Pressurization Plays An Important Role In Avoiding Mold

An island resort suffered moisture and mold damage in the guest room buildings. The damage was caused by warm, humid air being drawn into the conditioned spaces (from outside the conditioned space) and condensing on the cooler interior surfaces, including gypsum drywall and wood case goods. The source of the warm, humid air was air coming from the crawl spaces, attic spaces, and the outdoors (above grade). This warm, humid air was drawn into the conditioned spaces because the buildings were under negative pressure, induced by the toilet exhaust air fans, which then resulted in outdoor air entering the building. The negative pressure was the result of an HVAC system that did not consider the requirements for this type of building in its tropical climate zone, and it did not provide a source of conditioned air to replace that air.

Pressure Requires Pathways for Air to Flow

There are always pressures acting on and within buildings, but unless there are openings or holes, those pressures will not cause air to flow. The pathways for this humid air into the conditioned spaces included holes in the wall cavity side of the gypsum drywall, unsealed penetrations and cracks in the exterior envelope, the first-floor construction of old abandoned electrical outlets and conduits, new electrical work, the concrete masonry unit (CMU) demising walls, and openings to the attic in the second-floor walls and ceilings.

The above figure is a diagrammatic cross section of a typical first floor guest room and crawl space (not to scale). The crawl space is unvented and unconditioned and has an uncovered earth floor. The crawl space is warm/hot and humid. Negative pressure occurs in the guest room due to a bathroom exhaust fan that pulls air from the space and discharges it to the outside air (blue arrows). The negative pressure in the guest room draws hot humid air from the crawl space through pathways such as unfilled CMU cells and old electrical conduits (red arrows). The guest room is cooled by a fan coil unit (FCU). The warm, humid air condenses on the cooler drywall surfaces (blue water drops). The condensed moisture is absorbed into the paper facing of the drywall and results in moisture damage and mold growth.

Additionally, the first floor deck structure (plywood deck and joists) is cooled by exposure of the topside of the deck to the guest room, which is cooled by the FCU. There is no insulation under the plywood floor deck, so the cool surface of the plywood is exposed to the crawl space atmosphere. Warm, humid air in the crawl space condenses on the underside of the cooler first floor, resulting in moisture damage (dotted blue line).This same mechanism occurs on the upper floor as these same pressure forces act on the openings and holes at the attic to upper floor ceilings.

How Individual Exhaust Fans Become Continuous Exhaust

The HVAC system design for the guest room consists of a variable refrigerant flow (VRF) system of FCUs that are installed on the wall between the bathroom and bedroom area. This FCU is a ductless unit, which means it has no air duct or air outlet connections and therefore has no provision to receive an outside duct connection. The bathrooms are exhausted with individual fans controlled by a light switch, occupancy sensor and relative humidity sensor. The control of the exhaust fan included several sensors including a built-in humidity sensor that detects rapid rise in relative humidity (RH), automatically turning on the fan to remove moisture and helping to prevent mold and mildew. Typical of many guest rooms in tropical resorts, RH will rise and stay elevated because the guests will set the FCUs to very cold temperatures.

This is because the guest is trying to make the room feel comfortable in a room that often feels damp and clammy. When the guest drives the FCU temperature setpoint lower, it can drive RH higher as the FCU is able to make the room cold but not remove sufficient moisture through dehumidification. This caused the fan to operate nearly continuously, exhausting 80 cubic feet per minute (cfm) from the room without any make up air to offset the exhaust. This generated consistent negative pressure between the room and the outdoors. Currently, building code requires only 50 cfm if the bathroom fan in a guest room is intermittent (i.e., an individual exhaust fan) and 25 cfm if the bathroom is exhausted on a continuous basis. Inadvertently, the fan insulated in this room because of the built in humidity sensor, resulted in a room that was being exhausted at a rate that was more than three times the 25 cfm requirement.

Openings From Top and Bottom

Moisture damage led to mold growth on the cavity side of drywall paper facing because on the first floor, there were holes between the wall cavity and crawl space below and there were holes between the upper floor wall cavity and the attic above. Note apparent mold growth is at an old abandoned outlet which is visible behind the piece of removed drywall. Abandoned outlets as well as CMU cells were pathways for humid air from the crawlspace and attic, and condensation, as well as resultant moisture damage that led to apparent biological growth, would occur first on drywall surfaces near the humid air sources.

The ground surface in the crawl spaces was bare earth, without any presence of a vapor retarder. Without a vapor retarder, the moisture from the ground can easily enter the crawl space environment, creating conditions that can be more humid than the already humid outdoor (ambient) air.

Because the impact of air flow that travels into a guest room can be far reaching, we found that even the fire alarm strobes showed evidence of condensation. Air flow will travel long distances seeking openings in the room walls and ceiling cavities in order to reach where the negative pressure source is located. In this case, that source was the individual bathroom exhaust fan that instead of operating on an intermittent basis, operated continuously because of how it was being controlled.

How It Was Fixed

Remediation and build back was required of moisture-damaged materials and the HVAC design required modification by installing a separate makeup air system to positively pressurize the rooms. The pathways (e.g. gaps, holes, abandoned electrical conduits and boxes, and breaches in the CMU) through the walls and floors that allowed humid air to enter from the crawlspace, attic, and outdoors into the conditioned spaces, including wall cavities, were a secondary cause of the moisture problems. A dedicated outdoor air system was designed and installed and ducted to each guest room in order to provide positive pressurization with conditioned outdoor air. The crawl space was remediated and a vapor retarder installed.