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Moisture Generated in Buildings
A considerable amount of moisture is produced in a building and introduced into indoor air as water vapor. Sources of moisture include occupant respiration, food preparation, body cleaning (e.g., bathing), and maintenance (e.g., mopping floors).
TBL. 15 provides sources and quantities of moisture generated in a typical building. Although some moisture in a building is acceptable because it can maintain good relative humidity levels, too much moisture creates problems. Moisture (water) in various forms is a principal cause of building component deterioration, so it must receive considerable attention in building design.
Tbl. 15 moisture produced by selected tasks in a typical home. Data was collected from a variety of governmental sources.
Sources of Moisture:
Moisture Problems in Buildings
Over a half century ago, buildings in the United States and Canada were built with a fairly loose building envelope with respect to flow of airborne moisture. When building cavities got wet, air moved freely through and out of the building envelope, carrying excess water vapor with it. The introduction of thermal insulation in the 1940s and increased use of insulation in the building envelope in response to the energy crisis in the 1970s limited the ability of these cavities to breathe and expel moisture. Simply, water vapor movement inside a building envelope was not important until the introduction of thermal insulation.
It is a problem because when insulation is added, the temperature of the water vapor can drop very quickly as it's being isolated from the heat of the building (in the winter) or from the outdoors in the summer if the building is being cooled. With changes in construction techniques over the last decade, the number of buildings experiencing moisture-related problems has risen sharply.
Most building materials are adversely affected by moisture. Wood expands with increases in moisture content and con tracts with a decrease. Irregular levels of humidity can cause parts of a single wood member to swell and contract in different ways, which causes warping of the member (e.g., one edge may get wet and swell while the other edge remains at a constant moisture level). High humidity levels (above about 20% moisture content) can cause wood to begin to decay and rotting problems to develop. Exposure to water induces corrosion of irons and steels. Recurring freezing and thawing of water-saturated concrete and clay masonry products can lead to deterioration (i.e., cracking and spalling). When soluble salts are present in or in contact with concrete and masonry, moisture caused by condensation may contribute to efflorescence, a white, crystalline deposit that's aesthetically unpleasing. The performance of insulating and finish materials is greatly reduced by the absorption of water. Additionally, expansion from freezing water may also create structural problems.
Causes of moisture problems in buildings can be summarized in three areas:
• Highly insulated and sealed cavities that reduce air exchange in the assembly (i.e., wall, attic, and floor).
• Lighter construction materials that have less mass to store water and organic materials that are more susceptible to moisture deterioration.
• Improper use of vapor diffusion retarders and air barriers that trap moisture in by limiting moisture transmission.
Modes of Moisture Movement
The movement of moisture into and through building assemblies generally takes place by any of four modes:
• Liquid flow is the movement of water under the influence of a driving force (such as gravity, or suction caused by air pressure differences).
• Capillary flow is the movement of liquid water in porous materials resulting from surface tension forces.
Capillarity, or capillary suction, can also occur in the small space created between two materials.
• Diffusion is the movement of water vapor resulting from a vapor pressure difference.
• Moisture-laden air movement is to the movement of water vapor in air resulting from airflow through spaces and materials.
Liquid flow and capillary flow into the building envelope occur primarily with exterior source moisture (e.g., rainwater and groundwater) while movement of moisture into the building envelope by air movement and diffusion can occur with interior or exterior source moisture. Of the four modes of moisture movement, liquid flow and capillary flow are the most common and significant. As a result, builders and designers place their primary emphasis on control of rain and groundwater penetration into the building. Although less significant, air movement and vapor diffusion can also cause moisture problems.
Types of Moisture Problems
Moisture problems found in buildings can be caused by three factors: water intrusion, airborne water vapor infiltration, and water vapor diffusion. Water intrusion can be from penetration of water through the above-ground building envelope (the super structure) or through the building foundation. Airborne water vapor infiltration and water vapor diffusion are directly associated with the formation of condensation in cavities of the building envelope. These condensation problems are tied to development of mold and mildew and structural or material deterioration and decay.
