Building Legos in the Rain: Modular Construction and Moisture Intrusion

By Ed Rutkowski

Modular construction, a building process that connects prefabricated components to form larger structures, has a simple appeal: it seems to reduce the complexities of construction through the use of life-sized Lego bricks. But serious problems can arise during assembly, as Mike Smith, CIH, and Michelle David, MS, demonstrated during “Moisture Intrusions, Risk, and IH in Modular Construction,” a virtual session held June 2 as part of AIHce EXP 2020.

Smith began the presentation by summarizing the history of modular construction, from its post-World War I roots in the Bauhaus movement to the modern era’s use of cube-like components that are bolted into building superstructures. Some of today’s prefab components even come fully furnished, Smith said, which increases efficiencies. The speed with which modular buildings can be assembled offers a significant advantage over traditional methods. A recent example is a modular field hospital built in two weeks to house coronavirus patients in Wuhan, China.

Other benefits of modular construction, Smith said, include potentially safer construction conditions and minimal use of cranes. Ultimately, modular construction can be less expensive and result in structurally stronger buildings.

However, problems can arise when moisture gets trapped between and within components. Depending on severity, moisture intrusion can lead to insurance claims; costs in delays of occupancy, building completion, remediation, and restoration; as well as damage to the builder’s and designer’s reputations.

“Rain is one of the biggest problems we see in this kind of construction,” David said, and for that reason, the timing of projects is crucial. David, like Smith, works in Seattle at Wood Environment and Infrastructure, and she shared information about the effects of the Pacific Northwest’s famously wet weather on two modular construction projects.

The first, from October 2013, involved the use of wood-frame components assembled on site to form an apartment building. “They were really nicely furnished little modules,” David said. Built in a high, dry desert climate in Arizona, the units were trucked into Seattle, where problems began. Movement during transportation caused cracks in the elastomeric sealant applied to the tops of the units, and when the Seattle rain came, leaks appeared. “This caused a fair amount of failures that were first discovered at the temporary staging location,” David said. She and her team were asked to document moisture intrusion for insurance purposes.

Then in December, as the building was coming together, workers discovered another problem. “Gaps between modules were subject to a lot of water intrusion during the assembly process,” David said. “It was a mess.” Water leaked through balconies, doors, and windows. Puddles were trapped between units. In one area, a significant amount of water had accumulated between a unit and the concrete foundation. Drying manifolds were used to suck out the water. “Every once in a while, we got lucky” and only the top edges of a module got wet, David said, allowing her team to do focused drying. She estimated that up to 75 percent of the finishes in the building had to be replaced.

David’s second case study—“by far the most complicated project I’ve ever had,” she said—involved fully furnished, steel-frame modules, each one covered completely by heavy-duty Tyvek “burrito wrap” that extended all around the unit, even underneath. The modules began arriving at the staging area in August 2018, a time of dry weather in Seattle, which soon gave way to one of the wettest seasons on record. When assembly began in October, the general contractor and manufacturer noted evidence of water intrusion in some units at the staging location. Moisture was evident in window assemblies, in ceilings, and in the “belly skins”—the portion of the Tyvek wrap underneath the modules. The wrap acted as insulation that trapped in the cold. The insides of the units were like “a cross between a wet cooler and a terrarium,” David said.

By January 2019 the problems were multiplying. During assembly, water accumulated on top of temporary roofing; when the roofing was removed, water cascaded down onto the floors below. Because many of the units already had fixtures, furnishings, and appliances, David’s team had trouble reaching the surfaces they wanted to test for moisture. “We had really poor access to drywall in the units,” she said. “We were not able to test with the infrared camera in any of these modules, partly because it was so cold and insulated.”

The vinyl wallpaper in the units was especially problematic, preventing evaporation from the wet drywall behind it. Once the wallpaper was removed, David’s team had to test the surfaces with moisture meters in more than 250 occupant spaces. The humidity in some units was as high as 75 percent. Mold growth was common.

Some of the units’ ceilings, which were made of cementitious deck board, had been damaged by workers walking on top of them. Moisture entered through these cracks and through the holes for the “picking eyes”—devices that allowed cranes to lift each unit into place. “Basically these ceilings had their own weather system,” David said. “We drained about 3,100 gallons of water out of the ceiling assemblies.”

While removing wet drywall, David said, a worker pulled out a screw and water poured from the steel tube frame. “That was sort of the nail in the coffin,” David said. If it wasn’t removed, water inside the tubes would eventually corrode the screws and leak into the drywall. Contractors determined the problem was serious enough to warrant testing every piece of tube steel in the building. David estimated that around 8 percent of the tubes had water in them. Drying manifolds were used to remove it.

“We ended up gutting probably 90 percent of each module,” David said. The building’s opening was pushed back by a year.

While the projects David worked on encountered numerous difficulties, modular construction appears to be gaining steam. According to the Modular Building Institute (PDF), in 2018 the industry accounted for approximately nine billion dollars in construction activity and more than 4 percent of new construction starts among 207 manufacturers.

Ed Rutkowski is editor-in-chief of The Synergist.