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Moisture Control in Buildings: Putting Building Science in Green Building
Written By: Alex Wilson, Environmental Building News, Volume 12, Number 7, July 2003
Edited By: Cindy Meehan-Patton, Shelter Ecology
Addendum By: Cindy Meehan-Patton, Shelter Ecology
I am continually surprised by how little emphasis the green building movement places on building science. As we examine the many priorities of green building, from land-use planning to energy efficiency, material selection, and indoor air quality, the basic science of how we design and build structures to ensure long life and healthy indoor environments should be at (or near) the top of the list. Excess moisture is at the center of many problems relating to durability and indoor air quality, not just in homes but in all types of buildings in a wide range of climates. If we build structures that won’t rot or support mold growth, we will both increase the longevity of those buildings and reduce the health risks of living in them.
This article examines the physics of moisture in buildings and addresses design and construction strategies for (a) keeping buildings dry and (b) allowing those buildings to dry out if they do get wet. While the emphasis and examples are largely focused on houses, most of the ideas apply more broadly.
Moisture 101
Moisture exists in three commonly known forms or phases: liquid, gas (vapor), and solid (ice), along with an additional adsorbed state that is somewhat between liquid and vapor in characteristics. All these forms come into play in buildings. The movement of moisture from one place to another is governed by physical forces including:
• Gravity. Rain falls to earth; water runs down roofs and walls.
• Capillarity. Water can be pulled through thin air spaces or pores through a process called capillarity. Capillarity occurs because intermolecular forces cause water to stick to itself (surface tension) and to many other materials. (These same intermolecular forces allow water to cling to the bottom of a joist, running along horizontally until a gap is reached where the water accumulates into a drop large enough that gravity becomes the dominant force.) Capillarity is a powerful force strong enough to raise water hundreds of feet into the air (counteracting gravity) in tall trees. This is the primary mechanism of moisture movement through porous materials, such as concrete floor slabs and wood siding.
• Convection. Airflow can carry moisture in both liquid and vapor forms. This movement occurs through air pressure differences. Wind blows rain into the walls of buildings, for example, and forced or passive airflow carries water vapor (humidity) with it.
• Diffusion. Water vapor can diffuse through permeable materials. This process is driven by partial pressure (vapor pressure) differences across the material. Vapor pressure as a driving force is confusing. In any mixture of gases, such as air, each separate gas has its own concentration or vapor pressure. If the vapor pressure of a gas in one area is greater than in another area, those vapor pressures will try to equalize through the movement of gas molecules from one air mass to the other. This is why it’s very difficult to keep part of a building dry through air-conditioning if other parts of the building operate at ambient humidity levels. (Experts once thought diffusion was the primary mechanism through which moisture got into wall cavities, and we installed poly vapor retarders to block that diffusion; they now believe that airflow –convection-is a much greater source of such moisture migration.)
• Temperature differential. Moisture is known to move from hot to cold within a material. When brick siding gets soaked from rain, for example, and then the sun heats the outside of the brick, moisture in the brick is driven through the wall to the interior.
Integral to the moisture dynamics in a building is the issue of relative humidity and the phase change between liquid and vapor. Humidity is a measure of the moisture content of air -the concentration of water-vapor molecules in a particular air mass. The amount of water vapor a given volume can hold is dependent on temperature -more water vapor can exist in a space if it is warm than if it is cold. As a mass of air and water vapor is cooled, the relative humidity increases, until the mass reaches 100% RH, when liquid water condenses out (changes phase from gas to liquid). This point is known as the dew point.
This process has a huge bearing on moisture problems in buildings. If warm indoor air flows into the wall cavity through cracks in the drywall during cold weather, for example, that air mass may cool enough to reach the dew point within the wall cavity. When this occurs, the insulation in the wall cavity or the inner surface of the exterior sheathing gets wet-and that can cause big problems.
Risks of Moisture in Buildings
Excess moisture in buildings is bad. Decay of wood and other cellulosic materials is dependent on moisture and temperature. With moisture content in framing lumber over about 20% (28% according to some experts) and typical room temperatures, decay is possible.
In addition to causing wood decay, moisture can corrode-and eventually destroy-steel structural members and fasteners. Steel does not have to be wet to begin rusting. Corrosion can occur whenever the relative humidity is above 70-80%, though corrosion problems worsen when pipes or fasteners actually become wet through condensation, leaks, and so forth.
Keeping humidity levels within an acceptable range is an important strategy both for ensuring a long life for buildings-longer-lasting buildings are greener buildings-and for ensuring health of the occupants. The relative humidity should not be lower than about 25% (to prevent dry eyes and throats, shrinking of wood flooring, and static electricity problems on carpeting) or higher than about 60% in the center of a room. The 60% level is intended to keep the relative humidity from exceeding 70% at surfaces, such as walls and floors. The relative humidity near surfaces is typically higher than it is in the center of a room. When the relative humidity at surfaces is above 70%, mold growth can occur. Things dry out more slowly when the humidity level is high. Thus, at higher relative humidity levels in a building, even a fairly small source of liquid water can become a big problem.