Water intrusion is leakage of moisture into the building envelope. It can cause degradation of underlying materials inside wall, ceiling, and floor assemblies. Intrusion can occur through and /or around building components such as windows, doors, gable vents, penetrations, flashings, and construction details. It can also take place through foundation walls in basements and crawl spaces. Additionally, water from leaking pipes, plumbing fixtures, and bathtub and shower surrounds can leak unnoticed into wall and floor cavities. Water can enter behind cladding or roofing materials, become trapped, and dampen unprotected substrate (building sheathing) and in some case the wood structural members. Depending on construction of the building assembly, trapped moisture may not easily escape, so materials may not readily dry out. If concealed water intrusion continues to occur, it can accumulate to levels substantial enough to cause moisture damage (e.g., mold, rot, and so forth).
Airborne Water Vapor Infiltration
Water vapor in moist air leaks into wall, attic, or floor cavity openings with infiltrating air currents and by heat transfer.
Pressure differences from wind and mechanical system operation can drive airflow. Additionally, pressure differences from temperature gradients inside the building envelope cause air to move through a heat transfer mode called natural convection; that's , warm air is less dense and thus more buoyant than cool air, so warm air rises. In the heating season, then, warm air exfiltrates through the upper area of the building and infiltrates through the lower area.
Air travels by the easiest path possible, which is generally through any available opening in the building envelope.
Moist air can also be mechanically introduced through the improper discharge of bathroom exhaust fans and clothes dryer vents that incorrectly discharge directly into building envelope cavities, crawl spaces, and attics.
Water Vapor Diffusion
Water vapor can pass through (not around) a building material and become trapped in a wall, floor, or attic cavity. Some building materials are fairly permeable to water vapor and others are nearly impermeable. The rate at which water vapor is transmitted through a material is based on the properties of the material and the pressure difference that drives vapor movement. These factors are discussed in the following:
Water Vapor Pressure
Vapor pressure in air is based upon the amount of water vapor that exists in the air. The constituent gases that make up a mixture of gases each exert a partial pressure that contributes to the total pressure exerted by the gas mixture. For example, atmospheric air consists of about 75% nitrogen, by weight, so 75% of the total pressure exerted by the air is due to the nitrogen constituent.
Thus under standard conditions (14.696 psia), the partial pressure of nitrogen is 11 psia, 75% of the total pressure. The partial pressure of a constituent gas in a mixture of gases equals the pressure it would exert if it occupied the same volume alone at the same temperature.
Atmospheric air contains some water vapor. Like all constituent gases in air, water vapor exerts a pressure, which is known as vapor pressure. Water vapor pressure is the contribution of water vapor to the total pressure exerted by a gas. In buildings, this gas is atmospheric air. Water vapor pressures at standard pressure at various relative humidities are shown in TBL. 16.
Vapor pressure difference is the principal driving force behind water vapor diffusion. When a water vapor pressure difference exists, water vapor will attempt to equalize the pressure with water vapor moving from a high vapor pressure region to one of low vapor pressure. In comparing the vapor pressures in TBL. 16, one can find that typically during the heating season water vapor is driven out of a building, and , during the cooling season, water vapor is driven into a building.
It is not only the vapor pressure difference in surrounding air that drives water vapor diffusion. Recent studies indicate that solar heating of wet cladding materials (e.g., siding, brick veneer) will create a vapor pressure force that drives water vapor inward into the building envelope.
Tbl. 16 water vapor pressures at standard pressure at various relative humidities and temperatures.
Moisture Transmission Rate
Water vapor permeability is the ability of a material to permit the passage of water vapor, the rate of water vapor transmission between two surfaces of a homogeneous material under a specified vapor pressure difference. Simply, it's the measure of the ease with which water vapor passes through a material. Units of permeability are gr-in/hr ft2 in Hg (ng-m/Pa s m2 ). The lower the permeability, the less water vapor will come through the material. Permeability is not dependent on the materials' thickness.
Water vapor permeance is the rate at which water vapor passes through a material with parallel surfaces when induced by a vapor pressure difference, per unit thickness. It is the ratio of water vapor flow to the differences of the vapor pressures on the opposite surfaces of a material or an assembly of materials with parallel surfaces. The unit of permeance is called the perm, which is defined as one grain of water passing through one square foot in one hour under the action of a vapor pressure differential of one inch of mercury. In U.S. Customary (I-P) units, a perm (gr/hr ft2 in Hg) is equal to the transfer of one grain (1 7000th of a lb) of water per square foot of material per hour under a pressure differential of one inch of mercury (1.134 ft of water). In metric units (SI), a perm (ng/Pa s m2 ) is equal to the transfer of one nanogram of water per square meter of material per second under a pressure difference of one Pascal.
