"Designing and Building Dry Buildings"
The President's Message, July 1999
by Linc Moss, Ramtech Building Systems, Inc.

For years modular as well as conventional construction has been plagued with moisture penetration of the building envelope. Still, the number one problem facing property management companies across the country today is roof, wall, window, and door leaks. Why is this such an issue? Designing shelter from the weather is nothing new. For gosh sakes, we've been doing this since we left the cave. Why haven't we yet figured out this most basic of needs?

Moisture penetration is a complex issue. Just ask any of our architect friends. There's a good reason that architects spend what seems to be an inordinate amount of time designing flashing and run-off details. They are aware of how critical this issue is due to constant feedback from users and building owners. As a result, they're continually refining ideas and designs that shed water away from building interiors. To understand this challenge, we should first determine the ways moisture can be introduced to a building's interior.

Water Sources
For our industry, the most common means of moisture penetration is the simple roof leak. Modular construction is normally more susceptible than conventional construction for several reasons. First, the integration of factory-applied roofing and field-applied flashings at matelines. This mixing of the work elements can often lead to confusion over material applications, surface preparation, etc. We've all heard the horror stories of improperly trained field personnel installing EPDM flashings with nothing more than roofing nails. Secondly, due to shipping constraints on module heights, most of the buildings we produce have a relatively flat roof that does not shed water as readily as roofs with a greater slope. Finally, many of the individuals utilized in our factories are not trained in the proper installation of the roofing materials. Typically more exotic roofing materials such as the variety of single-ply membranes we use, demand an understanding of the cleaning solvents, adhesives, and water stop mastics that are used. All of the roofing suppliers that cater to this industry offer factory-certified training for the installation of their membranes. To achieve an installation that is bondable, factory certified training for all plant and field installers is required.

The most common problem associated with waterproofing is a lack of proper roof drainage. The "rule of thumb" is for a roof to have 12 inches of overhang width for each floor level. A single-story building, for example, would have a roof that overhangs one foot.

Condensation is probably the second leading cause of moisture penetration. Since the energy crisis of the early 70's, construction as a whole has strived to build tighter and tighter buildings. This was done in an effort to reduce infiltration. The problem associated with this practice is a growing number of instances where moisture has formed on the inside of wall and/or roof cavities. In older buildings, the structural assemblies could "breathe" allowing water vapor to exit the building. In newer, tighter construction, this water vapor is trapped within the cavities, causing mold, mildew, odors, and wood rot. These are all the ingredients of a sick building. This phenomena occurs in both cold and warm climates whenever warmer humid air comes in contact with a cold surface.

Other sources of moisture into the building include introduced moisture. This addresses such things as improper venting of heat-producing appliances, rain and snow tracked into the building, open storage of liquids, even the respiration of the building's occupants. Poor maintenance practices, such as wet mopping of tiled floors, is yet another source of introduced moisture. This practice not only releases tremendous amounts of moisture into the building, the mop water can also wick up the wall coverings and penetrate cracks in the tile causing water damage to both the walls and floors.

Water Management
Designers and builders should take a holistic approach to water management of the building envelope. It is critical that all components; structural, outer and inner claddings, flashings, and barriers/retarders complement one another in the protection of the building's interior. Redundancy in design of critical water shed elements is required to properly protect the building envelope.

All building materials have a specific moisture content associated with their manufacturing process. Some materials release moisture, such as wood and concrete, allowing the material to shrink, while others, such as brick, absorb moisture and swell with age.

Often overlooked in the design and specification of a building are the forces of nature that are constantly acting upon the building. These changes happen quickly and cumulatively. The primary factors contributing to this dimensional instability are moisture and temperature. All building materials have a specific moisture content associated with their manufacturing process. Certain materials release moisture, such as wood and concrete, allowing the material to shrink, while others, such as brick, absorb moisture and swell with age. Direct connection of materials with a dissimilar moisture content can lead to undo stress on flashings and structural ties. Temperature changes also reek havoc upon the building envelope. Increases in temperature cause building materials to expand. Conversely, cooling of those same materials cause them to shrink. Temperature can cause dimensional changes in the opposite direction to a building as the sun rises against one elevation, crosses the sky, and sets on the opposite elevation. These movements combined with dead and live loads create dynamic loading to claddings and flashings that can lead to shortened life spans of the materials and eventually moisture penetration.