Controlling Moisture in Buildings
Strategies for dealing with moisture in buildings include the following: keeping water out, avoiding (or managing) plumbing leaks, avoiding condensation inside the building or within the building envelope, controlling the entry of humid outside air, controlling indoor sources of humidity, designing building assemblies to dry out, providing mechanical ventilation, and providing mechanical dehumidification.
Keeping water out
"Drain, drain, drain’, is the mantra echoed by building scientists Terry Brennan and Joseph Lstiburek. Keeping rainwater out necessitates quality flashing details at all window and door openings, roof penetrations, and roof-wall intersections. Less obvious but also very important is the need for drainage planes in walls to prevent water from getting through siding and sheathing. Capillary breaks are also needed to keep groundwater from moving through foundation walls and floor slabs. Rainwater can be kept away from the building walls with overhangs, properly installed gutters and downspouts, and an adequately sloped ground surface around the building.
Avoiding moisture problems from plumbing leaks
Lstiburek considers plumbing leaks to be "more significant than any other factor" in causing moisture problems in buildings. Based on his company’s experience, he estimates that one-third of all houses develop plumbing leaks within the first five years! Most building science experts recommend installing all plumbing in interior rather than exterior walls. Brennan even suggests leaving pipes fully exposed to provide full access-as he sometimes sees in older buildings.
In place of standard drywall, use one of the recently introduced mold-resistant drywall products (U.S. Gypsum’s Humitekª or Georgia Pacific’s DensArmorª), monolithic (non-paper-faced) drywall, such as USG’s Fiberock¨, or- where leaks and other moisture problems are most likely-cement board. If standard drywall must be used, at least raise the drywall panels a few inches off the floor and cover the space with baseboard (taking care not to compromise the airtightness of the wall). If practical, install clothes washers and water heaters in spaces with floor drains or in watertight pans, and provide clearly visible shut-off valves. Flooring in spaces prone to leakage should be water resistant, such as tile. Avoid carpeting in these locations.
Avoiding condensation
Condensation can occur in warmer months when humidity is high and cold-water pipes are significantly cooler than the air temperature. When such condensation is severe, water can accumulate and cause mold or decay. Risk of condensation can be reduced or eliminated through careful attention to energy-efficient building practices (high-performance glazings, proper wall insulation, insulative sheathing on steel framing, foundation and slab insulation, etc.) and by insulating all water pipes.
Controlling humidity
High humidity in buildings can come from either indoor or outdoor sources. The concern is typically much greater in summer months, when outdoor humidity levels are high. While experts used to recommend ventilating unheated basements during the summer months, most now discourage that practice in humid climates (even though codes may still require it), because the ventilation introduces more moisture than it removes.
Preventing the entry of humid outside air involves creating a tight building, installing a poly vapor retarder on the ground in crawl spaces, and avoiding conditions that will depressurize the building. The most common interior sources of humidity; showering and cooking-are best dealt with through spot ventilation at the source. Avoid storing firewood indoors, venting clothes dryers indoors, and using unvented gas heaters-the last because unvented heaters produce significant quantities of water vapor as well as other combustion products.
Designing building envelopes to dry out
Because we can’t always succeed in keeping moisture out of our buildings, the second line of defense should be to design our structures to dry out. Think of this as an insurance policy. Some strategies for improving a building’s drying potential have ancillary benefits, such as longer life for paint. How you should design a building envelope to dry out depends on the climate. Experts used to say that you should always put the vapor retarder on the warm side (on the interior in cold climates and on the exterior in warm climates), but Lstiburek now argues that in most climates the envelope should have some drying potential in both directions-because moisture-driving forces change dramatically throughout the year.
CertainTeed has begun marketing MemBrainª, developed by a German subsidiary of CertainTeed’s parent company, Saint-Gobain. This so-called "smart" air/vapor barrier, made from 2-mil nylon, becomes more permeable as the humidity increases. It may even be effective as an exterior housewrap product in hot, humid climates
Providing mechanical ventilation
Mechanical ventilation should always be part of the moisture-control strategy for building. The proposed ASHRAE 62.2 residential ventilation standard has just been approved by a key committee at ASHRAE and is awaiting publication-despite strong opposition by the National Association of Home Builders and the Gas Appliance Manufacturers Association. This standard, for those municipalities that adopt it, would require continuous mechanical ventilation rated at 7.5 cubic feet per minute (cfm) per occupant plus 1 cfm for every 100 ft2 of usable floor area- 3.5 liters per second (l/s) plus 0.5 l/s for every 9 m2.
Providing dehumidification
Experts recommend that indoor RH levels should be kept below 60%, and there may be conditions in which this cannot be achieved without some form of dehumidification. Unfortunately, some air conditioners do not dehumidify well, especially if the need for sensible cooling is small, which is often the case in energy-efficient buildings. When dehumidification is required, a high-efficiency dehumidifier or a high-efficiency air conditioner with a high moisture-removal rating should be used.