The corresponding unit of permeability is the "perm-inch," the permeance of unit thickness.
A material's permeance is dependent on thickness, much like the R-value in heat transmission. If a material has a perm rating of 1.0, then one grain of water vapor will pass through one square foot of the material when the vapor pres sure difference between the one side and the other side of the material is equal to one inch of mercury. One grain of water is equal to 1 7000th of a pound (about a droplet of water). For homogenous materials, doubling material thickness generally cuts water vapor transmission in half; that's , if 1 inch of a material has a perm rating of 2.0, then for 2 inches of the same material, the perm rating would be 1.0. With paints, however, adding a second coat more than halves the water vapor transmission.
Permeance is used to compare various products in regard to moisture transmission resistance: the lower the perm rating of a material, the more effectively it will retard diffusion and thus the more effective the material will be in controlling moisture vapor transmission. Reported perm ratings are based upon standardized dry and wet tests at a temperature of 73.4°F (23°C). Under the desiccant (dry-cup) test, one side of the material is maintained at 0% relative humidity and the other side is maintained at 50% relative humidity. Under water (wet-cup) test, one side of the material is maintained at 50% relative humidity and the other side is maintained at 100% relative humidity. The water test method is preferred when testing materials that are likely to be exposed to high humidity in service.
Reporting permeance of generic types of materials is difficult because these properties are sensitive to small changes in material composition and the moisture content of the material being tested. Hydroscopic materials adsorb (i.e., gather and store a substance on their surface) moisture and lose moisture with changes in ambient relative humidity and temperature.
Water vapor permeance of hydroscopic materials (e.g., wood and paper products) increases greatly with increase in relative humidity. For example, the perm rating of bitumen-impregnated kraft paper varies by a factor of about 10 between the desiccant and water permeance tests. Data provided in TBL. 17 is presented to permit comparisons of perm ratings of various building materials. Exact values for permeance should be obtained from the manufacturer of the materials under consideration or secured as a result of laboratory tests.
Condensation from Water -- Vapor Infiltration and Diffusion
In cold climates, components of the building envelope that are outside the insulation (e.g., roof and wall sheathings) are commonly cooler than the dewpoint temperature of air in a conditioned space. Under certain conditions, the dew point > _ _ temperature occurs within the insulation itself. If water vapor from diffusion or moisture-laden infiltration air comes into contact with a component at or below the dew point temperature of the air, the vapor will condense to a liquid (water droplets). If the temperature is below freezing, condensation will form as frost (ice crystals). Frost can collect in the cavity through the heating season and melt during the first thaw of the spring season.
Condensation can form within a wall, attic, or floor cavity, where it remains concealed. It can accumulate unnoticed, where it can cause rot of framing members and sheathing, to a point where structural collapse is possible; deterioration of interior room finishes, from gravity flow of water; corrosion and failure of metal truss plates and clips; and decrease of thermal resistance or deterioration of insulation. Thus, it's very important that unintended paths that infiltrating moist air may follow be permanently sealed. It is also important that an impediment exist that limits diffusion of moisture into the building envelope cavity because reducing the amount of water vapor entering a cavity will reduce the quantity of condensation in the cavity.
Tbl. 17 characteristic water vapor permeance (perm) ratings for selected building materials.
Data is presented to permit comparisons of perm ratings. Exact values for permeance should be obtained from the manufacturer of the materials under consideration or secured as a result of laboratory tests. Data was compiled from an assortment of manufacturer and governmental sources.
