3 rights make a wrong... Twice Graphic

by Richard Scott, AIA – Senior Forensic Architect

Everyone knows two wrongs do not make a right. But like “man bites dog”, three rights making a wrong is news, especially when it happened twice in two different buildings, in two different regions. And it really hurt when the wrong meant a seven-figure fix.

LBFG provided solutions to ductwork corrosion and mold problems in two large high-performance buildings, both with complex mechanical systems. One was located in a mid-Atlantic state and the other in the Southeast. Both experienced seven-figure ductwork damage during construction after dry-in. Both were victims of three rights making a wrong. 

three rights make a wrong twice covered ductwork pic

These three “rights” were good ideas related to green sustainable building goals (such as those found in LEED rating requirements) including low energy use and improved indoor air quality (IAQ):

  1. Use of low VOC duct mastic to seal joints in the ductwork. Low VOCs (volatile organic compounds) are desirable from an IAQ standpoint in sustainable buildings. Low VOC mastics are water-based and dry, releasing moisture into the air, instead of curing. Solvent-based mastics, which are out of favor due to the higher amounts of VOCs they release, cure with much less moisture release.
  2. Application of greater amounts of low VOC mastic to the ductwork joints. An important green building goal is low-energy usage and leaky ducts waste energy. Tin knockers apply even, thicker layers of mastic in order to reduce leakage and pass rigorous duct leakage tests.
  3. Sealing uncapped ends of duct runs with plastic. Limiting deposition of construction-related dust in ductwork is a typical green building goal. Therefore, many specifications call for sealing ends of open ductwork with plastic sheeting during construction to minimize dust inside the duct.

While these are all good goals, unfortunately, they worked together to cause mold growth on the mastic and, in some cases, corrosion to the metal ductwork. The low VOC mastic applied in thick applications released moisture into the sealed duct runs, causing terrarium-like conditions ripe for mold. The thicker mastic, which did not have the mold inhibiting properties of solvent-based mastic, also took longer to dry and became a base for mold growth. Even though the mastic was low VOC it still contains chemicals. Where mastic remained wet, the volatilized mastic chemicals caused corrosion of the duct severe enough to warrant replacement.

Miles of new ductwork required cleaning and remediation in many sections by hand wiping and scraping. Corroded ductwork was removed and replaced. The location of the ductwork, in interstitial spaces and/or intertwined with complex runs of piping, made this work difficult and expensive. LBFG provided investigation, protocols for cleaning/remediation, QA oversight, and legal/insurance recovery support for both projects.

What should owners/designers/contractors do to prevent these three rights from becoming a wrong for their buildings?

  • Consider using breathable filter fabric to seal ductwork open ends rather than plastic. This will allow some moisture to escape before it becomes a problem.
  • Monitor duct conditions during construction for high relative humidity (RH) conditions, especially during warmer, humid summer months. This may include installation of wireless sensors in a few of the larger duct runs, with such sensors programmed to send an email alarm if RH exceeds a specific setpoint.
  • Monitor application of low VOC mastic in order to prevent it from being applied too thick, especially in one application. Drying time for such products increases exponentially in relation to the thickness, with increasing moisture release.
  • Consider use of solvent-based VOCs, if they are available. Green building calculations for VOCs are often based on the total VOC load from all products used in the building. Therefore some higher VOC products could be acceptable if offset by lower VOC products used elsewhere. Unfortunately, the industry has migrated to water-based mastics due to green building demands and solvent-based mastics may not be readily available.
  • Be prepared to immediately react to water intrusion events that add water to the ductwork. Such reaction includes adding drying equipment, in addition to bulk water removal, such as shop vacs. In one of the buildings discussed above a roof issue allowed rain intrusion which added to the ductwork problems.
Richard Scott

With over 35 years of experience, Richard Scott is an expert in the areas of architecture, interior design, and building forensics, with a focus on moisture-related building problems. He is a registered architect in five states and a member of the American Institute of Architects/AIA Florida and the National Council of Architectural Registration Boards (NCARB). He has published over 30 articles and has lectured or presented at nearly 40 seminars or events. Mr. Scott has developed various training courses, including a 16-hour IAQ training course for NASA and an 8-hour water intrusion prevention training course for the Naval Facilities Engineering Command (NAVFAC).

Click here to find out more about Mr. Scott and his expertise.