Final Thoughts
By making use of today’s best building science, we can not only design and construct buildings that will last well over a century but also greatly reduce the risk of moisture-related health problems, including exposure to molds and other allergens. To apply building science, we have to address the interactions among components in a building--looking to manufacturers for solutions at the level of their individual product isn’t enough. There is rarely anyone filling the role of building scientist on design teams today. It is up to architects to either learn enough to play that role, or hire consultants who can work through every detail of the envelope and mechanical and plumbing systems with them.
For more information:
Building Science Corporation
70 Main Street
Westford, MA 01886
978-589-5100, 978-589-5103 (fax)
www.buildingscience.com
Addendum: Specific Facts about Mold and Moisture in WNC
WNC is in the EXTREME annual precipitation zone for North America. This brings us awesome natural beauty along with constant sources of mold and fungi. As the article above proves: when outdoor mold moves indoors, it becomes toxic to people and the structure itself.
A majority of days in mid to late spring, all of summer and portions of the fall months are humid beyond safe levels mentioned in the article above. This means that air conditioning and dehumidification are essential in all homes- leaky or tight, new or old. It also means that because a majority of the homes in WNC are not currently air-conditioned, that a majority of these homes have toxic mold in the structure as well as the interior of the home.
To elaborate on the Controlling Moisture in Buildings section of the above article as it specifically pertains to WNC’s moisture issues- it is more practical to design our new homes so that all interior space is conditioned, if possible. Instead of designing with crawl spaces, make that space usable as a basement that is properly constructed to deter moisture from coming in, is heated, air conditioned and dehumidified as needed. It also means the elimination of or conditioning of the attic, if it is a necessary part of your design. Put your HVAC, plumbing and any additional mechanical equipment in your conditioned space. This strategy will increase the life of your equipment, increase equipment efficiency and empower you to monitor any condensation or leaks easily. Creating a tight envelope with the use of mechanical ventilation also allows for you to control your indoor air quality that much more. Please refer to the article "Bridging the Gap between Solar and Healthy Home Design" for more details on this subject.
A tight building envelope enables you to control your indoor air quality, while a leaky envelope may allow you to maintain lower levels of mold, but makes it difficult to keep mold from proliferating. If a crawl space is unavoidable in the design of your home (existing or new), sealing it by using the building science methods (www.buildingscience.com) is recommended. Creating a climately controlled basement instead- by following proper moisture free construction methods (Builders Guide for Mixed Humid Climates by EEBA (www.eeba.org) is also a direction you can take.
I consult with people daily (from all over the South and Northeast) that suffer from Mold allergies and sensitivities. Yet I often hear that "they like to allow fresh air in by opening their windows". In a mixed humid / high humid climates (extreme moisture area) like WNC, natural ventilation such as this also allows high humidity in, thus allowing toxic mold in your home. Factually speaking, our outdoor air quality ranks right up there with Los Angeles, one of the more polluted cities in the U.S. Given these facts, it makes sense to create a home that is healthy by designing it to stay dry.
A few facts on closing:
• IAQ inquires concerning toxic mold have increased from less than 2% in 2001 to over 25% in 2002. Environmental Building News, Mold in Buildings and How to Keep it Out, Volume 10, Number 6- June 2001
• In 1991, The Business Council on Indoor Air (in a 10-year, 700 building survey) found microbial growth to be the #1 IAQ problem in the nation. Environmental Building News, Mold in Buildings and How to Keep it Out, Volume 10, Number 6- June 2001
• A 1999 Mayo Clinic article states that 93% of all chronic sinusitis is caused by mold and fungus AND over 37 million people in the US suffer from chronic sinusitis. Mayo Clinic Rochester News, Sept. 9, 1999
• A 300% increase in the asthma rate over the past 20 years has been linked to mold AND over 50% of homes have mold problems. USA Weekend, Dec. 3-5, 1999
Checklist: Design Strategies for Moisture Control
Keeping Water Out
• Provide proper flashing on all windows and doors: All components should be layered so that water is shed down and outward. There are numerous ways to flash windows flashing can be installed before or after housewrap or building felt drainage plane, specialized formable flashing can be used at the sill, and special measures are required with masonry walls and when rigid foam provides the drainage plane (see illustration, page 17, as well as the EEBA Water Management Guide, review page 19).
• Provide a rain screen behind siding: To facilitate drying of siding and to provide a capillary break between the siding and the sheathing, most experts now recommend rain screen detailing in wetter climates (see map, page 13). This can be provided with vertical strapping (minimum 3 8, 9.5 mm thick); a specialized rain screen product, such as Benjamin Obdyke's Home Slickerª; or, with brick siding, a bottom-draining air space behind the brick facing.