Vapor Retarders: Aluminum foil STEGO WRAP Under-slab polyolefin geo-membrane Polyethylene film Asphalt-impregnated kraft facing Building felt/tar paper (15#) Air Barriers Prime Wrap Tyvek Commercial Wrap Tyvek HomeWrap Insulation Polystyrene (extruded)
Polyurethane Cellulose Fiberglass Building Materials Built-up roofing Glazed structural (clay) tile masonry Wood (solid board) Oriented strand board (OSB) Plywood, Douglas fir (exterior glue) Plywood, southern pine (exterior glue) Brick masonry Concrete 4 Gypsum wallboard (painted) Concrete masonry Building felt/tar paper (15#) Hardboard (tempered) Hardboard (standard) Finishes Gypsum wallboard-knockdown texture, semigloss paint Gypsum wallboard, 2 coats latex flat paint Gypsum wallboard-knockdown texture, flat paint Gypsum wallboard (unpainted) Plaster on wood lath Plaster on metal lath Plaster on gypsum board lath Paint Asphalt paint on plywood Latex VDR paint or primer Oil-based paint on plaster 2
Moisture control in building envelope cavities is generally accomplished by eliminating (or minimizing) water intrusion, ventilation of indoor spaces where moisture is generated, and construction techniques that properly retard water vapor.
Eliminating Water Intrusion
Designing and constructing proper details and using appropriate materials will prevent the likelihood of water intrusion.
After construction, intrusion can still take place if maintenance of these details and other critical areas (e.g., caulk joints) is neglected. In addition, even slight structural movement as a building settles during its early life can create openings that were not there initially.
Damage can be significant if moisture intrusion goes undetected, so early detection of water intrusion is crucial to minimizing or preventing such damage. The location of water entry is often difficult to detect (e.g., damage to substrate and structural members behind the exterior wall cladding frequently can't be detected by visual inspection). In cases where intrusion is suspected, testing must be conducted. A moisture meter is used to detect elevated levels of water in building materials.
Two types of moisture meters should be used: a noninvasive moisture meter that scans through the assembly for the presence of moisture without penetrating and damaging the surface; and an invasive, probe-type moisture meter that penetrates the assembly surface and gives moisture readings of materials in contact with the probes.
In cases where water intrusion is discovered, repairs should be made promptly. The primary objective of any repair is to first eliminate water intrusion. Areas of elevated moisture with an absence of damage or decay may require no more than eliminating the source of water intrusion. It has been discovered that undamaged but wet substrate can dry out over time once the source of the water intrusion has been eliminated.
Where structural damage has occurred, replacement of decayed lumber is required.
One of the chief sources of water intrusion into a building interior is basement or under-floor crawl-space moisture. Water vapor from a wet or damp basement or crawl space travels into the indoor spaces and ultimately migrates into the building interior and envelope. Improper grading around the base of the exterior of the building foundation directs storm water towards the house.
Not extending downspouts so they discharge away from the foundation also contributes to the problem. Solutions to basement or under-floor crawl-space moisture are improving the slope of grade around the foundation and adding downspout extensions, which are extended as far away from the foundation as practical.
Ill. 22 Installation of a polyethylene VDR on the "warm" side of a framed wall in a building in a heating climate.
Ill. 23 Installation of a blanket of insulation. Attached to the bottom face of the blanket is a bitumen-impregnated kraft paper facing (installed by manufacturer) that will serve as a VDR. (
Fgr. 3A A vapor diffusion retarder (VDR) is a material of system that adequately impedes the transmission of water vapor through the building envelope. Shown is a properly positioned cold climate VDR.
Fgr. 3B A Properly positioned warm climate VDR.
Fgr. 3C Shown is an improperly positioned VDR for a warm climate With the VDR located near the inside wall surface, water vapor contained in moist outdoor air will infiltrate into the wall and condense on the cooler inner wall surface.
Retarding Vapor Diffusion
A vapor diffusion retarder (VDR) is a material or system that adequately impedes the transmission of water vapor through the building envelope. A VDR has a rating of 1.0 perms or less. The main reason to limit or stop transmission of water vapor is to prevent it from condensing to liquid water in the building envelope. VDR membranes commonly used in the building construction industry include thin, flexible aluminum foils, plastic (e.g., polyethylene) films, and bitumen-impregnated kraft paper.
Other products (i.e., CertainTeed's MemBrain SMART Vapor Retarder) are designed specifically for use as a VDR.
Many insulation batts, blankets, and boards come with a VDR attached to one face. A common example of this is aluminum- or kraft-paper-faced fiberglass insulation blankets.
