Air sealing LV conduit

A homeowner forum post describes a problem worth knowing about if your house has wiring run in flexible plastic conduit (often called "Smurf tube") between an indoor equipment area and an outdoor fixture, such as a basement AV cabinet wired to an outdoor TV mount. In the case described, water staining showed up around a low-voltage electrical box in the basement cabinet. Investigation found no leak in the wall itself. Instead, the conduit carrying HDMI, network, and audio cables ran from the basement box to an outdoor box, with both ends left open to the air. Warm, moist indoor air apparently traveled through the open conduit, hit cooler air near the outdoor end, and condensed, with the moisture dripping back down into the indoor box.

This kind of hidden air path can develop in newer, well-built houses too, since conduit for cables is often not sealed the way conduit for standard house wiring might be. The person investigating also suspected that some piece of mechanical equipment in the basement might be creating a slight negative pressure, pulling outdoor air through the tube and making the airflow worse.

If your home has similar low-voltage wiring running through conduit between conditioned space and the outdoors, unexplained moisture near an electrical box is worth a closer look. The underlying issue is an unsealed air pathway through the wall, not a plumbing or roof leak, and it's the kind of gap that can go unnoticed in an otherwise tight, energy-efficient house.