Behind the Drywall – The Psychrometrics of a Hidden Moisture Failure - Short #298 · HVAC School - For Techs, By Techs summary

Key takeaways

HVAC technicians must understand microclimates within walls and cavities to prevent moisture buildup and mold growth.
Tools like moisture meters and thermal imaging are valuable for detecting hidden moisture, even if technicians are not mold assessors.
Proper moisture management involves not trapping water vapor and understanding how water interacts with different building materials and insulation.
Monitoring for water leaks and condensation caused by HVAC systems is within the technician's role to maintain healthy indoor environments.
Building science principles, including the correct placement of vapor barriers and drying strategies, are essential knowledge for preventing long-term moisture problems in buildings.

Summary

Understanding Hidden Moisture and Microclimates in Buildings

This section discusses how moisture can become trapped within the microclimates of a home, such as cavities between walls, behind drywall, or inside insulation. Roman shares his personal experience with discovering mold in his house due to a plumbing leak that saturated the slab and created hidden wet zones. The discussion emphasizes that walls are not solid but contain various spaces that can support microclimates, which are often sealed and therefore prone to trapping moisture, leading to mold and potential structural issues.

Tools and Techniques for Detecting Moisture Issues

Roman recounts how a mold assessor used both pin-type moisture meters and pinless devices that use radio waves to measure capacitance and detect moisture behind surfaces like drywall and tiles. Thermal imaging is also highlighted as a useful method, showing temperature variations and cooling effects associated with evaporation. These tools help HVAC technicians identify unseen water issues, enabling them to advise clients to seek further evaluation without necessarily diagnosing mold themselves.

HVAC Responsibility in Moisture and Mold Prevention

The discussion clarifies the HVAC technician’s role in preventing moisture problems. While they are not mold assessors, they are responsible for ensuring that their equipment does not cause leaks or excessive condensation—such as overflowing condensate pans or poorly insulated ductwork. Recognizing water leaks and abnormal moisture conditions can help maintain a healthy indoor environment. Proper application of vapor barriers and understanding of building science principles are crucial to prevent long-term damage and health issues.

Building Science Insights and Practical Advice

Roman introduces principles from building science regarding moisture management, particularly the importance of vapor barriers and drying directions depending on the climate. The conversation emphasizes that moisture problems within walls cannot be resolved solely through HVAC interventions but require understanding of how water interacts with building materials, and when to refer to specialists. Using thermal imaging and moisture meters, technicians can effectively identify problem areas and advise clients accordingly.

Show notes

In this short episode, Bryan and Roman talk about the psychrometrics of a hidden moisture failure... behind the drywall. Roman had an indoor air quality and mold assessment in his own home, and he learned that there were active mold spores in his home; he shares what he learned from the experience.

Roman had a bulk water issue from plumbing (though bulk water issues can also occur outside the building or from condensate drainage problems). Walls are NOT solid; they have plumbing and insulation inside, and the air spaces between the walls support a microclimate when moisture is in there, regardless of whether it's water vapor or bulk water. In Roman's case, the bulk water failure came from the plumbing leak; it saturated the slab for half the house. The water was absorbed by the concrete, and any surface that touched the concrete transferred that water over time.

The mold assessor used two different devices. The pin-type meter punctured the surface of wood or drywall and measured the resistance between the probes (water transfers electrons between them); the measurement revealed the water quantity. A pinless meter determined the source of the failure via high-frequency radio waves, and it measured the capacitance to determine whether moisture was present within or behind the surfaces. The results showed that water was behind tile, baseboards, and drywall.

Thermal imaging has also proven to be a useful tool to see the cooling effect that occurs when water evaporates from liquid to vapor inside a wall, but the cold spots go away when the space thermally equalizes. While HVAC professionals cannot assess mold, we can use tools like thermal imaging to help determine when it's time for a customer to call someone who can help; we are still responsible for our customers' health and comfort in their homes.

Learn more about building science concepts, especially moisture and its effects on buildings and comfort, from the Building Science Corporation's many great resources at https://www.buildingscience.com/.

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Behind the Drywall – The Psychrometrics of a Hidden Moisture Failure - Short #298