Preventing Humidity Issues in Vacant Buildings

Posted by Stuart Heisey on Friday, April 3, 2020

Closed Building Sign

During these unprecedented times, schools, offices, restaurants, hotels, and many other businesses and organizations are shut down. One of the biggest questions now faced by facilities staff: what to do about vacated or low occupancy buildings? One thing you don’t want to end up with is mold and a remediation issue to deal with when building activities resume.

Most facility operators in the northeast region of the United States probably remember the summer of 2018, when record amounts of rainfall led to humidity issues across the region. With warmer, wet weather on the horizon, many facilities staff will be faced with controlling and minimizing the impact of humidity within buildings that were occupied in previous years, but now are left vacant.

Here are 8 Tips on Minimizing Humidity Issues:

1. Measure indoor humidity:

Balancing recommendations from The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRE) and the Environmental Protection Agency (EPA), a good "rule of thumb" goal should be to maintain relative humidity below 60%. You can only control what you can measure. If you don’t have humidistats as part of your building automation system, either add them or purchase standalone humidistats and locate them around your buildings for staff to monitor. Check with your controls vendor for options they might have as well.

2. Increase Vacant Building Set Points:

Overcooling a space alone will not decrease relative humidity unless you also then reheat the space. Occupant complaints should be lower than usual (since there are no occupants)! At a minimum, increase your cooling set points to 80 deg F. If you have a building where the humidity is creeping up, you might even want to increase your heating setpoints to 75 or 80 deg F to drive out the moisture. A little energy spent now could prevent a lot of mold remediation costs later.

3. Increase Air Movement:

Run your system fans (in recirculation mode) for several hours a day, one hour at a time, a few times a day. This will help move stagnant air pockets/zones within the building(s). Best time to do this would be during peak temperatures from around 2-7pm, but pay attention to how each building reacts. If you are keeping the building humidity in check, you can reduce the frequency of this strategy.

4. Selectively Increase Outside Air (OA):

Be careful on this one! When the outside air temperature AND humidity are low, introduce as much outside air to the building as you want. However, make sure you don’t forget to reduce or eliminate the OA flow when the humidity climbs. For this strategy, you should be paying attention to the outside air dewpoint.

  • If you are maintaining no less than 70 deg F in the building, OA dewpoint should be below 55 degrees before you over ventilate. If you are maintaining no less than 75 deg F, OA dewpoint below 60 deg F. If maintaining no less than 80 deg F, OA dewpoint below 65 deg F.
  • If you have a system with demand controlled ventilation (DCV), it should be disabled for the vacancy period so you can directly command the OA.

5. Dehumidifiers:

Rent/Purchase large commercial dehumidifiers. Place them in the corridors and commons areas and keep room doors open. (Keeping room doors open might not be possible until students return to clear out their rooms, but anything you can do to increase the air movement in the rooms will help.) Set the dehumidifiers to 50%. If possible, connect the drain pan/bucket with a hose directly to a storm drain or to grade (or to sanitary drain if no storm or grade-level drain point is available).

6. Fans:

Rent/Purchase large utility/box fans. This will help circulate the air within the buildings. This type of fan uses very little energy, so you might choose to run them continuously or have them turned on in the morning and off at the end of a shift. Inexpensive timer plugs can also energize intermittently.

7. Building Envelope:

Hopefully you don’t have any major building envelop failures causing rain water, air or vapor infiltration. If you do, major envelope repairs may be costly and currently unattainable, so focus on simple, cost-effective, temporary in-house solutions.

8. Deep Cleaning:

Many organizations are at least talking about giving their buildings a good deep cleaning during this period of vacancy. It is important to remember that any cleaning that involves water or cleaning fluids or wet-shampooing of carpets will add a significant amount of moisture to the building. It is imperative that when wet cleaning is performed, the building remains as open as possible, especially keeping interior doors open to allow air and vapor to migrate and equalize throughout the building. Consider opening windows and increasing airflow throughout the building until all of the moisture is dried out (but obviously not when rain is imminent or when outside humidity levels are high). If opening the building up to the outside is not an option, all of the first six items in this list are applicable to rapidly dry out the building!

Entech Engineering has helped many building owners address and correct humidity issues. If you have any questions, concerns, or other tips, please contact me.


Authored by:
Stuart Heisey, PE Photo

Stuart Heisey, PE


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Categories: Buildings & Campus

Tagged: Mechanical  |  Engineering  |  COVID-19  |  Building Humidity

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Dear Entech, Your posts are always well received by the community.
Posted by Meredith Buzacott on Sunday, March 12, 2023, 6:21 PM

Thanks Meredith! That is good to hear!
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