This article was originally published in the March/April 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online March/April 1998

Moisture Control In Bathrooms

By Marilou Cheple and Pat Huelman


Mold, mildew, failing grout, dislodged tiles, water stains, wet insulation, rotting walls--moisture in bathroom walls causes serious problems. One reason the issue of bathroom moisture is so hard to address is that these problems, and their solutions, vary from climate to climate. Things get more complicated as builders decipher contradictory information about strategies for air-sealing, ventilating, and insulating. 
Figure 1. In bathrooms, it is especially important for builders to control heat loss, air movement, and moisture movement. The hardest place to control these three is at a juncture or at a corner. All the same, many homes are built with tubs located in these difficult locations. At a cantilever, three or four angles are present in a short stretch of wall; two such angles are shown here. These must be carefully constructed and sealed to keep moist air from moving into or out of the house. This design is for hot, humid climates.

Moisture Vocabulary

Air permeability: The property of a building component to let air pass through when air pressure on one side is higher.

Airtight drywall approach: A construction procedure designed to stop airflow into and out of the building at the warm surfaces through careful detailing and gaskets at the top and bottom of the drywall panels and framing joints.

Capillary: The flow of liquid moisture without pressure or gravity, through small interconnected pores or spaces due to adhesion and surface tension.

Diffusion: Movement of water vapor between two areas caused by a difference in vapor pressure, independent of temperature and air movement. The rate of diffusion is determined by the difference in vapor pressure, the permeability of the material to water vapor, and the surface area of the material.

Permeability: The ability to transmit water vapor. Measured in perms. 

Relative humidity: The amount of water vapor in the air compared with the maximum amount possible at the same temperature. Expressed as a percentage.

Sone: A unit of loudness. Forty sones is twice as loud as twenty sones, making it different from decibels. Forty decibels is 100 times as loud as twenty decibels. A sone is about the volume of a refrigerator.

Vapor diffusion retarder: Any material of low water vapor permeability used to restrict the movement of water vapor due to vapor diffusion.

Vapor pressure: The pressure exerted by water vapor in the air. Water vapor moves from an area of high pressure to an area of low pressure.

These tiles and their substrate fell off the wall 18 months after installation. If moisture had been able to evaporate out the back of the substrate, it would have diffused back into the house through the paint at the top of the tiles.
Figure 2. Tiled showers on exterior walls are bound to suffer from moisture problems. One way to reduce moisture penetration into the wall is to isolate the tiled wall from the actual exterior wall with an air space. This prevents water from moving in through capillary action, and instead provides a space into which the tiles can dry out. Vapor from the drying tiles can get back into the bathroom by diffusing through the tile grout or through the paint at the top of the wall. The entire bathroom wall must have an air space.
When water has no way to evaporate, it can condense and run down the wall. In this tub, water built up into a reservoir where the tub meets the substrate and tiles.
Protecting bathroom walls from trapped moisture requires different practices in different climates. In cold climates, the moist side of the wall will almost always be the interior side while in hot, humid climates, both sides will be exposed to moisture. But in any climate, the goal is the same--minimize moisture intrusion into the wall, and then provide a way for the wall to dry out if it does get moist.

Different builders, researchers, and building guides give different techniques for dealing with bathroom moisture. The technique described below is unusual in that it does not require special materials or extra education for installers.

Build Tight, Ventilate Right Years ago, houses very rarely had interior moisture problems. It was the norm to have drafty rooms throughout the house. Curtains moved at the windows. Kids ran to the heat supplies to get warm in the morning. Nobody stepped out of a shower without shivering. Somewhere along the road, homeowners found out that if they had well-insulated, air-sealed walls and good windows, they were more comfortable. And they decided they liked it.

Today, homeowners demand the comfort of a tighter wall system. And in hot climates, they want mechanical cooling. These demands, along with improved building materials and methods, widespread energy concerns, and more stringent building codes, have led to increased house tightness. Builders can no longer count on leaky houses to provide the ventilation necessary to keep people healthy and prevent moisture damage. In many cases, the reduced airflow has also reduced walls' ability to dry out.

