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Moisture Control, Part of Indoor Air Quality Design Tools for Schools


Introduction

Uncontrolled moisture indoors can cause major damage to the building structure, as well as to furnishings and to finish materials like floors, walls and ceilings. Uncontrolled moisture can trigger mold growth which not only damages the school facility, but can lead to health and performance problems for students and staff. Mold is usually not a problem indoors unless there is excess moisture.

Primary causes of indoor moisture problems in new schools include:

  • Use of building materials that are repeatedly or deeply wetted before the building is fully enclosed
  • Poor control of rain and snow resulting in roof and flashing leaks
  • Wet or damp construction cavities
  • Moisture-laden outdoor air entering the building
  • Condensation on cool surfaces

NOTE: Controlling moisture entry into buildings and preventing condensation are critical elements of protecting buildings from mold and other moisture related problems such as pest infestation and damage to building components. Moisture migration in buildings is highly complex and depends on a variety of factors, including the climactic conditions where the school is being built. For this reason, in addition to the guidance below, designers should consult other references on moisture control.

Building Materials

Newly constructed buildings give off significant amounts of moisture during their first year as a result of moisture trapped within materials such as: fresh concrete, green lumber and "wet"-applied insulations.

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Keep building materials dry during construction. Building materials, especially those with moisture absorbing properties like wood, insulation, paper and fabric, should be kept dry to prevent the growth of mold and bacteria. If moisture is present, mold will grow on any virtually any material. 
Some building materials such as wood may arrive at the construction site with a high moisture content or may have been wetted before arrival or during the transport process. Wet materials need to be allowed to dry as much as possible as weather permits. Cover dry materials with plastic to prevent rain damage and if resting on the ground, use spacers to allow air to circulate between the ground and the materials.
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Dry water damaged materials as quickly as possible, preferably within 24 hours. Due to the possibility of mold and bacteria growth, materials that are damp or wet for more than 48 hours may need to be discarded.

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Precipitation Control

There are four primary methods for preventing rain and snow from causing moisture problems in buildings:

  1. sloped roofs
  2. ground slope
  3. covered entries
  4. air intakes.

Problems in these areas generally allow moisture to leak or be blown into the building.

  1. Sloped Roof — Over the life of a building, sloped roofs are less likely to cause major moisture damage from rain and snow as compared to flat roofs. The resulting space under a sloped roof can be used for HVAC equipment and ductwork and other mechanicals and electrical. Catwalks will allow year-round access to the equipment for easier and more timely maintenance.
  2. Ground Slope — Ensure that ground next to the building slopes away from the foundation. This is a well understood design practice, yet sometimes a newly finished building will have surface water ponding next to the building instead of quickly moving away from it.
  3. Covered Entries — Ensure that exterior entries have sufficient overhang to prevent rain or snow from being blown into the building, or large amounts of moisture collecting directly in front of the entry where it can be tracked into the building.
  4. Sloped Intake Plenum — Consider adding a section of sloped intake plenum that causes moisture to flow to the outside or to a drain if intake grilles are not designed to completely eliminate the intake of rain or snow.

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Building Envelope

It is extremely important to prevent uncontrolled moisture from entering the building envelope through window and door openings, seams, footings, roofs or other openings. In virtually all areas of the country, provide an exterior weather barrier to prevent moisture from entering construction cavities. Wet or damp construction cavities (e.g., spaces between interior and exterior walls), attics and plenums are major sources of mold and can contribute significantly to indoor air quality problems. In addition, moisture can damage the structure and degrade the performance of insulation, increasing energy and operating costs.

Water also enters construction cavities through a process of moisture migration. Moisture migrates from the warm and humid side of the construction assembly to the cold dry side of the construction assembly. Air pressure differentials can also drive moisture into interior and exterior walls, creating mold-favorable environments and they can do so in both heating climates and hot, humid climates.

In hot, humid climates, even slight negative interior pressures can pull hot outdoor moisture into chilled wall cavities during cooling periods if the building envelope is not properly designed and constructed. Similarly, during heating periods, positive pressures can push warm, moist air from indoors into chilled exterior walls. In either case, this uncontrolled moisture can lead to mold growth that impairs indoor air quality and damages building materials.

Thus, assuring the integrity of exterior walls (or interior walls where moisture migration may be a problem) is the first line of defense for preventing this problem in all climates. For hot, humid climates, a slightly positively pressurized building may also help, as a slightly negatively pressurized building may help in heating climates. However, in mixed climate areas, using positive or negative pressures can create problems as the weather changes. Ultimately, the most reliable solution is the proper construction of the building envelope.

