Envelope Failures and Forensics

10/01/2013 | By
Photos Courtesy of ARUP

Notice clues, identify evidence, and enlist investigators to solve the case

When something goes wrong with building enclosures, the detective work is daunting. It will take heavy lifting to crack the case of cracking masonry, and if water is pooling at your window sill, the true culprit is just as slippery. To zoom in on the root cause of your problem, you’ll need more than a magnifying glass.

Proper identification depends on seeing the red flags, keeping up with general maintenance, and recruiting the right specialists, if necessary. Building envelope failures are insidious and pinning them down is far from elementary. But if you take the following steps, this sleuth work may uncover the whole truth.

Understand Performance Criteria
When the envelope fails, it’s important to have an understanding of how the system works so you can consider what could have gone wrong. A basic knowledge of components and properties can help you lay the necessary groundwork and map the crime scene.

“From a building enclosure perspective, there are five key performance points to consider, and each of those leads to specific failure paths,” explains Stéphane Hoffman, principal and facade engineering practice leader at engineering firm Morrison Hershfield.

Water management. The first goal of the envelope is to keep water out, whether that’s wind-driven rain or general moisture. Water can cause mold growth on insulation and corrode structural elements, among other disasters.

“The number one failure is water penetration, typically due to lack of maintenance as opposed to poor construction or design,” says James Cohen, principal at engineering firm Arup. “There are multiple barriers to stop water – literally thousands of feet of sealant at windows, joints, doors, and roofs. The likelihood that they are all correct is very low, so it’s usually not a single-item problem.”

Air leakage. The flow of conditioned air out of the building or of outside air through the envelope can lead to excessive heat loss and high energy bills.

“There is also potential for condensation if humidified interior air goes out or you’re pulling humid outdoor air inside,” adds Hoffman. “Additionally, air leakage can cause pressure differences and wind-driven rain can be sucked inside.”

Vapor diffusion. Although a little harder to understand, this property is as important as the others, because it also presents the possibility of condensation.

“Imagine two rooms side by side that have different humidity levels,” Hoffman explains. “Unless there is a vapor barrier between them, the two humidity levels will seek to equalize, and the interior wall cavity will suffer.”

Insulation. The insulation layer acts as the thermal barrier. Its aim is to prevent excessive exchange of heat from one environment to another.

“If insulation is damaged or missing, it can lead to occupant discomfort, draftiness, and temperature differences,” Hoffman says.

Differential movement. This term refers to outside factors like wind loading and also entail the thermal expansion and contraction of certain building materials, such as masonry or concrete.

“Your enclosure is all tied back to the base structure, and it has to accommodate for some movement between itself and the base structure,” Hoffman explains. “Differential movement can lead to breaches in one of the other four barriers and cause performance problems.”

Equipped with a basic understanding of envelope science, you’ll be able to take your knowledge into the field and dig into possible failures.

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