Polyethylene, a thermoplastic sheet material, is the most commonly used VDR in cold climates. It is used over unfaced, vapor-permeable insulation (i.e., fiberglass and mineral wool) in wall and ceiling cavities. Coatings may be semifluid, mastic, or paint type, which are applied on a substrate such as gypsum wallboard. Ill. graphs of VDRs during construction are shown in Ills 22 and 23.
According to industry standards, a vapor barrier is essentially impermeable. It has a rating of 0.01 perm (0.60 metric perms) or less. A material with a perm rating of 1.0 perm (60 metric perms) or less is considered to be a nonpermeable VDR, which includes most polyethylene films, elastomeric coatings, oil-based paint, aluminum foils, solid glass, metals, and vinyl wall coverings. Materials rated at 10 perms (60 metric perms) or less are semipermeable VDRs, including solid and engineered wood products (e.g., plywood, OSB, hardboard), unfaced expanded polystyrene insulation board, latex paints, and bitumen impregnated kraft-paper facing on insulation. Those materials rated above 10 perms (60 metric perms) are permeable and are not really vapor retarders at all, such as unpainted gypsum wallboard, cellulose and unfaced fiberglass insulation, bitumen-impregnated cellulose sheathing boards, and building air barriers and house wraps.
Current building code regulations require placement of a VDR rated at 1.0 perm (60 metric perms) or less on the heated side of the building envelope, except in hot, humid climates.
Placement of a VDR in a construction assembly (e.g., a wall) is climate dependent. See Fig 3. The rationale used in deter mining VDR placement was that in cold climates water vapor pressure is typically higher inside the building, so placement of the VDR should be on the interior side of the wall insulation.
Having a single VDR on the side of the wall with the highest vapor pressure retards wetting and encourages drying of the cavity. Such placement makes sense in extremely cold climates.
However, this flawed rationale does not account for hot and /or humid weather and the use of air conditioning for cooling.
Under summer conditions, vapor pressure is typically higher outside the building, causing vapor to move to the inside of the building envelope and condense on the surface of the VDR, which is cooled by air conditioning. Because of such inconsistencies in climate, type and placement of a VDR can't be treated with a single comprehensive specification that covers all conditions in all climates.
The assembly of the building envelope plays a significant role in vapor diffusion control requirements in terms of the order in which layers of different permeance materials are arranged.
A single VDR is typically installed that limits the rate of vapor diffusion so moisture can move through the other components of the building envelope assembly more rapidly than it passes through the VDR. For example, in a cold climate a low-permeance VDR (e.g., a polyethylene film or bitumen-impregnated kraft facing) is installed on the interior side of a wall to reduce the transmission of water vapor through the building envelope. In a second example, a low-permeance layer (e.g., a roofing membrane) is installed on the outside surface of the building envelope in a cold climate. This outer layer will prevent water vapor from escaping to the exterior, which slows drying to the outside of the envelope. In this case, the permeance of interior layers must be considerably less than the permeance of outer layers. To prevent trapping moisture in a cavity, the cold-side material's perm rating should be at least five times larger than the value of the warm side (various rules place the ratio at 3:1 to as much as 10:1).
Local climatic conditions and heating and cooling needs of a building dictate whether, what type, and where a VDR should be installed in a building envelope. In most climatic regions, the building envelope assemblies must be designed to promote drying to both the interior and exterior, so a semi-permeable VDR is recommended on the heated side of the building envelope. The varying permeance of the bitumen-impregnated kraft facing on batt and blanket fiberglass insulation is low under dry winter conditions but its permeance rises significantly under very humid conditions, which allows water vapor to flow out of the cavity. In hot, humid regions (Florida and Gulf Coast) where vapor pressure is more often higher outside, use of a VDR is advised on the outside of the building envelope. In hot, dry regions, a VDR is typically not needed. In extremely cold climates, a nonpermeable VDR on the heated side of the building envelope is needed. A non-permeable VDR is also advisable be hind high moisture areas such as shower surrounds.
