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Technician checking duct temperatures. Carbon monoxide monitor at a furnace.

Mold - Mildew - Humidity - Condensation Experts!


It is not uncommon in new buildings to see the formation of mold and mildew. How can this happen?

Over the years I have observed the mold formation phenomenon resulting from a variety of causes, but generally all come down to poor construction technique. In one case, the staged drywall was allowed to get wet. In another, lined ductwork was left out in the rain before installation.

In each case, the construction materials were allowed to get damp or wet. After installation, the air conditioning was turned on, and the materials were exposed to low dewpoint temperature air. The result: dampness became liquid water, and spores grew into mold.


Most know that today's double pane windows save energy. But other factors contribute to the satisfactory performance of the window, including whether the window frame creates condensation.

In cold climates it is important to install a metal frame window that is "thermally broken". This means a frame with physical discontinuation between the exterior and interior. If the frame is not thermally broken, that is, continuous from cold outside to warm inside, the inside of the frame will be cold. That cold surface will cause condensation of airborne moisture in the space. Eventually, that "sweating" on the frame interior will cause the formation of mold and mildew.

Thermally broken metal frame windows should be installed in any climate where the winter temperature regularly falls to below 55 F.


In a 6 month old building, metal surfaces were rusting, and mold was everywhere. Vinyl wall coverings were coming unglued. What was the cause?

The air conditioning system in this building employed small (5 tons or less) direct expansion units. The fans units ran continuously, and included fixed outside air intakes. All of the equipment was sized for a peak daytime cooling load with high outdoor temperatures and sun. In many cases two small cooling units served a single space.

We took temperature and humidity measurements, and found that the indoor humidity level was well above 60%. I theorized that during moderate weather, the compressors would cycle on temperature, but the high humidity outdoor air would continue to be introduced to the space, as the fans ran all the time. When a compressor would come on, the cold air from the system would cause condensate of the highly humid air.

Our solution was to disable half of the cooling units. This allowed for longer operation of the remaining equipment, allowing for better dehumidification. After about an hour in this operational mode, the indoor humidity dropped from 63% to 51%, solving the problem.

After confirming the solution, we made some duct and temperature controls modifications, and the humidity and mold problem was solved.