Stay Dry, My Friends – Avoid Condensation in Your Building
Everybody has experience with condensation – On warm summer days, while enjoying a cold beverage, the outside of our red plastic cups have inevitably beaded up with moisture and dripped onto everything beneath them. The moisture on the outside of the cup is condensation that has formed from warm humid air contacting the cold surface of the cup. Although not environmentally responsible, if you use two cups with a napkin between them, you can create a layer of insulation that will reduce if not eliminate the condensation. A better solution is using a vacuum-sealed stainless steel or plastic cup. The vacuum between the inner and outer walls of the cup provides insulation that reduces condensation and keeps your drink cold.
A similar effect can happen to buildings. Moisture can and does manifest in walls and roofs as condensation, which can be exacerbated by other sources of moisture intrusion (e.g., storm damage, construction defects, inadequate maintenance, etc.).
For heated and air-conditioned structures, warm and moisture-laden interior air in the wintertime migrates through ceiling and wall coverings toward the exterior. In the summer, warm and humid exterior air migrates toward the cooler interior. Depending on the local climate, construction type and insulation, interior use, etc., condensation can form within the wall or roof assembly and deposit moisture on framing elements.
The formation of condensation within a wall or roof assembly (i.e., between the exterior and interior finished surfaces) is detrimental to most forms of building construction. For wood and steel framing, seasonal condensation results in long-term and cyclical exposure of the framing to moisture and subsequent decay or corrosion, respectively. For roof assemblies, condensation results in degradation of metal decking, wood sheathing, wood-fiber cover boards, insulation, and mechanical fasteners.
Modern building codes provide requirements for roof and wall assemblies to restrict the formation of condensation within the roof and walls. However, considerable care must be taken during the design, construction, and maintenance of buildings with wall or roof elements susceptible to degradation from moisture. Examples of condensation in locations investigated by EDT are as follows:
Moisture vapor in an indoor water treatment plant is prevalent due to extensive process tanks that are open to the interior environment. At such a plant in South Dakota, the fully-adhered roof assembly had become saturated with moisture. The preceding was despite fan-assisted removal of moisture-laden air from within the building, a lack of apparent cuts or tears in the white single-ply (i.e., thermoplastic polyolefin, or TPO) roof covering, and a roof system that had been installed with the appropriate vapor barrier and insulation required by the designers and roof covering manufacturer. However, moisture-laden air still permeated through the vapor retarder (likely at roof deck penetrations such as plumbing and HVAC vents) and condensed beneath the TPO surface. The condensation weakened gypsum cover boards throughout the facility. Consequently, the cover boards delaminated during two discrete windstorms with wind speeds of 60 miles per hour or less (i.e., not unusual wind speeds for South Dakota).
Due to the inherent porosity of some stone and most mortar, full stone masonry veneers retain moisture and in humid climates can cause moisture migration into an air-conditioned building. Although modern building codes allow the air space behind an anchored masonry veneer to be filled with grout, a building in Kansas City, Missouri was constructed with unintentional voids in the grout fill. The voids acted as reservoirs and increased the amount of moisture retained by the veneer. As such, and without a ventilated air space, water vapor from the veneer was driven toward the interior of the building during the humid summer months. The result was discoloration and biological growth on hard surfaces, such as cabinets, attached to the exterior walls throughout the building.
Even arid environments are susceptible to condensation. In the panhandle of Texas, a hotel pool building attic space became inundated with biological growth and wood decay because the pool owner had spray-foam insulation installed over soffit vents throughout the building. Although the building had a hip roof with ridge vents, covering the soffit vents with foam prevented exterior air from ventilating the attic space. [Note: Most modern building codes require hip roofs to have attic space ventilation with soffit and ridge vents.]
In addition, no vapor retarder was installed above the ceiling covering that would restrict the migration of interior air into the attic space. Therefore, warm, humid air from the pool area entered the attic space and condensed on uninsulated wood roof sheathing.
In summary, and although condensation on your red plastic cup is simply annoying at picnics and tailgates, condensation in your building could lead to serious damage. There is no condensation-prevention formula that is uniform for every building, and therefore specific considerations should be given before construction to design and assemble building components in a manner that restricts condensation within roofs and walls.
About the Author
Nicholas K. Pontillo, P.E. is a consulting engineer with our Kansas City, MO office. Mr. Pontillo provides structural evaluation and damage assessment of commercial and residential buildings and structures. You may contact Nick for your forensic engineering needs at npontillo@edtkc.com or (913) 859-9580
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