Roofs
The most common problem associated with waterproofing the building envelope is a lack of proper roof drainage. Low pitched roofs without gutters and eaves are so common to modular construction because of shipping constraints are the primary ingredients to poor roof design. Water draining from the roof should never be allowed to sheet down the wall surface. This is guaranteed to cause water related problems especially in high rainfall areas. The normal rule of thumb is for a roof to be provided with twelve inches of overhang width for each floor level. For example, a single-story building should have 12 inches of overhang, while a two-story building should have 24 inches of overhang. On any building other than a standard construction field office, consideration should be given to the use of overhangs. This gets the termination point of the roof and the transition of the horizontal plain to a vertical plain away from the envelope. One-time costs associated with either field installation of an overhang or utilization of an escort and shipping the building at a greater width can quickly be offset by repeated warranty calls addressing leaks.

Application of flashings and drip edges is critical to waterproofing the roof assembly. Particular care should be given to penetration in the roof for HVAC equipment, exhaust fans, drain waste and vent stacks. Conventional construction typically addresses these penetrations with pitch-pockets. These are reservoirs filled with tar that surround the pipe and isolate the roof material from any movement of the pipe. Modular construction typically relies on roof jacks to flash piping penetrations. Many times field repairs for leaking roofs are associated with flashing around roof jacks and HVAC curbs. All roofing companies have standard details on file for these applications and should be used as reference during the design process. The greatest single exposure associated with roofing application on modular buildings is the mateline flashing. Far and away, the vast majority of field problems can be traced to this single area. Adherence to fastening schedules, specifying the proper materials, and the correct application of those materials are essential to achieving a watertight connection between building modules.

Walls
Wall assemblies offer an even greater challenge as they combine normal water shed issues with greater condensation exposure. Wall assemblies utilized for modular construction can normally classified into two groupings.

First, a surface barrier design is the most common form of wall waterproofing. This focuses on making the exterior siding as watertight as possible. The siding itself is used in conjunction with sealants, flashings, and drip edges to repel all the water at the surface. Sometimes a vapor retarder will be used beneath the siding in case of a leak, but without a means to evacuate the water back to the exterior, this vapor retarder does little more than slow down the progress of the water towards the interior. Wood siding, hardboard siding, and most EIFS (exterior insulating finish systems) utilize a surface barrier approach to envelope protection.

Second, a little more sophisticated system is the drain wall design, which adds an interior air cavity and vapor retarder behind exterior siding. Any water that gets through the outer skin will drop into the air cavity, flow down the wall on the face of the retarder or back side of the siding, and be returned to the exterior via flashings and/or weep holes at the base of the assembly. High rib steel siding and brick masonry utilize this approach. The drain wall system provides an excellent back-up system for shedding water that may infiltrate the wall. An evolution of the drain wall approach is the rainscreen approach. This takes the premise of the drain wall and adds pressure relief ports to neutralize the pressure differentials inside and outside the wall that are often encountered during wind driven rain events. This approach is common in high rise construction but is also finding application in low-rise construction along coastal areas.

Condensation, as mentioned earlier, is proving to pose serious problems to designers and builders of wall assemblies. Water vapor is a constant that we all must deal with. Water vapor is always driven to cooler temperatures and surfaces. This translates to buildings in colder climates having warmer interior air driven to the colder exterior. Buildings in warmer climates have just the opposite problem. Warm moist exterior air is driven towards the cooler interior of the building. The temperature in which the water vapor condenses and forms a liquid is called the dew point. When the temperature inside the wall cavity reaches the dew point, water droplets form and soak the surrounding building materials.