Basements and under-floor crawl spaces should have a vapor retarder to prevent soil moisture from entering the building interior. In under-floor crawl spaces, a nonpermeable VDR should be installed on top of the earthen floor. Concrete is permeated by microscopic voids that allow the passage of water and water vapor so, in basements with a concrete slab, a non permeable underslab VDR should be placed just below the concrete at building sites with moderate to heavy annual rainfall, low-lying building sites, or those sites where soil testing reveals sufficient hydrostatic pressure. Foundation walls should also be treated with a VDR coating or membrane.
To be effective, an under-slab VDR must possess a number of key performance characteristics, including low permeance [usually less than 0.3 perm (17 metric perms)], high tensile strength, high puncture resistance, and resistance to chemical or environmental attack. Under-slab VDRs are classified by the industry as Class A, B, and C, with Class A being the best. The key distinction between the classes of materials is their differing ability to withstand the abuse of installation without being punctured or torn-that is, being capable of handling the rigors of job-site installation. Although 4 or 6 mil (1 mil is a thickness of 1>1000 in or 0.0254 mm) polyethylene film is commonly used below concrete slabs, it typically does not meet the classification standard.
Reducing Water Vapor Infiltration
Airborne water vapor infiltration accounts for the largest part of water vapor movement into building envelope cavities, yet it typically receives the least attention in design and construction.
In cold temperatures, moisture-laden indoor air can pass through construction openings (e.g., electrical boxes, unsealed joints, and plumbing penetrations) where moisture easily condenses in the building envelope cavity. In hot, humid conditions, damp outdoor air travels in the opposite direction through similar openings. Openings in the building envelope do little to reduce airborne water vapor infiltration and reduce, if not eliminate, the effect of a vapor retarder. Eliminating these paths is very important. Simply put, an effective VDR needs to be a continuous barrier, free of any holes or open seams. The VDR should be continuously lapped over all joints and should be carefully cut to size. The key to making this method work effectively is to carefully and permanently seal all openings, seams, and penetrations, including around windows, doors, electrical outlets, plumbing stacks, and vent fans.
Air barriers are breathable water vapor permeable materials intended to impede airflow into the building envelope. Air barriers are effective in controlling moisture from airborne water vapor infiltration by obstructing much of the air movement into a building envelope, yet still allowing water vapor that does collect in the cavity to diffuse back through the barrier material. Most air barriers are also effective in reducing water intrusion into cavities by providing an additional layer beyond the cladding. They have microscopic pores, small enough to resist flow of liquid water and air molecules, but large enough to allow smaller vapor molecules to pass through the barrier material.
Common materials used for this purpose are house wrap, plywood, drywall (gypsum) board, and foam board. See Ill. 24. Many of these materials serve a dual purpose in that they are used for insulation, structural purposes, and finished surfaces. The most common air barrier materials in use today are fibrous spun polyolefin plastic, matted into sheets (e.g., Tyvek HomeWrap or CommercialWrap®), and woven and coated polyethylene fabric with microscopic-size perforations (Prime Wrap®). These air barriers are sheets that are wrapped around the exterior of a building's sheathing before the siding or cladding is installed. Sealing all of the joints with a special tape improves the wrap's performance significantly.
Use of an air barrier is appropriate in all climates, but especially in hot, humid climates where keeping moist outdoor air from entering the building cavities is important during the extended cooling season. However, some house wraps react poorly to certain kinds of wood siding (e.g., redwood, cedar) when they get wet, such as in rainy or humid climates. Natural substances in wood (lignin) act as a detergent when moistened, which makes it easier for liquid water to pass through the barrier material. Additionally, most house wraps are susceptible to ultraviolet degradation, so they must be protected from pro longed exposure to sunlight.
Alternative Approaches to Reducing Water Vapor Infiltration
There are alternative approaches to the use of VDR and air barrier in controlling moisture in the building envelope. These include:
Airtight Drywall Approach
The airtight drywall approach ( ADA) uses drywall installed over gaskets adhered to building framing members and ordinary paint to control air and vapor movement.
In this approach, drywall must be carefully joined to provide continuity between drywall sheets. Gaskets installed between wall plates, rim joist, subfloor, and other members create an air barrier. Two coats of latex paint on gypsum wallboard behave in a manner similar to bitumen-impregnated kraft facing and provide a sufficient semipermeable vapor retarder.
