Due to energy code changes, better enforcement, and as a matter of good practice, we are making buildings more and more air tight. Humidity and condensation control in buildings
go hand in hand with exterior enclosure, air barriers and mechanical ventilation. This paper will focus on the increasing number of interior wall assemblies that are experiencing
excessive moisture damage due to combination of moisture diffusion and condensation. We discuss lessons learned from investigation and analysis of projects in California, where
we utilized WUFI model studies as well as actual data logger information.

Understanding and predicting moisture movement within and through exterior wall systems is one of the most important factors effecting envelope performance today. Condensation and moisture diffusion damage can occur with or without designed air barriers and it is important to understand the role ventilation plays in the design. This paper will include forensic studies of moisture movement and condensation control where data loggers were used to measure moisture movement alongside WUFI analysis of actual assemblies in occupied buildings. This allows us to understand how buildings behave in real time versus design parameters. The discussion will include mechanical ventilation involved in condensation control, including make-up air and exhaust
mechanisms.

This paper will also provide an analysis of code requirements for ventilation and actual system performance. The nature of these moisture intrusion failures includes construction defects and changes the industry has made for energy efficiency including air barriers and vapor retarders.

CASE STUDY #1

In 2017, Allana Buick & Bers, Inc. (ABBAE) was retained to investigate reported leaks and water intrusion at a 186-unit apartment complex in San Ramon, CA. The five-building, three-story apartment complex was built 2011. The wall assembly consists of wood sheathing over wood framing, two layers of 60-minute weather resistive barrier, and a three-coat stucco system.

CASE STUDY #1 – BACKGROUND

There were reports of mold and moisture damage on the interior walls of one stack of three units. Water damage and biological growth were reportedly uniform in nature and varied between heavy damage on the 1st floor and less damage on the 3rd floor. Management believed there was a deficiency with the cement plaster so they hired a contractor to replace all the plaster assembly in a three-story high unit stack of the façade. The interior and exterior were fully remediated prior to ABBAE’s involvement so the original evidence was lost. Approximately one year before our investigation, the same stack of units that were originally remediated failed, repeating the development of mold and biological growth on the inside face of the exterior walls. In the second go-around, the client and contractor hired us to perform an investigation to understand the factors that were not understood at the time.

As part of our initial study, we were given access to eight apartment units and we performed visual inspections of the interiors. All the units selected for visual inspection were vacant units.

We were not given access to the original stack of units that had the previously repaired damage. Out of the eight units that we were given access to, we observed damage in one unit in Building 1500, at a wall that matches the same elevation at the Northwest corner as the previously  repaired unit stack in Building 1400. Our staff documented high moisture readings (96% RH and 73-degree temperature) in this unit even though it had been vacant for several years.

We began by investigating around the windows (as is our standard protocol). Next we investigated the corner areas of the units. The contractor who did the original repairs suspected that the gutter penetrations were causing the water intrusion. Once we noticed how extensive the damage visible damage was, we extended our investigation.

We opened up gypsum board in the various units available to us and observed severe damage to the exterior oriented strand board (OSB) sheathing. Based on our visual observations, we identified several potential sources of wall and window leaks and proceeded to perform water testing to identify the source of the leaks. We removed of the interior gypsum board and observed severe damage inside the wall cavities. Most of the damage was on the backside of the exterior wood sheathing as seen below.

ABBAE staff spent 2 days performing extensive water testing and invasive forensic testing of the windows and wall assemblies. Windows were tested under ASTM E-1105 under differential pressure and the exterior stucco façade was tested with a spray rack with no differential pressure however, we did not observe any liquid water penetrating the interiors of these units.

We tested wood moisture levels with Delmhorst probe before, during and after the 6 hours of testing, the moisture levels did not change.

Based on not finding any water or moisture directly coming through the stucco walls, we investigated the slabs on grade to see if they were contributing to the elevated humidity levels. We performed concrete coring and calcium chloride testing of the slabs on grade to understand if the under-slab vapor barrier was installed and to check for water travelling under the slab. Calcium chloride tests indicated an elevated moisture content of between 3.7 and 5.7 pounds/1000 square feet.

Our investigation found that the foundation wall was allowing water from the landscape irrigation system to migrate under the slab, increasing vapor movement through the slab and potentially contributing to high interior humidity. Based on water testing the foundation wall, liquid water migrated on top of the vapor barrier (as seen in Graphic 2). We observed improper termination of the under-slab vapor retarders.

