of wood increases to at least 30%.

So wood rot has to be associated with exceptional conditions of humidity or directly access to moisture through leaking. A crawl space can create the conditions for wood rot without direct leakage although direct leaking from pipes will certainly help.

Most often wood rot is associated with water leaks into the structure, through roof of leaks or through the siding, or within the structure   or wicking of water in through wet sill boards.

Texas has no reprieve from rot and mold seasons.

Many articles you read talk about parts of the country where the weather is so cold in winter that rot does not start until the weather warms up and then if the wood can dry out quickly, rot will not occur. In Texas in zone 3, if your wood is wet as defined, rot will grow.

There are variations in this zone of destruction by quality of lumber and ability to dry out water, but the Texas temps are always just fine for growing woodrot fungi. It takes temps over 60 degrees F for long periods to grow rot but Texas homes are not as well insulated and with homes heated in winter, this temperature range covers a wide range of exterior walls or crawl spaces or attics. Wood root gives up after 100 F but in the zone of cool summer homes, the over hundred degrees starts just the other side of the brick cladding.

You might find a wood sorption chart interesting. My studying the x and y axis you will see the range of humidity level where a particular type of fungi flourishes.

A sorption chart for wood helps us understand wood moisture in relation to relative humidity.

A sorption chart shows the relationship between relative humidity and the impact on wood.

”AThere are many physical properties acting on the moisture content of building materials. Wood and many building materials as hygroscopic – basically attracting free water vapor molecules to their surfaces. As humidity increases, further moisture will be stored in cracks in the materials, and in wood pores. The availability of higher levels of humidity provided more loosely attracted moisture to be available to mold first and then wood rot fungi.
Moisture in wood can hit levels of 25 to 30% moisture at 98% relative humidity. At dewpoint, moisture actually condenses on wood, and cold wood will attract dew as relative humidity approaches 100%.
At much higher levels of moisture where liquid water is present, capillary suction will pull water along the length of boards just like a living tree, and spread moisture.

I am getting a bit in the weeds here but if you are interested in this topic there is not a more authoritative site that Building Sciences Corp.

How do we minimize our risks to rot and mold?

When we keep wood below 80% humidity, when we keep wood from getting wet with water leaks, and when we provide a path for wet wood to dry properly, we will not have wood rot problems.

Wet wood if allowed to dry will not rot. It takes months of ideal moisture content and temperature levels to create rot. You have seen old barns standing against the weather for decades looking weathered and majestic. They are able to dry moisture out easily and rot is not a factor in their demise.

Of course, we want to live in homes that are tight to air leaks so we are comfortable, and so we need homes that are built to keep water out, control humidity properly and allow pathways for any trapped moisture to dry out.
There have been problems with EIFS – Exterior Insulation and Finish system, a stucco without a venting layer, has had problems with rot when water gets behind the weathering surface and then has no space or method to dry out.

There have been serious problems in North Texas homes with vapor barriers in walls that then trap moisture condensing behind cooled sheetrock surfaces.

No vapor barriers in Texas ever, and this includes vinyl wallpapers against exterior walls. You might be able to break this rule of thumb by careful attention to air circulation in your walls, and knowing where water vapor can escape, otherwise avoid vapor barriers. Make sure you or your contractor understand vapor permeability of materials.

Keeping mold out of your home.

Well, we don’t keep out mold spores completely they are part of the natural environment to compost nature’s waste,  but we keep the mold down to healthy levels. Since our houses are built with wood products and paper faced drywall, food for mold is always present. Mold can also thrive on dust, cockroach and dust mite feces, skin flakes and food particles. Our only control factor that does not also impact us, is low relative humidity. We must understand how we control humidity in our homes.

Molds can be perpetrated in the microenvironments of our home where humidity levels are higher and air circulation is less. High humidity levels in your home help perpetuate these problem areas. We discuss this battle here. High humidity micro-environments can be created when vent fans and bathroom fans do not vent out moisture properly. Or went a clothes dryer is vented into an attic or crawlspace. We look at those here.

And then air leaks over and off of bathrooms can allow moisture to collect on cold poorly insulated sheetrock walls and structural wood members that bathrooms, air spaces off of bathrooms where high humidity comes in contact with cooler building components from thermal bridging. These types of issues need to be air sealed and properly insulated and the home kept at a safe range of relative humidity that creates a moisture sink from these high humidity trouble spots.

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