Humidity Gettin’ High in “Grow Rooms”

More than just a topic for lively debates, medicinal marijuana is now gaining professional acceptance as a viable treatment for a host of conditions including cancer, AIDS, Parkinson’s disease, and overall pain management.

Legalized in 17 states, (including Washington, D.C.) and pending legalization in 7 more states, “grow rooms” (dispensaries) are growing, literally, across the U.S. to keep up with high consumer demand. Since the operation of a “grow room” is federally illegal but legal by state law, grow rooms are often shrouded in secrecy for fear that the federal government will intervene and prosecute.

Moreover, the facilities that house grow rooms are often leased and not properly designed to be used for this occupation. Ranging in size from a residential closet to a commercial warehouse, a number of poorly-designed grow rooms are operating in poorly-conditioned spaces – creating costly building damage which property owners may not even be aware of.

The process of growing marijuana involves operating in conditions of high humidity. This atypical environment can wreak havoc on a structure built for other enterprises. In most cases, there is no easy repair. By the time the problem is discovered, it may be too late. This makes proper building envelope design very important!

Let’s take a closer look at “grow room” conditions:

  • Environment is nearly identical to that of an indoor pool with temperatures between 75º and 85°F (24°C to 30°C).
  • Relative humidity (RH) values are between 60% and 65% or higher due to the elevated dew point temperature associated with the transpiration of the plants themselves.

Now compare the two photos below.

humidity damage photo

Photo #1: Very high moisture content. The surface of
the wood was wet.

humidity damage photo 2

Photo #2: Adequately ventilated space has moisture
content well within acceptable range.

The photo to the left, illustrates a poorly-ventilated space with high levels of relative humidity creating condensation on building components.  Notice the glue migrating out of the plywood, which will cause delamination and loss of strength. Untreated wood should never be subjected to moisture levels over 20%, due to deterioration. The moisture probe reading in this photo is 40%, the highest reading this equipment model can indicate. The photo to the right shows an adequately ventilated space with a moisture content well within acceptable range.

Most buildings have not been designed to handle the resulting temperature gradient, moisture migration via air movement, and vapor diffusion from interior to exterior space. Elevated temperatures, together with the higher RH, are even more detrimental in cold climates where winter temperatures are cooler for longer periods of time.


  • Elevated temperature and RH can produce an ideal environment for the propagation of biological growth and an increased likelihood of building material deterioration.
  • Deteriorated structural components can not only cause health issues from poor indoor air quality, but can make the structure susceptible to further damage from the elements.


  • Property owners should require tenant(s) to build out the space to minimize the possibility of long-term structural damage.
  • Hire a qualified building envelope consultant with experience in temperature gradients, hygrothermal analysis, and design of structures (i.e. indoor pools and refrigerated buildings).
  • Install a properly insulated building envelope with a vapor retarder or barrier on the “warm-in-winter” side of the insulation. Hygrothermal calculations should be performed on the proposed system to assure the dew point does not occur at the vapor retarder.
  • Incorporate a primary plane of air tightness in the design/construction so as to significantly limit the amount of moisture-laden air transport.
  • Properly ventilate the space between the grow room wall and the exterior wall.
  • Install a properly designed mechanical system to effectively ventilate and/or dehumidify the air within the grow area and minimize the humidity.

There is no one-size-fits-all recommendation for the retrofit design of a space to successfully maintain a grow room operation. However, with proper design, most grow rooms can easily be retrofitted into most buildings, making for a good tenant, a healthy building, and properly medicated clientele.

Read the full article, Originally published by RCI.

About Dustin Smoot:

Smoot, RRC, RRO, LEED AP,  is a forensic specialist with Pie Consulting & Engineering, providing forensic engineering services involving building envelope components. Smoot’s technical expertise includes forensic investigation and assessment of damage from weather, construction defects, and product failures. His experience also includes the preparation of bidding documents, peer reviews with recommendations, and quality assurance observation. By working as a contractor, an observer, a consultant, and a designer, Smoot has gained extensive knowledge in building envelope systems design, construction, and failure analysis.

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