Passive Housed Durability – Condensation

When we think of Passive Houses, the first things that usually come to mind are airtightness (0.6 ACH50), above-average insulation levels, and whole-house ventilation. However, there are some additional subtleties to Passive House certification that are often overlooked.

One of those subtleties that can be problematic is the Minimum Temperature Factor. This is a unitless value that indicates the likelihood that a location in the building has the potential to form condensation. Short-term condensation is generally not a problem. Long-term condensation, which might last for a few weeks, can eventually lead to mold, mildew, and potentially the rotting of building components.

Table 1 shows the Minimum Temperature Factors for the seven climate zones defined by the Passive House Institute. These numbers, called fRsi values, describe the internal local temperature in relationship to both the room and external temperatures. A value of one means that a location has the same temperature as the room, typically 68 degrees Farenheit. A value of zero means that the location has the same temperature as outside. To minimize condensation-related damage, you want the interior temperatures to be as close as possible to that interior setpoint.

fRsi values are typically lower than you would like when there is a thermal bridge between the inside and outside of a building. If you have a well-insulated building, with some of the insulation wrapping the outside, the only potential trouble spots are most likely going to be door and window edges.

To make sure that there are no long-term moisture issues, fRsi calculations must be done for all door and window types in a certified Passive House. If you install PHI-certified doors and windows, you can usually get fRsi values from the manufacturer. Otherwise, someone on the project team has to derive fRsi values. This is typically done using finite-element software like THERM or Flixo.

In a Warm or Warm-temperature climate zone, like much of California, dipping below the recommended fRsi value may not seem like a cause for concern. However, that’s not necessarily the case. Condensation can form at fairly high temperatures depending on the indoor relative humidity.

Here’s a simple example from a project in San Luis Obispo, a fairly Warm location. The client insisted on a specific bifold patio door. At first glance, this looks like a pretty good door—it’s got triple-pane glazing and decent gasketing. However, it’s got an aluminum frame and is mounted on a concrete slab. The combination of aluminum and concrete clearly screams out “THERMAL BRIDGE”–aluminum is a notorious thermal conductor. It’s generally not suitable for a Passive House door or window frame.

The initial fRsi analysis, shown in Figure 1, doesn’t look very good. The fRsi value is 0.30, clearly too low. The second analysis, which includes exterior insulation that covers the bottom edge of the threshold, doesn’t do much better at 0.31 as show in Figure 2.  The best fRsi obtainable using other measures such as increasing the insulation levels and insulating under the threshold, are only able to achieve a 0.35 fRsi (not shown). These results indicate that this door is not suitable for a Passive House.

Changing the aluminum frame to uPVC, adding rigid insulation under the threshold, extending the exterior insulation below grade, and filling the frame members with foam results in a fRsi value of 0.51, as shown in Figure 3. This is still below the minimum threshold for a warm climate zone. This poor value is partially due to the large air gap between the threshold and the door frame, suggesting that the door design, not just the choice of aluminum. In fact, the fRsi notation in Figure 3 points to the gap, indicating the location of the lowest temperature.

If you want to avoid certification issues when designing a Passive House, it’s important to pay close attention to all thermal bridges, especially those around doors and windows. Avoid high-conductance materials, and consider using only Passive House-certified doors and windows. This issue should be considered early in the design stage. Once you order doors and windows, it can be very difficult to make changes.


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