• Seal wood and fiber-cement siding: Porous siding materials should be sealed on all sides. Pre-priming is recommended prior to installation, with multiple coats as needed, especially on end-grain. Sealed siding will not absorb moisture, thereby reducing moisture migration driven by solar heat.
• Provide a capillary break above footing: Paint the top of the footing with a dampproofing coating or install an impermeable layer before installing the foundation wall to block the upward migration of soil moisture.
• Provide drainage layer and poly vapor retarder under concrete slab: Before pouring a concrete floor slab, install a minimum 4 (100 mm) layer of crushed stone (no fines) and a poly vapor retarder. The con crete should be poured right on top of the poly; without a layer of sand between the poly and slab.
• Provide perimeter drainage at footing: Install crushed stone (no fines) and perforated drain pipe around the footing. The drainage pipe should be slightly pitched but extend neither below the bottom of the footing nor above the top of the footing. Sections of plastic pipe should extend through the footing every 6 to 12 feet (2_4 m) to drain the space under the slab. Wrap the layer of crushed stone with geotextile filter fabric to keep fines out.
• Paint outside of foundation wall with dampproofing layer: A durable dampproofing should be painted on the outside of the foundation wall. Several layers are suggested, with the minimum thickness depending on the material (follow manufacturer's recommendations).
• Install free-draining layer next to foundation wall: Install a specialized drainage layer (free-draining insulation, kinked nylon mesh, corrugated plastic, etc.) against the foundation wall, or backfill against the wall with crushed stone, or do both. When using a layer of crushed stone, protect it with a geotextile filter fabric to keep fines out.
• Slope ground away from building and provide impermeable cap: The ground should slope away from a building at a minimum pitch of 5% (6 per 10', 5 cm per m). Provide a low-permeability (high-clay-content) soil cap extending 6 feet (1.8 m) from the building to reduce infiltration and direct surface runoff away from the structure.
• Provide a roof overhang to keep rainwater away from building: The longer the roof overhang, the greater the protection of the house. A minimum 24 (600 mm) overhang is recommended in most climates; 36 (900 mm) is preferable. Porch roofs and awnings also drain water away.
• Provide self-sealing ice and water barrier on roof: To protect against water penetration if ice dams occur in cold climates, install a self-sealing protective layer (e.g., Grace Ice & Water Shield¨) under roofing.
• Install gutters and downspouts: Install durable gutters and downspouts to direct rainwater away from the building. Use screening to keep leaves and other debris out of downspouts, and instruct building owners to keep gutters and downspouts clean. At the bottom of downspouts, water should be channeled as far from the building as possible. Do not connect downspouts to footing drains.
Designing Building Assemblies to Dry Out
In most climates, provide drying potential to both interior and exterior: Building Science Corporation now recommends that in all but the coldest climates, above-ground walls should be designed to dry to both the exterior and interior. This means avoiding a poly air/vapor retarder and using permeable or semipermeable layers in the wall system.
In the coldest climates, install poly vapor retarder on interior: BSC recommends that polyethylene vapor retarders should be used only in the coldest climates. In such climates (over 8,000ºF [3,400ºC] heating degree days), install a poly vapor retarder on the interior wall, under the drywall. The vapor retarder should be carefully sealed at all overlaps, edges, and penetrations. This arrangement will allow the wall cavity to dry to the exterior only.
Design basements to dry to interior: In all climates, basement walls should be designed to dry to the interior. Insulation should be permeable or semipermeable, such as expanded polystyrene (EPS) or fiberglass with a moisture-resistant gypsum board product, such as Fiberock¨ or HumiTechª, and vapor-permeable latex paint on the interior.
Provide vented rain screen on exterior walls: In climates with more than 40 (1 m) of rain per year, BSC recommends an exterior wall detail with a minimum 3 8 (9.5 mm) air space behind the siding, with screened vents at top and bottom. This rain screen detail both provides a capillary break and allows drying to the exterior. In climates with fewer than 40 (1 m) of rain per year, this vented rain screen may not be necessary. See map, page 13.
Use plywood rather than OSB to aid in drying: Recent research shows that while both plywood and OSB have low permeability when dry, plywood becomes significantly more permeable than OSB when moisture content rises; this will aid drying to the exterior. With a fully vented rain screen detail, using OSB should be satisfactory.
Consider perforating sheathing: The June 2003 issue of Energy Design Update reported on Canadian research demonstrating the effectiveness of drilling 3 (75 mm) holes through the exterior sheathing at the top and bottom of stud bays. Some builders drill lots of smaller holes through the sheathing to aid in drying.
Provide a vented roof assembly: In all climates, a vented roof assembly will assist in drying and is generally recommended. In cold climates, a vented roof also helps to prevent ice dams by keeping the roof surface cold. Full-length soffit and ridge vents are the preferred venting strategy. Keep roof geometry simple to aid in venting. Unvented (hot) roofs can be successful, but only with great care in the construction; avoid this approach unless working with an expert.