In cold climates, when you see moisture or frost on windowpanes, you are witnessing the condensation of the warmer interior air upon a cold surface. Unfortunately, this same phenomena is occurring inside the wall cavity. To reduce this problem, the wall cavity must be kept as warm as possible. This is done in an effort to move the dew point further away from the building interior. Since the energy crisis, it is common for wood framed construction in northern climates to utilize 2x6 studs with R-19 insulation. While this makes for a sound wall structurally, without combining the R-19 insulation with an insulative sheathing board, this wall design actually contributes to the condensation problem. The reason is that the greater insulation thickness actually cools the sheathing assuring that the dew point is within the wall cavity. Either the surface of the sheathing or the outer layer of the insulation is where moisture condensation will occur. By using an insulative sheathing board in conjunction with less batt insulation, the dew point is moved away from the interior towards the air cavity. Condensation that forms in this area can drain out as mentioned in the drain wall design.

In warm climates, the situation is compounded by the greater amount of moisture that the exterior air contributes and the fact that the dew point is at the backside of the interior wall covering. As with the cold climate, the objective is to move the dew point as far away from the interior as possible. Once again, this can be achieved by utilizing an insulative sheathing board. Critical in warm climates is the addition of a vapor barrier on the outside of the sheathing. This provides a barrier at the dew point and helps evacuate water that does indeed form. Only recently have we reviewed the belief that the warm side of the wall is always toward the interior. In warmer climates, this practice has contributed to the problem by trapping the water vapor exactly where you want it the least, at the interior wall covering.

A building's mechanical system can also affect water management. In extreme cases, when outside air should be preconditioned and steps are not taken to remove a portion of moisture from the air prior to introduction to an HVAC unit, the unit can short-cycle. As a result, humidity may not be removed as desired.

HVAC
Finally, the design of the mechanical system can affect water management of the building because of the amount of moisture imported via the mandated fresh air requirements. Along the gulf coast where warm temperatures are combined with greater amounts of humidity, the amount of outside air and its latent moisture provides a serious concern for designers. The problem is that the amount of outside air per occupancy group is dictated by the American Society of Heating, Refrigerating, and Air Conditioning Engineers. For example, educational buildings typically demand 15 cfm per occupant of fresh air. With the typical classroom occupant per square foot ratio, this equals approximately 75 cfm of outside air per 100 square feet of floor area. Office occupancies are typically set at 20 cfm per occupant, rendering approximately 20 cfm of outside air per 100 square feet of floor area. This requires designers to walk the fence in an effort to minimize the amount of damaging moisture while still complying with the code. Ideally, from a water management standpoint, the building should have a slight positive pressurization to keep water vapor from being sucked into the building through joints, cracks and holes in the outer skin. The introduction of fresh air in moderation provides this positive pressurization. In extreme cases this outside air must be preconditioned to remove a portion of its moisture content prior to introduction into the primary HVAC plant. In those instances without some sort of preconditioning, much of the main cooling coils' capacity is used to remove moisture from the air, resulting in a larger capacity HVAC unit to meet it's original intent, which is to cool the air. As a result, the HVAC unit short-cycles and does not remove the humidity as designed. Unfortunately, this vicious cycle allows indoor humidity levels high enough to support condensation and all of its resulting problems.

As you can see, this is a complex issue. In this article we have merely scratched the surface of water management in building construction. From initial discussion with the customer, to design of the building and all its systems, through production and finally installation in the field, controlling water and its damaging effects should be a prime concern. Remember, very few things can get a customer more upset than being rained upon while sitting at their desks.

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Bibliography

Graham, Charles W. "Water Management of the Building Envelope." Texas Architect. 1997.

Gurnee, James. "Moisture Problems in your Homes." Building Material Dealer. 1999.

American Society of Heating, Refrigerating, and Air Conditioning Engineers. ASHRAE Handbook Fundamentals. 1993.

Copyright © Modular Building Institute, July 1999.