Before homes were tightened up, the moisture problems that did happen were in three areas: the basement, the kitchen, and the bathrooms. Of these, bathroom moisture led to some of the worst problems. Builders have come a long way in building better houses with better products, but moisture problems in bathrooms persist.

Today's builders must consider how to remove water vapor from bathrooms. If it stays and condenses into moisture, it can support mold growth, force ceramic tiles free of grout holdings, and even cause wall decay. Builders must insulate, air-seal, and ventilate correctly. The old adage, Build tight, ventilate right, isn't just an option--it's a necessity.

Why Tight is Right A good building envelope is the starting point for a good house. The most recent builder guides and research projects suggest that the most efficient, most economical walls will have the following components: exterior finish, drain plane behind the cladding material, wind protection layer, full coverage insulation, an appropriate structural wall system, air barrier, vapor retarder, interior wall material, and finish. In cold climates, the air barrier and vapor retarder should be placed on the interior, or warm, side of the insulation before the interior wall material is installed. In hot climates, particularly those which are humid, it is critical to have a vapor retarder and air barrier on the exterior side of the insulation. In general, the wall system must keep water and moisture out, must resist heat flow in either direction, and must retard air and vapor flow from the moist side.

It may be easy enough to build a flat vertical wall that meets these criteria. But some design features--dropped soffits without air barriers at the ceiling plane, cantilevers, and bays--can challenge wall integrity. Unfortunately, these features are often found in the bathroom, the most moisture-prone room in the house (see Figure 1).

Moisture Fundamentals There are four strategies for controlling interior moisture--eliminate the source of the moisture (clearly not an option for bathrooms); mechanically dehumidify the air; exhaust moist air; or dilute moist air by ventilating. The point is to protect the wall cavities from moisture.

In a bathroom, builders need to fulfill many apparently contradictory needs--they must get rid of the excess vapor while controlling heat loss, air movement, and moisture movement. This is accomplished with the help of vapor retarders, air barriers, insulation, and ventilation.

The terms vapor retarder and vapor barrier are used interchangeably, although the correct term is vapor retarder, because almost no material is truly a vapor barrier. The term refers to the permeability of a material--that is, how much vapor is allowed to pass through it. The lower the permeability, measured in perms, the less vapor can pass through. Vapor retarders are intended to control the diffusion of vapor through building materials. Vapor can move slowly through permeable materials such as drywall and wood, but it is retarded by polyethylene sheeting, for example.

Air barriers are designed specifically to retard convective air flow. Air barriers are sometimes referred to as air retarders, but the two terms mean the same thing. Water vapor that is carried in the air is blocked by air barriers.

In cold climates, warm, moist air inside the house must be kept from getting into the wall cavity and condensing on a cold surface. In hot, humid climates, moist exterior air must be kept from getting into the wall cavity where it might condense on the cooler interior surface, especially if the home has mechanical cooling. In any climate, if moisture can get into a wall, there must be a way for it to dry out.

In hot, humid climates, a house needs vapor retarders and air barriers outside the insulation. In fact, recent research has led some building scientists to suggest that these homes should have vapor barriers on both sides of the thermal boundary. Many homes use sealed foam or other impermeable sheathings. Polyethylene is typically used as the vapor retarder in cold climates, although kraft-faced products, foil-backed drywall, and vapor retarder (typically oil-based) paint, could be used. Many cold-climate homes use an airtight drywall approach with gaskets.

During cold weather, ventilation is pretty good at reducing moisture. Cold outside air usually carries less total moisture than warmer indoor air. When ventilation blows indoor air to the outdoors, some water vapor gets carried along. Meanwhile, it draws colder, drier air into the house. If ventilation depressurizes the house and sucks air in through leaks, the infiltration of drier exterior air won't cause moisture problems in the walls. Of course, the infiltrating cold air can cause serious comfort problems--especially if it is leaking into the bathroom.