Attention to detail is necessary during installation of interior walls, window openings, electrical boxes, plumbing and electrical penetrations, around bathtubs, at split levels and stairs on outside walls. The barrier created reduces air leakage and moisture penetration into cavities in the building envelope.
Dynamic Buffer Zone Ventilation Systems
A dynamic buffer zone (DBZ) ventilation system consists of an exterior wall or roof of a building coupled with an air-handling system that's arranged so that the cavities in the building envelope are mechanically ventilated with preheated dry air during the winter to prevent and control of cavity condensation. A DBZ system typically includes supply and exhaust fans, temperature, relative humidity and pressure sensors and controllers, sealed cladding components, and a sealed interior plane of air tightness. There are two types of DBZ systems: the ventilated cavity DBZ system and the pressure-controlled cavity DBZ system.
In the ventilated cavity DBZ ventilation system, the cavities in the building envelope are ventilated with dry outdoor air and pressure relieved or controlled through a return and exhaust system. In some cases, the cavity air is re-circulated if the dew point temperature of the air is low and the cavity air requires supplementary heat. In this system, it's the ventilation air that dilutes and evacuates any cavity humidity.
In the pressurized cavity DBZ ventilation system, cavities in the building envelope are pressurized slightly above in door air pressure with preheated outdoor air but without a pressure relief or return air system. In this system, it's the cavity pressure generated by the system fans, which prevent further contamination of the cavity air with humid indoor air.
These systems are less expensive to design and build and more efficient at controlling construction cavity moisture conditions.
A DBZ system is used when efforts to fully seal an existing building envelope to prevent condensation have not been successful. It is best suited to restored buildings that are upgraded to higher indoor humidity levels.
Need for Indoor Ventilation
Excessive water vapor levels in air must be removed by ventilation of indoor air. In restrooms, bathrooms, kitchens, laundries, indoor swimming pools, and other moisture-producing areas, exhaust vents are normally necessary and called for by most building codes. All vents must terminate and discharge to the exterior of the building.
In tight structures with heavy moisture loads, mechanical dehumidification may be required. Humidification during dry winter months and heavy use of evaporative (swamp) cooling are not recommended.
Need for Attic Ventilation
In cold and extremely cold climates, attic ventilation has been effective in controlling moisture problems where the objective is to maintain cold attic temperatures in winter to avoid ice dams created by melting snow and to vent moisture that moves from the conditioned space into the attic. Ice dams develop in conditions when air in the attic space is at above-freezing temperature (if insulation is insufficient or warm building air enters the attic) and melting snow or ice on the roof surface backs up at the roof overhang and seeps through the shingles into the roof and damaging materials.
Sufficient attic ventilation is usually defined as having a net free ventilating area equal to 1>150 of the attic floor area.
When an attic vapor retarder is used, ventilation requirements are cut in half with a net free vent area can be 1>300 of the attic floor area. The net free ventilating area of a roof vent is usually least than 50% of the actual size of the vent area. The vent area should be divided evenly between the top and bottom of the attic space to effectively promote airflow by natural convection.
Attic ventilation may cause moisture problems in other climates: In hot, humid climates, moist outdoor air that comes in contact with cold surfaces in the attic will condense, particularly if low interior temperatures are maintained with summer air conditioning. Recent research suggests that in hot, humid areas, the best approach to avoiding moisture condensation in attics may be to keep the moisture out of the attic completely by sealing the attic from the outdoors. A continuous non-permeable VDR can prevent condensation problems in the attic by pre venting moisture diffusion through the ceiling and infiltration into the attic cavity. It must serve as a barrier to airborne water vapor infiltration and vapor diffusion.
Roof systems without attic spaces (e.g., low-slope roof structures, cathedral ceilings) are characteristically not vented and are protected by roof coverings with very low vapor permeance. The roof covering does not diffuse moisture well. In this case, moisture control in the roof cavity is extremely difficult.
A continuous non-permeable VDR can prevent condensation problems in cathedral ceilings by obstructing flow of water vapor. The VDR must be completely sealed to prevent moisture diffusion through the ceiling and infiltration into the roof cavity. It must serve as a barrier to vapor diffusion and airborne water vapor infiltration. Particular attention is necessary around skylights, paddle fan outlets, recessed light fixtures, and other potential openings in the barrier.
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