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Written By: Alex Wilson, Environmental Building News, Volume 12, Number 7, July 2003
Edited By: Cindy Meehan-Patton, Shelter Ecology
Addendum By: Cindy Meehan-Patton, Shelter Ecology
I am continually surprised by how little emphasis the green building movement places on building science. As we examine the many priorities of green building, from land-use planning to energy efficiency, material selection, and indoor air quality, the basic science of how we design and build structures to ensure long life and healthy indoor environments should be at (or near) the top of the list. Excess moisture is at the center of many problems relating to durability and indoor air quality, not just in homes but in all types of buildings in a wide range of climates. If we build structures that won’t rot or support mold growth, we will both increase the longevity of those buildings and reduce the health risks of living in them.
This article examines the physics of moisture in buildings and addresses design and construction strategies for (a) keeping buildings dry and (b) allowing those buildings to dry out if they do get wet. While the emphasis and examples are largely focused on houses, most of the ideas apply more broadly.
Moisture 101
Moisture exists in three commonly known forms or phases: liquid, gas (vapor), and solid (ice), along with an additional adsorbed state that is somewhat between liquid and vapor in characteristics. All these forms come into play in buildings. The movement of moisture from one place to another is governed by physical forces including:
• Gravity. Rain falls to earth; water runs down roofs and walls.
• Capillarity. Water can be pulled through thin air spaces or pores through a process called capillarity. Capillarity occurs because intermolecular forces cause water to stick to itself (surface tension) and to many other materials. (These same intermolecular forces allow water to cling to the bottom of a joist, running along horizontally until a gap is reached where the water accumulates into a drop large enough that gravity becomes the dominant force.) Capillarity is a powerful force strong enough to raise water hundreds of feet into the air (counteracting gravity) in tall trees. This is the primary mechanism of moisture movement through porous materials, such as concrete floor slabs and wood siding.
• Convection. Airflow can carry moisture in both liquid and vapor forms. This movement occurs through air pressure differences. Wind blows rain into the walls of buildings, for example, and forced or passive airflow carries water vapor (humidity) with it.
• Diffusion. Water vapor can diffuse through permeable materials. This process is driven by partial pressure (vapor pressure) differences across the material. Vapor pressure as a driving force is confusing. In any mixture of gases, such as air, each separate gas has its own concentration or vapor pressure. If the vapor pressure of a gas in one area is greater than in another area, those vapor pressures will try to equalize through the movement of gas molecules from one air mass to the other. This is why it’s very difficult to keep part of a building dry through air-conditioning if other parts of the building operate at ambient humidity levels. (Experts once thought diffusion was the primary mechanism through which moisture got into wall cavities, and we installed poly vapor retarders to block that diffusion; they now believe that airflow –convection-is a much greater source of such moisture migration.)
• Temperature differential. Moisture is known to move from hot to cold within a material. When brick siding gets soaked from rain, for example, and then the sun heats the outside of the brick, moisture in the brick is driven through the wall to the interior.
Integral to the moisture dynamics in a building is the issue of relative humidity and the phase change between liquid and vapor. Humidity is a measure of the moisture content of air -the concentration of water-vapor molecules in a particular air mass. The amount of water vapor a given volume can hold is dependent on temperature -more water vapor can exist in a space if it is warm than if it is cold. As a mass of air and water vapor is cooled, the relative humidity increases, until the mass reaches 100% RH, when liquid water condenses out (changes phase from gas to liquid). This point is known as the dew point.
This process has a huge bearing on moisture problems in buildings. If warm indoor air flows into the wall cavity through cracks in the drywall during cold weather, for example, that air mass may cool enough to reach the dew point within the wall cavity. When this occurs, the insulation in the wall cavity or the inner surface of the exterior sheathing gets wet-and that can cause big problems.
Risks of Moisture in Buildings
Excess moisture in buildings is bad. Decay of wood and other cellulosic materials is dependent on moisture and temperature. With moisture content in framing lumber over about 20% (28% according to some experts) and typical room temperatures, decay is possible.
In addition to causing wood decay, moisture can corrode-and eventually destroy-steel structural members and fasteners. Steel does not have to be wet to begin rusting. Corrosion can occur whenever the relative humidity is above 70-80%, though corrosion problems worsen when pipes or fasteners actually become wet through condensation, leaks, and so forth.
Keeping humidity levels within an acceptable range is an important strategy both for ensuring a long life for buildings-longer-lasting buildings are greener buildings-and for ensuring health of the occupants. The relative humidity should not be lower than about 25% (to prevent dry eyes and throats, shrinking of wood flooring, and static electricity problems on carpeting) or higher than about 60% in the center of a room. The 60% level is intended to keep the relative humidity from exceeding 70% at surfaces, such as walls and floors. The relative humidity near surfaces is typically higher than it is in the center of a room. When the relative humidity at surfaces is above 70%, mold growth can occur. Things dry out more slowly when the humidity level is high. Thus, at higher relative humidity levels in a building, even a fairly small source of liquid water can become a big problem.