While both the stucco assembly and the interior slab were not causing direct water intrusion in the units, they were contributing to an increase in interior humidity levels. The original design of the HVAC system showed that the building was designed to have a fresh air intake duct connected to the fan coil unit. We wanted to fully understand the source of the continued high humidity in the units so we installed data loggers to track temperature, humidity and actual moisture levels in exterior sheathing of several units. 2 to 3 data loggers were installed in 3 units both inside the wall cavity as well as within living space to measure ambient levels. We also installed data loggers in the exterior covered hallways to measure exterior temperature and humidity levels. Analysis of the exterior data loggers allowed us to pinpoint areas of high moisture and humidity where mildew and mold can grow. We also calculated moisture contribution from human activity as part of our investigation. We finally, used WUFI modeling to analyze the as-built and as-designed conditions.

Based on the data logger information showing high humidity and poor air circulation, we decided to further study the individual unit’s HVAC system.

Our investigation included; review of the original mechanical construction documents, onsite verification of the mechanical systems installed, air flow measurement of the bathroom exhaust fans, air flow measurement of the fan-coil unit outside air intakes, and blower door air barrier tests. The construction drawings indicated a fresh air duct connected to the interior fan coil unit. We performed additional destructive testing to verify if the fresh air duct had been properly installed and if it was functioning properly.

We found that the installed indoor fan coil system did not comply with the existing mechanical construction documents. The system did not allow for a fresh air duct to be connected to the fan coil unit. The fresh air intake duct as designed by the mechanical engineer was not installed. Had the ducted fresh air inlets been per design, it would have resulted in a ventilation rate of 1.5 to 1.7 Air Changes per Hour (ACH).

Furthermore, the bathroom exhaust fans were found to be undersized and also did not match the mechanical construction documents’ specifications. The original design exhaust fans were specified to flow at 110 cfm (two exhaust fans per residential unit). Our calculations indicate that a minimum 1.5 air ACH can be achieved with the originally designed 110 cfm fans (at full rated flow). The 80 cfm units installed can only achieve 0.8 ACH at full rated flow as built.

CASE STUDY #1 – WUFI MODELING

WUFI (Wärme Und Feuchtetransport instationär or Transient Heat and Moisture Transport) is a software suite designed to realistically calculate heat and moisture transport in multi-layered building components, like exterior walls. It simulates the accumulation and dissipation of moisture through the building materials over a period of time.

We began our WUFI hygrothermal analysis by modeling the existing construction using project specific interior and exterior climate data.

WUFI MODEL – AS-DESIGNED

This model shows the hygrothermal behavior of the building exterior as designed. If the mechanical ventilation had been installed per the construction documents, this model would be expected to match the collected data logger information.

We chose the north facing elevation for all our WUFI runs because north facing elevations typically take longer to dry out and are more exposed to rain and environmental conditions.

The as-designed model uses the following parameters:

The as-designed model generated the following outcomes:

The results of the as-design WUFI Run were good. This raised a red flag since the results did not match field measurements of the sheathing moisture content and observed mold growth.

While investigating this discrepancy, we discovered that the ventilation systems were improperly installed. This also led us to run additional WUFI models to include alternative sources of moisture to account for actual conditions per the data logger information.

WUFI MODEL – RUN 1 (AS-BUILT)

WUFI Run 1 models the hygrothermal behavior of an exterior, as-built, north facing wall assembly. We modeled the ventilation based on the understanding that the mechanical ventilation designed for the project had not be properly installed. Therefore, the air changes per hour we used were significantly below the original design intent.

This model also added to the interior source moisture to account for occupant activity since this issue was not isolated to the vacant units investigated.

This model uses the following parameters:

This model generated the following outputs:

The graphic below shows the mould (or mold) index for this building exterior configuration. The mold index predicts mold growth based on the building materials, temperature, and relative humidity. A mold index below 1.0 indicates low mold growth probability. With a mold index of 1.0, this model indicated a medium probability for mold growth.

The graphic below shows the modeled moisture content of the OSB sheathing. We can see that the OSB moisture content went as high as 15% during the winter season according to the model.

The WUFI results for Run 1 look good, but do not represent actual site conditions. This is an example where “typical or default” input into the software will output results that do not represent real-world conditions.