In hot, humid climates, exhausting air from the bathroom depressurizes the house and draws hot, humid air into the wall cavities. This makes it important to have powered makeup air. Makeup air systems blow cooled and/or dehumidified air into the house with a powered air supply unit. This system usually creates a positive pressure in the house. While this could create problems in a cold climate, it is a recommended strategy for hot, humid climates.

Bathroom Walls--Cold Climate Showering produces about a half a pint of moisture for each five minutes that the shower is on. To keep water from condensing and building up in the walls, the bathroom needs a way to expel moisture. A good exhaust fan alone will expel the moist air, but what about the liquid water that is absorbed by products in the room? Grout between the tiles is permeable. It will allow water to pass through by capillary action. If a lot of water gets into the grout, it can stay wet long enough to cause mold and mildew growth behind the tile. This may loosen the bond of the tile. If water in the grout moves toward the drier outdoors, as it would in a cold climate, it can cause deterioration of the wall itself. This usually happens only on exterior walls. On interior walls, the water vapor can evaporate fairly well in the empty wall cavity.

The easiest way to avoid this problem is to avoid placing tubs and showers on exterior walls. But sometimes, that placement is the only choice. The next best solution is to put the shower or tub on an exterior wall and use a prefab unit or paint to cover the wall, instead of tile. But sometimes, the homeowner will insist on tile. In this situation, there are a few guidelines to reduce the chance of moisture problems. The most basic is this: Do not use drywall. Any kind of drywall. Blue, green, white, pink. Doesn't matter. Don't use it. It likes water and will hold on to water for a long time. Do use cement board. Cement board will take on water, but it won't deteriorate easily when wet. It will also give up water at a faster rate.

Figure 2 shows a way to build an exterior wall that can support a tile tub or shower. The theory is simple. The space behind the cement board provides a cavity for drying and prevents capillary action from carrying water deeper into the wall system. Water vapor can evaporate back into the room if the wall above the tub is covered with a permeable finish, such as latex paint. Tile at the tub lip can be finished as usual with grout and caulk.

Sometimes, the seam between the tile and the tub allows capillary action to carry water back up behind the tile. If there is no air space, the seam can become saturated, and the moisture has no way to evaporate out, leading to deterioration and tile failure. An air space allows the water to pass through and be evaporated out to the back. When creating an air space above the tub, the entire exterior wall of the bathroom must be constructed in the same manner. If it is not, the potential for failure increases (see Figure 2).

The gap between the tub and the wall behind the tub should be insulated. This spot is often missed, as the tub is typically installed before the insulation.

Bathroom Walls--Hot Climate In hot, humid climates, many of the same principles apply. But in hot climates, it is especially important to prevent moist air from getting into the wall cavity--from either inside or out. The wall needs to be moisture-sealed on the exterior side to keep outdoor moisture from diffusing in. And it must be sealed on the interior side to keep bathroom moisture out. This means that the sealing must be perfect--if vapor condenses inside the wall, it won't have any easy way to dry out.

To address this problem, start with the same basic suggestions as in a cold climate. Avoid putting showers and tubs on the exterior wall; use a good exhaust fan with powered makeup air; don't use drywall; do use cement board.

In hot, humid climates where air conditioning is in use most of the time, houses are supposed to have an air barrier and vapor retarder on the exterior wall surface. Any drying must be to the inside, where the air conditioning can remove the moisture in the air. Exterior walls of a bathroom with a tub or shower must also have vapor barriers on the interior. This means that the builder must create a tight wall where no air can move in or out. To avoid future problems, it must be built very carefully. This wall needs the same airspace as the wall in recommended for cold climates. A furred-out cavity will prevent capillary action from drawing water deep into the wall, and will increase drying potential. The water is able to evaporate to the air space, where vapor can diffuse out through the permeable paint at the top of the tile. In this type of wall, builders should take special care to seal the wall at the sole- and top plates.