Controlling Moisture in Buildings
Strategies for dealing with moisture in buildings include the following: keeping water out, avoiding (or managing) plumbing leaks, avoiding condensation inside the building or within the building envelope, controlling the entry of humid outside air, controlling indoor sources of humidity, designing building assemblies to dry out, providing mechanical ventilation, and providing mechanical dehumidification.
Keeping water out
"Drain, drain, drain’, is the mantra echoed by building scientists Terry Brennan and Joseph Lstiburek. Keeping rainwater out necessitates quality flashing details at all window and door openings, roof penetrations, and roof-wall intersections. Less obvious but also very important is the need for drainage planes in walls to prevent water from getting through siding and sheathing. Capillary breaks are also needed to keep groundwater from moving through foundation walls and floor slabs. Rainwater can be kept away from the building walls with overhangs, properly installed gutters and downspouts, and an adequately sloped ground surface around the building.
Avoiding moisture problems from plumbing leaks
Lstiburek considers plumbing leaks to be "more significant than any other factor" in causing moisture problems in buildings. Based on his company’s experience, he estimates that one-third of all houses develop plumbing leaks within the first five years! Most building science experts recommend installing all plumbing in interior rather than exterior walls. Brennan even suggests leaving pipes fully exposed to provide full access-as he sometimes sees in older buildings.
In place of standard drywall, use one of the recently introduced mold-resistant drywall products (U.S. Gypsum’s Humitekª or Georgia Pacific’s DensArmorª), monolithic (non-paper-faced) drywall, such as USG’s Fiberock¨, or- where leaks and other moisture problems are most likely-cement board. If standard drywall must be used, at least raise the drywall panels a few inches off the floor and cover the space with baseboard (taking care not to compromise the airtightness of the wall). If practical, install clothes washers and water heaters in spaces with floor drains or in watertight pans, and provide clearly visible shut-off valves. Flooring in spaces prone to leakage should be water resistant, such as tile. Avoid carpeting in these locations.
Avoiding condensation
Condensation can occur in warmer months when humidity is high and cold-water pipes are significantly cooler than the air temperature. When such condensation is severe, water can accumulate and cause mold or decay. Risk of condensation can be reduced or eliminated through careful attention to energy-efficient building practices (high-performance glazings, proper wall insulation, insulative sheathing on steel framing, foundation and slab insulation, etc.) and by insulating all water pipes.
Controlling humidity
High humidity in buildings can come from either indoor or outdoor sources. The concern is typically much greater in summer months, when outdoor humidity levels are high. While experts used to recommend ventilating unheated basements during the summer months, most now discourage that practice in humid climates (even though codes may still require it), because the ventilation introduces more moisture than it removes.
Preventing the entry of humid outside air involves creating a tight building, installing a poly vapor retarder on the ground in crawl spaces, and avoiding conditions that will depressurize the building. The most common interior sources of humidity; showering and cooking-are best dealt with through spot ventilation at the source. Avoid storing firewood indoors, venting clothes dryers indoors, and using unvented gas heaters-the last because unvented heaters produce significant quantities of water vapor as well as other combustion products.
Designing building envelopes to dry out
Because we can’t always succeed in keeping moisture out of our buildings, the second line of defense should be to design our structures to dry out. Think of this as an insurance policy. Some strategies for improving a building’s drying potential have ancillary benefits, such as longer life for paint. How you should design a building envelope to dry out depends on the climate. Experts used to say that you should always put the vapor retarder on the warm side (on the interior in cold climates and on the exterior in warm climates), but Lstiburek now argues that in most climates the envelope should have some drying potential in both directions-because moisture-driving forces change dramatically throughout the year.
CertainTeed has begun marketing MemBrainª, developed by a German subsidiary of CertainTeed’s parent company, Saint-Gobain. This so-called "smart" air/vapor barrier, made from 2-mil nylon, becomes more permeable as the humidity increases. It may even be effective as an exterior housewrap product in hot, humid climates
Providing mechanical ventilation
Mechanical ventilation should always be part of the moisture-control strategy for building. The proposed ASHRAE 62.2 residential ventilation standard has just been approved by a key committee at ASHRAE and is awaiting publication-despite strong opposition by the National Association of Home Builders and the Gas Appliance Manufacturers Association. This standard, for those municipalities that adopt it, would require continuous mechanical ventilation rated at 7.5 cubic feet per minute (cfm) per occupant plus 1 cfm for every 100 ft2 of usable floor area- 3.5 liters per second (l/s) plus 0.5 l/s for every 9 m2.
Providing dehumidification
Experts recommend that indoor RH levels should be kept below 60%, and there may be conditions in which this cannot be achieved without some form of dehumidification. Unfortunately, some air conditioners do not dehumidify well, especially if the need for sensible cooling is small, which is often the case in energy-efficient buildings. When dehumidification is required, a high-efficiency dehumidifier or a high-efficiency air conditioner with a high moisture-removal rating should be used.