Based on our data loggers, we were able to monitor changes in moisture and humidity conditions before and after rain events. Changes in humidity during a rain event allowed us to model how much “leakage” was occurring through the exterior wall assembly. While the leakage was not in liquid form passing through the interior finishes, there was moisture diffusing through the exterior sheathing due to the weather resistive barrier getting damp from leakage through the stucco. Damp building paper was acting like a wet towel against the OSB sheathing and the moisture gradually absorbed through the OSB and started to dry through the wall cavity. Humidity levels in the stud cavity increased and diffused through the gypsum board and increased the interior humidity levels.

Based on the data loggers, we were able to adjust the WUFI model to include an exterior wall vapor permeability (or air “leakage” rate) to model the actual conditions. Once wall leakage rates were known, we could model how much ventilation was needed to dry out the high humidity levels inside the apartments to mitigate condensation. We modeled a number of ventilation rates including those mandated by building code to see if it was adequate to mitigate condensation. We also modeled the ventilation as designed by the mechanical engineer to see if that would adequately mitigate the interior condensation.

WUFI MODEL – RUN 2

In Run 2 we modeled the impact of the reservoir cladding by adjusting the leakage rates of the stucco and the concrete floor slab until the WUFI model mirrored the site conditions established by data loggers, moisture meters, etc. A reservoir
cladding is a cladding that stores rainwater such as stucco, brick, wood, etc.

This model shows the hygrothermal behavior of the same exterior, as-built, north facing, wall assembly and uses the same parameters and Run 1 – with the addition of exterior source moisture to account for the air leakage.

This model uses the following parameters:

This model generated the following outputs:

The OSB moisture content for this model peaked at 18% during winter seasons. The mold index for this model peaked at over 1.7, indicating a medium probability for mold growth.

The results of WUFI Run 2 represent the final output from numerous incremental adjustments in exterior source moisture. Run 2 results matched the conditions we verified within the test units for moisture content monitored by the data loggers
over time.

WUFI MODEL – RUN 3

Once we were confident that our WUFI model was replicating the real-life existing conditions occurring at the project, we began running cases that implemented corrective solutions. Run 3 models the units with added exhaust and with new fresh
air intake as components of the repair solution.

This model uses the following parameters:

This model generated the following outputs:

This model indicates that with the higher ventilation, there is no probability for mold growth. The mold index for this model is 0.0005, well below the index 1.0 threshold. The OSB moisture content for this model peaked at a safe 14% during winter seasons.

The results of WUFI Run 3 represents the final output from numerous incremental adjustments in the air changes per hour. Run 3 results indicate that the moisture issues can be totally addressed through mechanical ventilation alone.

The analysis shows that by maintaining residential unit ventilation at a minimum of 1.5 [ACH] a significant reduction in moisture accumulation and moisture related damage to the exterior wall components would occur. We concluded that the construction defects introducing moisture through the exterior walls and floor slabs, combined with missing ducted fresh air intakes and undersized and underperforming exhaust fans, is what was causing the high levels of moisture to build up in these units. The high humidity caused condensation in the exterior walls during the winter, causing significant damage.

WUFI MODEL – RUN 4

We then began running cases where a hybrid approach of controlling the moisture load into the unit in addition to improvements in the ventilation. The impact of the stucco cladding as a moisture source was addressed by modeling the stucco installed in a rain-screen configuration.

Run 4 models the hygrothermal behavior of the exterior, rain screen configured, north facing, wall assembly. Since this model is already changing the building structure by adding a rain screen, we also modeled the planned upgrading of the sheathing system to exterior grade plywood since it provides superior durability than OSB.

The rain-screen WUFI modeling is based on Modeling Enclosure Design – 2016 by Building Science Corp.

This model uses the following parameters:

This model generated the following outputs:

The mold index for Run 4 was 0.35, well below the 1.0 index threshold. This indicates no probability for mold growth with this configuration. The moisture content of the OSB sheathing moisture content was 15% during the winter season.

The analysis shows that by controlling the exterior source moisture, the ventilation requirements to manage moisture were dramatically reduced. This illustrates the need for reservoir claddings to be installed in a rainscreen configuration over vapor
permeable water resistive barriers. Otherwise, the ventilation design for the space needs to anticipate the impact of exterior source moisture and be designed accordingly.

CASE STUDY #1 WUFI SUMMARY

CASE STUDY #1 – VENTILATION SOLUTIONS

Improvement to the existing ventilation was proposed in two options. The goal was to achieve the best performance possible combined with those repairs having the least impact on each rental unit and managing cost.

