Construction Generated Moisture: The Cost of Failure

In construction, the concept “the cost of failure” is the risk and amount of repair that might be necessary from performing needless repairs and replacement due to poor planning or management. Some issues are minor and can be managed, but some have the potential of being catastrophic. We need to keep this concept in mind when considering vapor moisture concerns.

In the simplest terms water vapor movement is from a warm humid location to a cool dry location. The point between these locations is the equilibrium point and is referred to as the “dewpoint.” Once vapor moisture passes this point, it seeks out a cool surface so the vapor moisture can condense and become condensation liquid. To control the movement, a vapor retarder needs to be on the warm side of this equilibrium point, so the water vapor stays a vapor. Vapor is not the issue to be addressed in this scenario; it is the liquid from condensation that is the problem.

For a vapor retarder to function, the material needs have a perm rating of 5 or less; be placed on the warm side of the insulation; and be sealed at all overlaps, penetrations, and perimeter gaps.

There are two forms of interior generated moisture commonly causing concerns for the roofing assembly; Occupancy Generated Moisture (OGM) and Construction Generated Moisture (CGM).

OGM is the result of activity within the building creating interior high humidity, such as food processing; like fruit juices; interior swimming pools and saunas, storage/distribution warehouses that may have tropical fish or certain fruits, and specialized buildings that need high humidity; such as data centers. This generated vapor moisture can be typically controlled by the design professional and his mechanical engineer through specifying the incorporation of a vapor retarder on the warm side of the roof insulation and using proper HVAC equipment.

CGM is the result of products and practices during the actual construction phase. This type of vapor moisture is harder to control because of the typical time crunch of a project during cooler months of the year. This vapor moisture could be generated by numerous sources, such as freshly poured concrete floors, gas heaters, mudding and painting of walls, or heavy rains that wash into the interior from incomplete building enclosure. These variables, with proper understanding, can be mitigated; but all parties need to understand the potential impact on the roofing assembly, and ultimately, the building.

The negative results vary, but if this moisture is not addressed, vapor moisture will travel into the roof assembly by either diffusion through materials and/or travel between the material joints, penetrations, or perimeter gaps. This vapor comes into contact with the underside of the membrane, which would be the coldest location in the cooler months. The vapor moisture then condenses into a liquid, and if outside temperature is cold enough, it then turns to ice. Since the vapor will continue to move, this ice will build up until the weather turns to warmer temperatures. When temperatures rise, the ice melts to a liquid and drips back into the structure, with detrimental effects to the building and the interior.

The best way to control this moisture is timing the project phases correctly, such as high moisture work being accomplished in the warmer months. Waiting for products to reduce their moisture content, such as waiting for concrete curing, is also another tactic to help control moisture. But if none of these steps can be taken to protect the roof assembly, a vapor retarder should be considered to help mitigate this potential temporary high humidity condition. The use of a vapor retarder should be specified by the design professional, and the cost of material and installation of a vapor retarder will avoid a much more costly repair or replacement of the roof assembly.

In the same way, temporary roofs should be avoided, if at all possible, during cooler months. Most temporary roofs have no insulation above them, so this material becomes the coldest location during the cooler months and could cause condensation to develop. This would be a costly repair if not addressed and could lead to needing a complete roof replacement.

When considering the “Cost of Failure,” the inclusion of a vapor retarder within the roof assembly during construction is much less costly than the worst case scenario of a complete roof replacement.

If you have any questions, please contact your Manufacturer's Representative.