Final Thoughts
By making use of today’s best building science, we can not only design and construct buildings that will last well over a century but also greatly reduce the risk of moisture-related health problems, including exposure to molds and other allergens. To apply building science, we have to address the interactions among components in a building--looking to manufacturers for solutions at the level of their individual product isn’t enough. There is rarely anyone filling the role of building scientist on design teams today. It is up to architects to either learn enough to play that role, or hire consultants who can work through every detail of the envelope and mechanical and plumbing systems with them.
For more information:
Building Science Corporation
70 Main Street
Westford, MA 01886
978-589-5100, 978-589-5103 (fax)
www.buildingscience.com
Addendum: Specific Facts about Mold and Moisture in WNC
WNC is in the EXTREME annual precipitation zone for North America. This brings us awesome natural beauty along with constant sources of mold and fungi. As the article above proves: when outdoor mold moves indoors, it becomes toxic to people and the structure itself.
A majority of days in mid to late spring, all of summer and portions of the fall months are humid beyond safe levels mentioned in the article above. This means that air conditioning and dehumidification are essential in all homes- leaky or tight, new or old. It also means that because a majority of the homes in WNC are not currently air-conditioned, that a majority of these homes have toxic mold in the structure as well as the interior of the home.
To elaborate on the Controlling Moisture in Buildings section of the above article as it specifically pertains to WNC’s moisture issues- it is more practical to design our new homes so that all interior space is conditioned, if possible. Instead of designing with crawl spaces, make that space usable as a basement that is properly constructed to deter moisture from coming in, is heated, air conditioned and dehumidified as needed. It also means the elimination of or conditioning of the attic, if it is a necessary part of your design. Put your HVAC, plumbing and any additional mechanical equipment in your conditioned space. This strategy will increase the life of your equipment, increase equipment efficiency and empower you to monitor any condensation or leaks easily. Creating a tight envelope with the use of mechanical ventilation also allows for you to control your indoor air quality that much more. Please refer to the article "Bridging the Gap between Solar and Healthy Home Design" for more details on this subject.
A tight building envelope enables you to control your indoor air quality, while a leaky envelope may allow you to maintain lower levels of mold, but makes it difficult to keep mold from proliferating. If a crawl space is unavoidable in the design of your home (existing or new), sealing it by using the building science methods (www.buildingscience.com) is recommended. Creating a climately controlled basement instead- by following proper moisture free construction methods (Builders Guide for Mixed Humid Climates by EEBA (www.eeba.org) is also a direction you can take.
I consult with people daily (from all over the South and Northeast) that suffer from Mold allergies and sensitivities. Yet I often hear that "they like to allow fresh air in by opening their windows". In a mixed humid / high humid climates (extreme moisture area) like WNC, natural ventilation such as this also allows high humidity in, thus allowing toxic mold in your home. Factually speaking, our outdoor air quality ranks right up there with Los Angeles, one of the more polluted cities in the U.S. Given these facts, it makes sense to create a home that is healthy by designing it to stay dry.
A few facts on closing:
• IAQ inquires concerning toxic mold have increased from less than 2% in 2001 to over 25% in 2002. Environmental Building News, Mold in Buildings and How to Keep it Out, Volume 10, Number 6- June 2001
• In 1991, The Business Council on Indoor Air (in a 10-year, 700 building survey) found microbial growth to be the #1 IAQ problem in the nation. Environmental Building News, Mold in Buildings and How to Keep it Out, Volume 10, Number 6- June 2001
• A 1999 Mayo Clinic article states that 93% of all chronic sinusitis is caused by mold and fungus AND over 37 million people in the US suffer from chronic sinusitis. Mayo Clinic Rochester News, Sept. 9, 1999
• A 300% increase in the asthma rate over the past 20 years has been linked to mold AND over 50% of homes have mold problems. USA Weekend, Dec. 3-5, 1999
Checklist: Design Strategies for Moisture Control
Keeping Water Out
• Provide proper flashing on all windows and doors: All components should be layered so that water is shed down and outward. There are numerous ways to flash windows flashing can be installed before or after housewrap or building felt drainage plane, specialized formable flashing can be used at the sill, and special measures are required with masonry walls and when rigid foam provides the drainage plane (see illustration, page 17, as well as the EEBA Water Management Guide, review page 19).
• Provide a rain screen behind siding: To facilitate drying of siding and to provide a capillary break between the siding and the sheathing, most experts now recommend rain screen detailing in wetter climates (see map, page 13). This can be provided with vertical strapping (minimum 3 8, 9.5 mm thick); a specialized rain screen product, such as Benjamin Obdyke's Home Slickerª; or, with brick siding, a bottom-draining air space behind the brick facing.
