Case Study: Two Mold Scenarios in Unconditioned Spaces (Attics and Crawlspaces) in St. Louis Homes
Written by: Sean Struckmeyer | Tech Inspect Home Services LLC
In a month’s time in 2025, Tech Inspect Home Services encountered mold growth in unconditioned spaces that lacked adequate ventilation, not once but twice. Both spaces had a little extra mechanical help that was incorrectly installed or had degraded over time, that were contributing moisture to support the found mold growth. In both cases, having mold grow in these spaces was entirely preventable, and underscores the need for a home inspection anytime a home is bought or sold. The good news for you is we have pictures and videos to show what NOT to do!
Ventilation in Attics and Crawlspaces:
First let’s get the boring stuff out of the way. We’ll start at the bottom of the home and work our way up. Around St. Louis, crawlspaces and Attics are generally unconditioned spaces. The simple definition of a conditioned space is any area within a building that is actively heated, cooled, or mechanically ventilated for human comfort. If you see any home inspection videos on YouTube from Texas or other parts of the country, sometimes they have conditioned attic spaces or crawlspaces. This generally tends to be geographically dependent on building codes.
Crawlspaces:
An unconditioned crawlspace is an unfinished, shallow area beneath a building that is not actively heated or cooled by the home’s heating, ventilation, and air conditioning (HVAC) system. The standard requirement for Crawlspace ventilation typically requires a minimum net free area of 1 Square Foot of ventilation for every 150 square feet of underfloor area. Openings must be placed at least on 2 opposite sides to allow cross-ventilation, with the vents located within 3 feet of each corner of the crawlspace. Vents must be corrosion resistant and screened to prevent rodent entry. This requirement can vary based on the presence of a vapor barrier and other considerations.
The crawlspace we’re discussing in this article had none of that.
Attics:
An unconditioned attic is simply an attic space that is not mechanically heated or cooled by your home’s HVAC system. It is exposed to outdoor temperatures, meaning it will be hot in the summer and cold in the winter. In Missouri, that can mean that the attic is 70* in December and 100-130* in July.
Ventilation is important as it helps keep the attic from overheating. If an attic overheats it can damage asphalt shingles, which is the primary roof covering material used in our area.
The most effective form of attic ventilation is a soffit and ridge vent combination. It allows fresh air to enter the attic system at it’s lowest point and rise up through the attic exiting at the top moving heat and moisture up and out of the system. See the following diagram.
While a soffit and ridge vent system is the most effective, possible the most common in St. Charles County Missouri especially in the older homes is a gable vent setup. This is extremely common on ranch style homes. This system does not evacuate moisture and air as effectively.
A gabled louver ventilation system is what was installed in the attic system we’ll discuss in this use case.
Case Study Scenario #1: Mold in the Crawlspace
The Outside of the Crawlspace:
During our home inspection, the inspector discovered multiple defects in the crawlspace of the home. It should be noted that this crawlspace was under a House built in 1918, making it 107 years old at the time of the inspection. Older homes were not built to the same standards, and while neighboring houses had their crawlspaces closed off with vents installed this one had ‘siding’ materials in place to ‘close off’ the space. However, the space was not closed off, and while a significant amount of fresh air could enter the space, there was no way to make it move through the crawlspace. When a crawlspace has opposing ventilation installed, air is forced to move through the vents, essentially drawing fresh air through one vent and discharging through the other, this allows heat and moisture to more effectively vacate the area.
The following pictures show the various materials and holes were present in the crawlspace. It was NOT completely closed off.
The Crawlspace Issue:
Once inside the crawlspace, the Inspector encountered issues with the HVAC ductwork running through the crawlspace. It was severely corroded and at the plenum one of the supply ducts was slightly separated and not sealed. This was allowing conditioned are to enter what should have been unconditioned space. This combined with the lack of cross ventilation, and natural humidity that we get in Missouri, proved a recipe for disaster.
When an HVAC duct is not completely sealed, it can allow condensate and moisture to enter the area. See the following diagram as an illustration.
Pictures of the Defective Ductwork:
While the detached duct at the plenum, isn’t detached by much, what was found directly above it, shows exactly why this was a problem.
Crawlspace Mold:
First the Picture then the video to show the extent of the problem:
Video of the Crawlspace Mold:
Could this have been prevented? Possibly.
If this house had a proper vapor barrier installed, adequate ventilation in the crawlspace and more frequent monitoring of this space, this scenario might have been prevented. This scenario really underscores the need to have a home inspection performed. While none of these defects are in themselves, “major” when they are combined, they can have larger consequences that will be more difficult and expensive to resolve.
