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Case study

An interesting study that may pertain to the current conditions that you are dealing with.

Location:
The following report chronicles interior building condensation issues at a facility in the southeast U.S.

Background:
A metal roof system on the aforementioned facility had previously received an application of an industrial grade, reflective white elastomeric coating on the 52,000 sq.ft. roof system several years earlier.  A few years following the application, this particular region of the country began to experience colder than normal winter weather and the building interior experienced water spots on the floor and moisture laden areas on roof insulation facing and exposed metal components & skylights.

Inspection:
An inspection of the facility was arranged and conducted that included a walk-around the inside of industrial building.

Observations:
The manager explained that moisture was accumulating on the interior of the building and that the occurrence coincided with times when it was very cold outside and no rain was present.  He went on to say that the observed moisture during these conditions was in the form of precipitation on interior: exposed metal roof panels, skylights, ridge ventilators and other non-insulated components.  The result was small drips of moisture onto the floor below.  The manager went on to say that he could only qualify one roof leak associated with the moisture incursion problem.  Understanding this basic information was critical in analyzing the problem, and led us to hypothesize that condensation would be the most reasonable explanation for the cold weather precipitation occurrences.

Physical characteristics noted:

  1. The pre-engineered metal building was covered with a conventional, “thru fastened” metal roof
  2. The conventional metal roof had received the application of a reflective roofing coating several years earlier
  3. The building was fitted with a number of suspended, un-vented gas heaters
  4. The roof was fitted with a significant number of un-insulated fiberglass skylights
  5. The roof configurations included a number of un-insulated ridge ventilators
  6. The interior fiberglass roof insulation and vapor barrier was detached from contact with the metal roof panels in a number of locations
  7. The 6’ wide runs of vinyl faced fiberglass roof insulation, installed between the roof panels and structural purlins were observed with gaps or separation at some adjacent seams
  8. Some facility processes utilized an un-vented water spray.

Discussion of observations:

  1. It was calculated that the un-vented gas heater byproducts of combustion would generate approximately 500 lbs of water vapor, per operating day that would be discharged as water vapor into the building interior environment.
  2. Un-insulated fiberglass skylight material has essentially no thermal “R” value to speak of.  Outside & inside surface film temperatures, Tout and Tin have only a modest differential.
  3. The ridge ventilator metal surfaces were without thermal breaks or insulation.
  4. The locations where missing/detached roof insulation exposed large areas of bare metal roof panels effectively maintained the same temperature as the outside roof panel surface film temperature.
  5. Where the vinyl faced fiberglass insulation and vapor barrier seams were gapped open, mitigation of warm, moist inside air was unencumbered to flow into the insulating cavity under the cool roof panels.  Vapor drive in this manner is always strongest during the winter months from a warmer environment to a colder environment.
  6. The manufacturing processes used also generated water vapor, to some degree, that would be absorbed into the internal atmosphere of the building.

Review:
In the southeast the average outside ambient temperature typically remains above 40º F during January and February however the past few winters were anything but normal with regard to cold temperatures and snow fall.  Additionally, the new white elastomeric coating applied to the metal roof system effectively reduced the roof surface temperature to ambient temperature during the winter months.

The amount of water vapor being generated on the interior of the building by the suspended gas fired, un-vented space heaters, and any residual water evaporation associated with the manufacturing processes, collectively create a warm, relatively high humidity environment internal to the building.

Thin fiberglass skylights, and exposed bare metal roof panels & ridge ventilators, inside the building, each have un-insulated surfaces that are essentially near outside ambient air temperatures.

The combination of these characteristics culminate into a moisture laden environment where warm, moist air rises coming into contact with structural components & roofing materials, dew point temperatures are breached, and the formation of condensation takes place on the exposed surfaces.  Eventually the condensation makes its way onto the floor.

Summation:
Cold outside ambient air temperatures, lower roof surface temperatures at exposed roof system components perpetuated by, and a function of a white reflective roof and lack of thermal breaks & integral vapor barrier with warm, relatively high humidity interior air all serve as a manifestation of condensation.

The condensation events occur when the warm, high relative humidity air comes into contact with the inside roof component surfaces that are at a much lower in temperature than normal previous to the coating application.  Warm air can hold a significantly higher percentage of water vapor than cold air.  Thus, as the warm, moist air cools when contacting the exposed roof component surfaces, the dew point is reached and water vapor is released in the form of condensation.

Recommendations:

  1. Replace suspended, un-vented gas fired heaters with thru-roof vented units.
  2. Consider installing a modest size exhaust hood & exhaust fan, ducted through an adjacent wall to eliminate the internal “water vapor” generating process.
  3. Re-insulate / reattach missing/detached roof insulation at identified locations.
  4. Consider the installation of several suspended circulating fans.
  5. It is not recommended that skylights or ridge ventilators be removed.
  6. Clean & tape vapor barrier seams in a few locations were the gaps are prominent and cause drips.

The previously applied reflective white elastomeric roof coating not only serves to preserve the integrity of the roof panels from premature oxidation, it also exhibits high reflectivity & emissivity properties that provide for more comfortable inside working environment during the warm months which constitute a majority of the year.  The slightly cooler roof surface temperatures that a reflective white roof coating will exhibit during cold winter months can be effectively marginalized by reducing inside air relative humidity conditions.  Controlling the inside relative humidity of the air will mitigate most all condensation problems.

Moral to the story is that building owners should have a full understanding of their facility characteristics, both internally and externally before considering the application of a roof coating.