Where does all of this moisture go? In a typical code level apartment building with moderate to high levels of air leakage, water vapor has two year-round exit pathways: exfiltration through the façade and dedicated kitchen or bathroom mechanical exhaust. In the summer, moisture is also removed via condensate from the cooling system.
Let’s now put this in the context of a highly energy-efficient apartment with very low levels of air leakage (about 5 to 10 times less than the code-compliant unit), and balanced ventilation with energy recovery. The first means of moisture removal, façade exfiltration, is virtually nonexistent given the building’s superior airtight design. Next is mechanical exhaust ventilation in the kitchens and bathrooms. Because the unit has balanced ventilation and energy recovery, the exhaust airstream in a Passive House project typically passes through the energy recovery core. Depending on the core selection, a large percentage of the interior moisture may be retained in the apartment air despite the constant mechanical air exchange.
There are two basic types of core:
HRV, in which a certain percentage of sensible heat is recovered (transferred from the exhaust airstream to the supply airstream) while no moisture is recovered.
ERV, in which a certain percentage of sensible heat and a certain percentage of moisture in the air is recovered.
Table 1 breaks break down the moisture-related pros and cons of ERVs and HRVs in the context of a high-density Passive House building.
Traditionally, the key factor in deciding between an ERV and an HRV for a high-efficiency building has been the project’s climate. However, as internal moisture loads begin to exceed exterior moisture loads in high-density projects, the decision between ERV and HRV must be looked at more closely for each project regardless of climate.
Our Passive House team at Steven Winter Associates, Incorporated, closely studied this dynamic relationship between interior humidity levels and the ERV vs. HRV decision for a 277-unit high-density, affordable housing Passive House project in New York City. The project team originally elected for decentralized ERVs, one unit per apartment, to reduce summertime cooling loads, resulting in utility savings for the buildings’ tenants. Given this decision, we underwent a major modeling effort to assess the risk of high wintertime interior humidity levels, an ingredient for increased condensation potential on the project’s windows. Two primary questions needed to be answered:
Will interior relative humidity (RH) levels get high enough in the wintertime to cause condensation potential on the windows?
Is the ERV’s airflow boost capacity enough to mitigate high interior humidity levels?
This modeling exercise yielded the following takeaways:
Takeaway #1: Weekday interior RH levels will peak anywhere from 50% to 63% in the morning and early evening. The largest peak will occur during the morning breakfast hours.
Takeaway #2: Weekend interior RH levels will remain between 50% and 70% over the course of the day.
Takeaway #3: Boost flow on the ERV during high-humidity conditions will only slightly reduce peak humidity levels.
Takeaway #4: Due to high predicted interior humidity levels, localized supplemental dehumidifiers may be required.
Takeaway #5: Some ERV cores can have a much greater moisture recovery rate in the winter season than in the summer. These seasonal differences are not typically reported by manufacturers and should be confirmed for each project.
As the project progressed, a decision was made to switch to four centralized ERVs. The model was then revised to account for a normalized amount of air mixing that would occur with the centralized design. See Figure 1 for a schematic of this concept. In other words, not all apartments will be generating high levels of moisture at any given time. Therefore, the supply air going back to each apartment will result from a blending of the average humidity conditions across the project’s 70 units that each centralized ERV services.
The revised moisture model, which was based on using centralized ERVs, yielded a noticeable reduction in apartment interior RH when compared to the decentralized model. On a typical weekend day in winter, the model was predicting that average apartment interior RH percentages would remain in the low to mid-50s for only a few hours during those times when domestic moisture generation peaks.
While the centralized ventilation approach largely reduces the risk of high moisture levels in the apartments, additional controls are being specified for the centralized ERV units to further alleviate this risk. During periods of cold exterior air temperatures and high return air humidity levels, the ERV will have the ability to partially bypass the enthalpy wheel to flush the moisture out of the apartment air. Figure 2 depicts the substantial effect of incorporating this ERV cotrol.