Option 1: (Preferred Solution)

    • Ducted supply to existing fan coil unit
      • From covered walkway side of building
      • With in-line blower fan
    • Controls with humidity sensor
      • To trigger fan coil, in-line fan, and bathroom exhaust fan
    • Larger bathroom exhaust fan
      • Existing 4” duct to remain

Option 2:

    • Larger bathroom exhaust fan
      • Existing 4” duct to remain
    • Controls with humidity sensor
      • To trigger bathroom exhaust fan
    • Passive outside air vent
      • From outside of building, ideally into at least 2 rooms

CASE STUDY #2

In 2015, ABBAE was retained to investigate water intrusion at a 60-unit apartment complex in Redwood City, California. The complex was originally built in 2006 and  consists of two three-story apartment buildings and a single-story clubhouse. The wall assembly consisted of wood framing, wood sheathing, exterior gypsum board, and stucco or panel siding, depending on the elevation or floor.

CASE STUDY #2 – BACKGROUND

Water intrusion was reported by the building owners at 12 of the residential units and at several areas of the Clubhouse. ABBAE performed interior visual surveys to review damage at windows, walls and private balcony decks. As part of this investigation, we pulled carpet back to observe any damage to carpet tack strips. There was very little damage found in the unit interiors during visual observations, limited to damaged carpet tack strips under a few windows.

As part of our building investigation, we removed the interior gypsum board and batt insulation in several units as part of our protocol for water intrusion testing of the windows and walls. In one unit stack in particular, dark staining and biological growth was observed on the backside of the wood sheathing and batt insulation prior to testing the windows.

Upon removal of the interior gypsum board in unit 209, we still did not observe much damage. It was once the batt insulation was removed that damage was uncovered on the backside of the wood sheathing. The wood sheathing had biological growth on the surface and there were severely corroded fasteners through the exterior wood sheathing.

We performed several water intrusion tests on the exterior wall and no leaks were found on the interior to correlate to the extensive damage found.

Observations on the exterior showed possible signs that water could get behind the siding and stucco panels (leakage to the WRB). We observed open sealant joints, window head flashings that were improperly installed, and open transitions between siding and stucco. We proceeded to remove layers of the wall assembly in search of leak sources. Removal of the siding panels exposed the weather resistive barrier over the exterior gypsum board. Removal of the weather resistive barrier exposed an exterior gypsum board (DensGlass) with a fiberglass facer that did not show signs of damage. The fasteners, however, were severely corroded when pulled out of the gypsum board indicating that while the sheathing is capable of years of wetting and drying cycle, the fasteners were obviously not.

The overdriven and corroded fasteners indicated that the board had been wet for long periods of time. We removed the gypsum board and exposed the wood sheathing, which had heavy staining and damage. The damage was most evident around windows, where water was most likely to get behind the siding. The gypsum board served as a “reservoir” for water, preventing direct water intrusion to in the interior finishes, despite the fact that the water had gone through the designed and installed weather resistive barrier.

We proceeded to install data loggers in several units’ wall cavities around the property to measure humidity, temperature, dew point and moisture in the wood sheathing. Correlating the data with weather events showed that the humidity and moisture increased during heavy rains. There was also some correlation of higher moisture with North facing units that were shaded for larger portions of the day. Below is part of a data set that analyzes the effect of weather events in determining moisture levels inside the units. The cumulative nature of multiple rain events during the winter months increased the humidity inside the units to the point where mold and mildew were allowed to grow.

CASE STUDY #2 – WUFI MODELING

We began our WUFI hygrothermal analysis by modeling the existing construction using project specific interior and exterior climate data.

WUFI MODEL – AS-DESIGNED

This model shows the hygrothermal behavior of the building exterior as designed. If the mechanical ventilation had been installed per the construction documents, this model would be expected to match the collected data logger information.

As with Case Study #1, we chose the north facing elevation for all our WUFI runs because north facing elevations typically take longer to dry out and are more exposed to rain and environmental conditions.

The as-designed model uses the following parameters:

This model generated the following outputs:

The results of the as-design WUFI Run were good. Just like Case Study #1, this raised a red flag since the results did not match field measurements of the sheathing moisture content and observed mold growth.

While investigating this discrepancy, we discovered that the ventilation systems were undersized and underperforming. This also led us to run additional WUFI models to include alternative sources of moisture to account for actual conditions per the data logger information.