As If You Don't Have Enough to Worry About ... A well designed, well built bathroom will keep moisture out of the walls. But some framing and design choices challenge the bathroom's ability to control moisture. The most common culprit in today's new houses is the recessed light. In some states, codes require that these lights be the airtight type. No matter what the specifications say, none of the lights on the market today is truly airtight--some air will get through. The only way to be sure that these lights are not the direct cause of warm, moist air getting into attic spaces is to build an airtight box around the light fixture and to insulate the box. An even better way to solve the problem is to house such a light in a dropped soffit with an air barrier at the ceiling plane.

Making the lights airtight is especially important in cold climates, where the warm, moist air condenses and sometimes freezes on the first cold surface--typically the attic rafters or sheathing. The problem is compounded when the sun heats up the roof, causing water to melt down on the attic insulation. The insulation gets wet and mold begins to grow.

One of the riskiest things a builder can do is to place a tub or shower in a cantilevered space. Such designs are increasingly popular in custom and luxury homes. Cantilevered spaces are very difficult to air-seal perfectly. And perfect is what you need. It can be done, but there is a great chance of future failure. Why take the risk? The homeowner is likely to complain about the cold and drafts, not to mention frozen pipes. Better to avoid this design in the first place.

One of the easiest ways to help prevent excessive window condensation in a cold-climate bathroom is to choose high quality windows that have a high R-value and warm-edge technology. These windows offer the best chance at eliminating condensation problems. The occupants will be more comfortable, too. Choosing high-quality windows alone will not take care of all moisture problems in a bathroom, but it will help. Keep windows away from areas where condensation is most likely. For example, don't place them high on bathroom walls, and don't install skylights in cold climates.

A good exhaust fan is critical to moisture control in bathrooms in both hot and cold climates. Choose one that is quiet and can exhaust at least 70 CFM. To ensure that any fan is capable of moving air against the friction of the duct run and the pressures caused by typical duct leakage, choose a fan rated at 0.2 inches of water column (WC), rather than the more common 0.1-inch WC rating. Putting the controls on a timer will help. Homeowners should be instructed to leave the fan on for at least 15 to 20 minutes after showering. A timer means that they can simply set it and leave. We have found people are much more likely to use the fan with this feature.

Another important feature for a bathroom fan is its sone rating. A sone is measurement of loudness. One sone is about the loudness of a quiet refrigerator running in a quiet room. There are fans available today with ratings of less than one sone--so quiet it is difficult to know when they are running. The biggest reason people do not use fans is that the noise is offensive. This just shouldn't be an issue anymore.

Fan installation is also important. Minimize the use of flexible duct. If it is used, make sure runs are short, the duct is fully stretched, and it is properly supported to minimize restrictions. For best results, use a sealed hard duct such as duct board or steel, and take the shortest possible route.

More Than Just Good Looks Losing a tile off a bathroom wall might be more than an annoyance; it might indicate a health risk to a house's occupants. Asthma and respiratory problems are rising rapidly. This may be because we spend 90% of our time indoors, and indoor air quality (IAQ) is not what it could be. One way to improve IAQ is to keep the inside dry. Mold and mildew cannot grow without moisture, but once moisture is available, mold and mildew will follow quickly. If surfaces are dry, mold and mildew won't start.

Get the moisture out, and keep it out. This is the basic principle of problem-free bathroom walls. Using the wall systems and ventilation systems described here will help keep a building and its occupants healthy.

The back of a bathroom wall should not be covered in mold. In this case, 60% of the wall has been consumed (left). The cause was clear--condensation began to appear on the wall within five minutes when the shower was turned on (right). The vapor got through the grout and substrate by direct diffusion.

Marilou Cheple is senior program manager at the Energy Efficient Building Association in Minneapolis, MN. Pat Huelman is associate professor, Department of Wood and Paper Science, University of Minnesota.


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