• Seal wood and fiber-cement siding: Porous siding materials should be sealed on all sides. Pre-priming is recommended prior to installation, with multiple coats as needed, especially on end-grain. Sealed siding will not absorb moisture, thereby reducing moisture migration driven by solar heat.
• Provide a capillary break above footing: Paint the top of the footing with a dampproofing coating or install an impermeable layer before installing the foundation wall to block the upward migration of soil moisture.
• Provide drainage layer and poly vapor retarder under concrete slab: Before pouring a concrete floor slab, install a minimum 4 (100 mm) layer of crushed stone (no fines) and a poly vapor retarder. The con crete should be poured right on top of the poly; without a layer of sand between the poly and slab.
• Provide perimeter drainage at footing: Install crushed stone (no fines) and perforated drain pipe around the footing. The drainage pipe should be slightly pitched but extend neither below the bottom of the footing nor above the top of the footing. Sections of plastic pipe should extend through the footing every 6 to 12 feet (2_4 m) to drain the space under the slab. Wrap the layer of crushed stone with geotextile filter fabric to keep fines out.
• Paint outside of foundation wall with dampproofing layer: A durable dampproofing should be painted on the outside of the foundation wall. Several layers are suggested, with the minimum thickness depending on the material (follow manufacturer's recommendations).
• Install free-draining layer next to foundation wall: Install a specialized drainage layer (free-draining insulation, kinked nylon mesh, corrugated plastic, etc.) against the foundation wall, or backfill against the wall with crushed stone, or do both. When using a layer of crushed stone, protect it with a geotextile filter fabric to keep fines out.
• Slope ground away from building and provide impermeable cap: The ground should slope away from a building at a minimum pitch of 5% (6 per 10', 5 cm per m). Provide a low-permeability (high-clay-content) soil cap extending 6 feet (1.8 m) from the building to reduce infiltration and direct surface runoff away from the structure.
• Provide a roof overhang to keep rainwater away from building: The longer the roof overhang, the greater the protection of the house. A minimum 24 (600 mm) overhang is recommended in most climates; 36 (900 mm) is preferable. Porch roofs and awnings also drain water away.
• Provide self-sealing ice and water barrier on roof: To protect against water penetration if ice dams occur in cold climates, install a self-sealing protective layer (e.g., Grace Ice & Water Shield¨) under roofing.
• Install gutters and downspouts: Install durable gutters and downspouts to direct rainwater away from the building. Use screening to keep leaves and other debris out of downspouts, and instruct building owners to keep gutters and downspouts clean. At the bottom of downspouts, water should be channeled as far from the building as possible. Do not connect downspouts to footing drains.
Designing Building Assemblies to Dry Out
In most climates, provide drying potential to both interior and exterior: Building Science Corporation now recommends that in all but the coldest climates, above-ground walls should be designed to dry to both the exterior and interior. This means avoiding a poly air/vapor retarder and using permeable or semipermeable layers in the wall system.
In the coldest climates, install poly vapor retarder on interior: BSC recommends that polyethylene vapor retarders should be used only in the coldest climates. In such climates (over 8,000ºF [3,400ºC] heating degree days), install a poly vapor retarder on the interior wall, under the drywall. The vapor retarder should be carefully sealed at all overlaps, edges, and penetrations. This arrangement will allow the wall cavity to dry to the exterior only.
Design basements to dry to interior: In all climates, basement walls should be designed to dry to the interior. Insulation should be permeable or semipermeable, such as expanded polystyrene (EPS) or fiberglass with a moisture-resistant gypsum board product, such as Fiberock¨ or HumiTechª, and vapor-permeable latex paint on the interior.
Provide vented rain screen on exterior walls: In climates with more than 40 (1 m) of rain per year, BSC recommends an exterior wall detail with a minimum 3 8 (9.5 mm) air space behind the siding, with screened vents at top and bottom. This rain screen detail both provides a capillary break and allows drying to the exterior. In climates with fewer than 40 (1 m) of rain per year, this vented rain screen may not be necessary. See map, page 13.
Use plywood rather than OSB to aid in drying: Recent research shows that while both plywood and OSB have low permeability when dry, plywood becomes significantly more permeable than OSB when moisture content rises; this will aid drying to the exterior. With a fully vented rain screen detail, using OSB should be satisfactory.
Consider perforating sheathing: The June 2003 issue of Energy Design Update reported on Canadian research demonstrating the effectiveness of drilling 3 (75 mm) holes through the exterior sheathing at the top and bottom of stud bays. Some builders drill lots of smaller holes through the sheathing to aid in drying.
Provide a vented roof assembly: In all climates, a vented roof assembly will assist in drying and is generally recommended. In cold climates, a vented roof also helps to prevent ice dams by keeping the roof surface cold. Full-length soffit and ridge vents are the preferred venting strategy. Keep roof geometry simple to aid in venting. Unvented (hot) roofs can be successful, but only with great care in the construction; avoid this approach unless working with an expert.
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