WUFI MODEL RUN – 1

Run 1 models the hygrothermal behavior of an exterior, as-built, north facing, wall assembly.

We modeled the ventilation based on the understanding that the mechanical ventilation designed for the project was underperforming. Therefore, the air changes per hour we used were significantly below the original design intent.

This model uses the following parameters:

This model generated the following outputs:

The mold index for this building exterior configuration. The mold index predicts mold growth based on the building materials, temperature, and relative humidity.
A mold index below 1.0 indicates low mold growth probability. With a mold index of 1.7, this model indicated a medium probability for mold growth.

The modeled moisture content of the OSB sheathing. We can see that the OSB moisture content went as high as 18% during the winter season according to the model.

WUFI MODEL RUN 2

Run 2 models the hygrothermal behavior of an exterior, as-built, north facing, wall assembly.

We modeled the impact of the reservoir cladding by adjusting the porosity (and leakage rates) of the stucco until the WUFI model mirrored the site conditions established by data loggers, moisture meters, etc.

This model uses the following parameters:

This model generated the following outputs:

The OSB moisture content for this model peaked at 18% during winter seasons. The mold index for this model peaked at over 1.8, indicating a medium probability for mold growth.

The results of WUFI Run 2 represent the final output from numerous incremental adjustments in exterior source moisture. Run 2 results matched the conditions we verified within the test units for moisture content monitored by the data loggers
over time.

WUFI MODEL RUN – 3

We initially began running WUFI cases with ventilation improvements only. We started with the ‘as originally designed’ ventilation as a starting point. We adjusted the ventilation further, increasing air exchanges, until we achieved performance
that managed the impact of moisture and significantly reduced mold potential.

Run 3 models the hygrothermal behavior of an exterior, as-built, north facing, wall assembly.

This model uses the following parameters:

This model generated the following outputs:

This model indicates that with the higher ventilation, there is no probability for mold growth. The mold index for this model is 0.0005, well below the index 1.0 threshold. The OSB moisture content for this model peaked at a safe 15% during
winter seasons.

The results of WUFI Run 3 represents the final output from numerous incremental adjustments in the air changes per hour. Run 3 results indicate that the moisture issues can be totally addressed through mechanical ventilation alone.

The analysis shows that by maintaining residential unit ventilation at a minimum of 1.5 [ACH] a significant reduction in moisture accumulation and moisture related damage to the exterior wall components would occur. We concluded that the
construction defects introducing moisture through the exterior walls and undersized and under-performing exhaust fans, is what was causing the high levels of moisture to build up in the units we investigated. The high humidity caused condensation in the exterior walls during the winter, causing significant damage.

WUFI MODEL RUN – 4

We then began running cases where a hybrid approach of controlling the moisture load into the unit as well as the ventilation. The impact of the stucco cladding was addressed in two corrective approaches. One was to review the impact of incorporating a moisture absorption reducing coating and the other was reviewing the impact of the stucco being installed in a rain-screen configuration. This approach allowed a reduction in ventilation rates.

As with Case Study #1, we upgraded that the sheathing system to exterior grade plywood since it provides superior durability than OSB and we are already adding a rainscreen system.

Run 4 models the hygrothermal behavior of an exterior, rain screen configured, north facing, wall assembly.

Note that rain-screen WUFI modeling based on Modeling Enclosure Design – 2016 (by Building Science Corp.)

This model uses the following parameters:

This model generated the following outputs:

The mold index for Run 4 was 0.425, well below the 1.0 index threshold. This indicates very low probability for mold growth with this configuration. The moisture content of the OSB sheathing moisture content was 15.5% during the winter season.

The analysis shows that by controlling the exterior source moisture, the ventilation requirements to manage moisture were dramatically reduced. This illustrates the need for reservoir claddings to be installed in a rainscreen configuration over vapor
permeable water resistive barriers. Otherwise, the ventilation design for the space needs to anticipate the impact of exterior source moisture and be designed accordingly.

CAST STUDY #2 WUFI SUMMARY

BUILDING CODE REQUIREMENTS

Designing to building code will not always prevent condensation and incidental moisture.

The International Building Code (IBC), and therefore the California Building Code, requires vapor permeable water-resistive barriers be installed over wood sheathing in two independent layers. Further, the layers must be installed such that each layer provides a separate continuous plane and that any flashing intended to drain to the water-resistive barrier is directed between the layers.