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To prevent moisture migration, framed walls, floors and roofs should have a vapor/air retarder on the warm, moist side of the insulation. In mixed or cooling climates, special care must be taken in the placement of vapor/air retarders. Vapor retarders are available as part of most insulation products. Common examples would be asphalt impregnated paper or metal foil. Care should be taken during construction to ensure that this vapor retarder is continuous, tightly secured at the framing members and not damaged. Special care should be taken in lockers, showers, food preparation areas and other spaces that are likely to have high humidity.
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For floor slabs in immediate contact with the ground, consider installing a 3-6 mil continuous vapor barrier under the slab to prevent moisture entry through the slab.
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In addition to correctly installing a vapor barrier, it is important to provide adequate ventilation of spaces where moisture can build up. Most building codes require that attics and crawl spaces be ventilated and some require a minimum one-inch clear airspace above the insulation for ventilation of vaulted ceilings. Even the wall cavity may need to be ventilated in extreme climates.
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Pay close attention to both the design and proper installation of roof flashing, pan flashing for windows and doors, capillary breaks for footings and other areas where moisture can enter the building envelope. Even the best design can be thwarted by mistakes and shortcuts in the actual construction process, so special attention to moisture control details in the construction and commissioning processes is warranted.

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Controlling Moisture in Ventilation Air

Because schools have a large number of occupants, large amounts of outdoor air must move through the building to assure proper ventilation. In states east of the Rocky Mountains, even small amounts of moisture in the outdoor air can lead to too much moisture indoors and moisture-related problems during the spring, summer and fall, if the air is not properly conditioned.

Indoor air can become too dry for comfort and health during the heating season, particularly in northern and mountain states, even though outdoor air may be high in relative humidity. Virtually all schools are designed with heating or cooling equipment to closely control indoor air temperature, but very few schools are designed with equipment dedicated to controlling moisture. As a result, indoor relative humidity can range from less than 10% to over 90%.

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To protect health, comfort, the school building and its contents, it is important that indoor relative humidity be maintained below 60%, ideally between 30% and 50%.
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Avoid over-sizing the HVAC system, particularly in high humidity climates.
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Consider specifying energy recovery ventilation equipment.
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If humidification equipment is installed in very dry climates, care must be taken to avoid microbiological problems associated with excess humidity and to ensure maintenance of humidification equipment.

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Summer Breaks and Humidity Control

Summer breaks often end with significant mold problems in schools, not only in the southeastern U.S., but in most states east of the Rockies. This is due to several factors:

  • higher humidity in the outdoor air during the summer;
  • lack of cooling system operation because school is out;
  • and extra indoor moisture due to special activities such as deep cleaning of carpets and painting.

These conditions do not need to exist for the whole summer before expensive mold remediation and clean-up is required - only a couple of weeks can result in cleanup costs ranging in the tens to hundreds of thousands of dollars. Therefore, moisture control during summer break, even if the building is not occupied, is essential.

For new schools, the HVAC system designer can specify controls that will close outdoor air intake dampers while still allowing the air conditioning system to operate for moisture removal. As with occupancy sensors that control office and restroom lighting, automation of the outdoor air dampers is the preferred approach. Another simple solution is to install a manual switch or timer that would close the dampers. However, this is the most problematic solution because it is easy to forget to turn the switch on, potentially resulting in mold problems, or to turn the switch off when school begins in the fall or for special summer classes, causing the dampers to remain closed and resulting in poor IAQ.

The energy management system could be set so that in the summertime, carbon dioxide (CO2) sensors in each zone control the outdoor air damper for that zone. If background (outdoor) CO2 levels are detected, dampers would remain closed. However, if elevated CO2 levels occur, indicating occupancy, dampers would open until the CO2 levels return to background levels. Use of such a demand controlled ventilation system in these circumstances can help ensure that potential mold problems are significantly reduced, energy costs are reduced and occupants are protected - all automatically - using current CO2 technology that is inexpensive, easy to install and self-calibrating.

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Consider cycling the air conditioning system (if present) several hours every day to reduce indoor moisture, or run portable de-humidifiers.
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Consider establishing policies that moisture generating activities (e.g., carpet cleaning) not be conducted in the summer in humid climates unless air conditioning or other moisture removing equipment is available.
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Consider manual thermostat overrides. A manual override on each thermostat should be provided so that teachers and staff can easily activate the HVAC system during non-standard hours when the energy management system has the HVAC off, such as evenings, weekends and holiday breaks. A simple push of the override button would allow a preset amount of operating time, typically 30 to 60 minutes, thus providing thermal comfort and outdoor air.
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Consider CO2 sensors in each zone to control outdoor air dampers during summer periods when occupancy may be intermittent.

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Condensation

In winter, water vapor in the indoor air can condense onto cold surfaces such as windows, walls and the underside of roof decks. In summer, condensing can occur on cool surfaces like water pipes and ducts conveying cold air.

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Fully insulate all cold water pipes and fittings and condensate drain pipes, with appropriate insulation. There should not be any gaps or unsealed seams in the insulation and all fittings, including valve stems, should be insulated.

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Fully insulate all cold refrigerant lines similar to cold water pipes.

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Fully insulate cold-air supply ducts.

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Minimize thermal bridging in the building shell.

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If renovating, add appropriate levels of insulation to prevent cool interior surfaces of windows, walls, ceilings and floors.

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Specify windows and doors with frames and spacers around edges that have thermal breaks.

Problem

Cool surfaces are not properly insulated causing condensation.

Solution

Completely cover all surfaces with insulation materials having non-moisture absorbing properties

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References and Resources

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