Case Study Scenario #2: Mold in the Attic
In our second scenario underscoring the importance of ventilation, we had an attic on a ranch style home with gabled louvers for ventilation. The house was built in the 50’s, so this was an extremely common setup. Based on the size of the home with no other factors involved, this attic was probably adequately ventilated. It should be noted that turtle box vents had also been added with a previous roofing job to provide increased ventilation.
The Attic Issue:
This attic unfortunately had a little bit of help. The house had a single bathroom with a non-operational exhaust fan, but the fan looked like it could have been original with the home. The problem was that the fan was not ducted directly into the outside of the home. The home did not have soffits, so the only option would have been to vent it directly through the roof with an appropriate cap and flashing, or out through the gable wall. Unfortunately, this was not the case, and mold was found at the ridge above the exhaust fan.
Exhaust air from bathrooms, toilet rooms, water closet compartments, and other similar rooms shall not be:
- exhausted into an attic, soffit, ridge vent, crawlspace, or other areas inside the building; or
- recirculated within a residence or to another dwelling unit.
Failure to properly vent a bathroom exhaust fan can lead to mold growth or other moisture related issues dependent upon where the fan exhausts to.
There are multiple acceptable configurations for exhausting a fan to the exterior of the home (see included diagrams for some examples). It’s important that any installation has all of the ductwork completely sealed and that the pipe is insulated. This prevents condensation forming on the pipe and/or conditioned/humid air from entering the unconditioned space.
The following diagrams show 2 common installation configurations, one through the roof and one through the gable wall.
Attic Mold:
First the Picture then the video to show the extent of the problem:
Video of the Attic Mold:
Could this have been prevented? Yes.
I believe that the attic was properly vented especially with the addition of the box vents, however, with the heat and humidity coming directly out of the bathroom when the shower was ran, it created a breeding ground for mold. How long it took for this to grow, is unclear, but properly exhausting the fan would have saved the homeowners a significant amount of money by having it remediated.
Conclusion: Protect Your St. Louis Home from Mold with Proper Ventilation
Mold prevention in St. Louis homes starts with understanding the critical role of proper ventilation in unconditioned spaces. As these two real-world cases demonstrate, both crawlspace mold and attic mold are entirely preventable when homeowners address ventilation deficiencies and ensure mechanical systems are correctly installed and maintained.
Whether you’re dealing with an older home built in the early 1900s or a mid-century ranch, inadequate crawlspace ventilation and improperly vented bathroom exhaust fans remain leading causes of moisture buildup and mold growth in St. Charles County homes. The combination of Missouri’s natural humidity, temperature fluctuations, and small installation errors creates the perfect environment for mold to thrive in your attic or crawlspace.
The good news? A comprehensive home inspection can identify these issues before they become costly mold remediation projects. From detecting separated HVAC ductwork in crawlspaces to finding bathroom fans exhausting directly into attic spaces, professional inspectors catch the warning signs that homeowners often miss.
Take Action: Schedule Your St. Louis Home Inspection Today
Don’t wait for mold to become visible before taking action. Whether you’re buying a home in St. Charles County, selling a property in the greater St. Louis area, or simply want peace of mind about your current home’s condition, Tech Inspect Home Services can help protect your investment.
Our comprehensive home inspections include:
- Thorough crawlspace evaluations checking for proper ventilation and vapor barriers
- Complete attic assessments examining ventilation systems and potential moisture sources
- HVAC ductwork inspection for leaks, separation, and corrosion
- Bathroom exhaust fan verification to ensure proper exterior venting
- Detailed reporting with photos and videos of any deficiencies found
Serving St. Louis, St. Charles County, and surrounding Missouri communities, our experienced inspectors understand the unique challenges that local climate and older home construction present. We’ve seen firsthand how small ventilation problems lead to expensive mold damage, and we’re committed to helping homeowners avoid these preventable issues.
Contact Tech Inspect Home Services LLC
Ready to ensure your home’s attic and crawlspace are properly ventilated? Schedule your professional home inspection today and gain the confidence that comes from knowing your home is protected from mold growth.
Remember: Mold remediation costs significantly more than prevention. A single home inspection can save you thousands of dollars and protect your family’s health.
Don’t let poor ventilation compromise your St. Louis home. Schedule your inspection online at www.Techinspecthome.com for expert crawlspace and attic inspections that identify problems before they escalate into mold disasters.