IBC Section 2510.6: Water-Resistive Barriers reads as follows:

2510.6 Water-resistive barriers. Water resistive barriers shall be installed as required in Section 1403.2 and, where applied over wood-based sheathing, shall include a water-resistive vaporpermeable barrier with a performance of at least equivalent to two layers of water-resistive barrier complying with ASTM E2556, Type I. The individual layers shall be installed independently such that each layer provides a separate continuous plane and any flashing (installed in accordance with Section 1404.4) intended to drain the water-resistive barrier is directed between the layers.

Exceptions:

    1. Where the water-resistive barrier that is applied over woodbased sheathing has a water resistance equal to or greater
      than that of a water-resistive barrier complying with ASTM
      E2556, Type II and is separated from the stucco by an
      intervening, substantially nonwater-absorbing layer or
      drainage space.
    2. Where the water-resistive barrier that is applied over woodbased sheathing in Climate Zone 1A, 2A or 3A, a ventilated
      air space shall be provided between the stucco and the
      water-resistive barrier.

This revised application requirement creates reverse laps within the water resistive barrier and more opportunities for trapped water within the wall system. Water will collect at locations where the water-resistive barrier bonds directly to the plaster. Water is also held between layers at crinkles, folds, creases, and puckers, of the water-resistant barrier.

Additionally, by directing the water between the two layers, instead of over the two layers, the Code has effectively reduced the protection provided from two layers down to a single layer of Grade D paper standing between the water and the exterior sheathing.

There are two exceptions allowed by the California Building Code that allow the installation of a single layer of water resistive barrier. The first exception is the use of an intervening substantially non-water-absorbing layer or drainage space and the second is a ventilated air space. Both of these exceptions are superior solutions to the base requirement.

Ventilation: The Building Code does not require taking into account moisture movement through exterior walls. Exterior source moisture entering the wall assembly is not specifically addressed by standard ventilation design.

CONCLUSIONS

These case studies illustrate the real-life impact of cladding performance on the moisture movement and management within building enclosures. The impact of not properly controlling the amount of wetting of the wall assembly is exhibited by the moisture readings, biological growth and rot shown throughout the photos included. The case studies also demonstrate the impact of interior air exchanges in controlling moisture build up within a building.

Traditional claddings installed compactly over weather resistive barrier(s) and sheathing, as allowed by the Building Code, leads to excessive moisture trapped between the cladding and WRB and result in the issues illustrated within this paper. The water absorbed by the reservoir claddings transfers through each layer of the assembly by capillary action, absorption, diffusion, and/or air exchange. This is compounded when the cladding is highly absorptive and has a significant moisture storage capacity (e.g. gypsum sheathing and stucco.)

Additionally, leaks that allow water through the cladding (installed compactly over weather resistive barriers,) will be stored within the WRB and between the WRB and the cladding and sheathing.

Managing Moisture: One method of controlling the vapor transport in to a wall assembly with a reservoir cladding is with a vapor control layer that blocks that moisture movement. This control layer is installed between the cladding and the sheathing. However, in climates where an impermeable control layer cannot be used, the introduction of a rain screen application or an air space or a drainage layer between the cladding and the waterresistive barrier is the best option to control moisture absorption and diffusion, provide drainage, and allow drying air to circulate from within the wall assembly. Based on the climate conditions of the two case studies – drying is desired in both directions.

Installing the cladding in a rainscreen or drainage configuration (see Graphics 10 and 11 above) enhances drainage of bulk water and can allow ventilation behind the cladding. This ventilation will allow airflow behind the cladding that will accelerate drying. The efficiency of the ventilation requires combining the potential for air movement with vents at both the top and the bottom of the wall. The stack effect and wind pressure differentials will effectively move air through the air space.

Building Code Impact: The benefits of a drainage layer or air gap between reservoir claddings and water-resistive barriers has been understood for some time. However, the current California Building Code not only ignores the need to uncouple this assembly, but has recently reduced the water-resistive barrier performance with revised installation requirements.

Ventilation required by the California Building Code does not require including moisture load from the building enclosure in the engineering calculation. If the proposed design allows significant moisture gain within the building that is not properly addressed with ventilation, then high moisture content, humidity, and biological growth can be expected. With building being more efficient and air tight, there is an increased need for less incidental leakage in the exterior wall assembly. Furthermore, while increasing ventilation can reduce the harmful impact of mold formation, it will continue to damage the exterior sheathing and additional ventilation will increase energy consumption.