Building Humidity: Issues, Problems, & Causes

Posted by Stuart Heisey on Friday, July 31, 2020

Building Humidity 101

Why is humidity suddenly such a common problem?

Are you a building owner or facility manager that has experienced high levels of relative humidity in your buildings or high volumes of complaints about humidity? You’re not ALONE. Many buildings, and likely most of the ones you operate and spend time in, have no explicit humidity control. But that is not a new phenomenon, so why is everyone suddenly all hot and bothered about humidity?

NOAA Precipitation Map July 2018

Wet weather conditions in the past few years have tipped the humidity scales in many buildings and highlighted the dehumidification deficiencies of their comfort control systems. During the summer of 2018, the Mid-Atlantic region was experiencing record levels of rainfall. During the period of May-July, Maryland experienced the wettest 3-month stretch in state history (since 1896), and in July 2018 (per, Pennsylvania recorded the wettest July in the last 124 years. Similar conditions existed for Delaware and New Jersey.

As a building owner or facility manager, high humidity is often your biggest concern, which makes sense. High humidity causes conditions that damage building materials and that occupants can see and smell, increasing complaints. While focusing on high humidity in this series of blogs, we will also address the implications on occupant health in buildings with seasonal low humidity.

What’s the big deal?

Whether you are in an office, classroom, laboratory, natatorium or sauna; humidity control is a critical factor for the occupants. In fact, pick any occupied space. In every case, the comfort and health of the occupants is impacted by humidity levels. Too wet and you have mold growth, odors, increased dust mite populations, etc. Too dry and you have increased respiratory concerns, increased spread of viruses, dry skin and rapid dehydration, etc.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 55 focusses on occupant comfort and recommends that indoor humidity be maintained at or below 65% RH. But ASHRAE also notes the importance of keeping humidity above 30% RH to prevent “skin drying, irritation of mucus membranes, dryness of the eyes, and static electricity generation.” Numerous other organizations (like the EPA) also recommend humidity levels for indoor environments be maintained between 30-60% RH.

The impacts of low indoor relative humidity have come into focus recently due to an expanding body of research showing the human health implications of very low relative humidity (less than 30% RH). If you haven’t seen a presentation by Dr. Stephanie Taylor on this subject, I highly recommend you do!

Critera for Human Exposure to Humidty Chart

While ASHRAE Standard 55 focuses on occupant comfort, the chart provides a simple but powerful graphic of the importance of relative humidity for occupant health. Of note, low humidity significantly increases the R-naught (contagiousness) of viruses and respiratory infections.

So humidity control (both high and low) is a big deal!!!

Identifying Potential Causes for High Humidity in Buildings

While low humidity has its own set of health impacts, for most building owners and facility operators, high humidity is a more significant and immediate problem. So let’s identify the main causes of high indoor humidity issues.

Where to start if you are experiencing high humidity issues in your building?

First step is to identify (or confirm) if you have a problem. Setting up inexpensive digital hygrometers (you can buy these for as little as $10/each) in various spaces throughout your building will help you identify and validate if you have a humidity issue.

If you are having humidity concerns, begin to identify possible causes (see the list below). On our next blog post, we will be discussing possible corrective actions you can take to control your humidity problem.

Common Causes of High Humidity in Buildings:

  • Ventilation
    • Too much outside air: While ventilation (fresh outside) air is both required and necessary for occupied buildings, bringing in more outside air than necessary can contribute to high indoor humidity, particularly during summer months when temperatures and humidity are at peak levels.
    • Too much exhaust: Similar to bringing in too much outside air, if you are exhausting too much of your inside air, the make-up air has to come from somewhere – outside! All of the exhaust from the building will be replaced by infiltration air drawn into the building. Not only is excessive exhaust air costly, but the infiltration air it causes can significantly affect both relative humidity in your building and the condition and longevity of your building envelop.
    • Untreated Outside Air: Some ventilation is an absolute must, but it is important to control and treat the outside air you are introducing into your buildings.
  • Mechanical Systems
    • Oversized Cooling Equipment: No building owner wants to install an undersized cooling system. However, oversized cooling equipment is one of the major reasons buildings have poor dehumidification capabilities. An oversized cooling system will satisfy the cooling demand with short run-times which does not allow the condensation to build on the coil surface enough to drain away. When the cooling system cycles off, the airflow re-evaporates the condensation on the coil and you are back where you started.
    • Wrong Mechanical System: Did the building usage change during a large renovation, but the mechanical system wasn’t addressed? Or do you have terminal cooling equipment or constant volume systems? Does the system have any active dehumidification modes of operation?
    • Single Stage Cooling Equipment: Any cooling system that has a simple on/off control of the cooling capacity will struggle to maintain proper humidity levels, especially in commercial buildings where continuous ventilation of occupied spaces requires continuous fan operation. But even when the fan cycles on/off with the cooling, single stage equipment is oversized for nearly every hour of the year since it is sized to handle the worst case cooling load of the year. Dormitory Fan Coil Units are a key example of this type of deficiency.
    • Inadequate Air Circulation: Proper circulation of air is critical in preventing localized stagnant zones and maintaining even space temperatures and humidity.
    • Poor Control Strategies: Even if your mechanical systems have the capability of better humidity control, your controls systems might not be operating the equipment for optimal performance. Many systems have some incremental staging capability, but the controls need to utilize these stages correctly! One notorious culprit is allowing the cooling system supply air temperature to reset too high rather than reducing the system airflow.
  • Building Envelope:
    • Rain Water Penetration: While unusual, some building envelops have a failure of this most basic function and allow rain water into the wall cavity through the exterior skin. These failures can be hard to detect due to their infrequency and they can be even harder to fix.
    • Ground Water Penetration: Basement or lower level building spaces often suffer from some form of ground water penetration. In many cases, the issue does not show up as a wet surface or running water, but as a vapor pressure through the foundation wall, acting as a giant humidifier for the space. Again, this can be difficult and expensive to correct the root cause (sealing the foundation) and costly and inconvenient to deal with by treating the symptoms (dehumidifiers and air circulation).
    • Air and Vapor Barriers: A good building envelop functions to complement what the mechanical systems are doing with regard to proper building ventilation and pressurization. A well-designed air and vapor barrier located in the correct location of the cavity wall system is critical.
    • Summer vs. Winter: For centuries, buildings were only heated, not cooled. As a result, building envelopes evolved with designs for indoor conditions that were nearly always warmer and drier than the outdoor conditions. With the introduction of cooling in the 20th Century, buildings suddenly had summer conditions flipped from the centuries old norms. Building envelop designs are only starting to address the needs of the seasonal flip-flop in vapor pressure and air pressure.
  • Occupant Behaviors
    • Opening Windows: It is really nice to open a window and get some fresh air on a breezy warm spring day or on a crisp fall afternoon, and usually occupants inherently recognize when the outdoor conditions are good for this. However, windows are often left open for a longer period of time, past the “window” of opportunity (sorry, I couldn’t resist!). This can cause a spike in the local relative humidity. In the worst cases, a window can be left open unattended just in time for a passing shower...
    • Driving down the space temperature: The key understanding here is the difference between relative humidity and absolute humidity. Lowering the space temperature will not fix high relative humidity!!! Driving the space temperature down ALWAYS results in higher relative humidity. (If you want to get nerdy, find someone to help you “read” a psychrometric chart – we would be glad to help!)
  • Other
    • Burst Pipes: A burst pipe can go unnoticed for hours, days, or even in some cases weeks. Hopefully, an explanation of why that might be bad for humidity isn't needed here.
    • Poorly Insulated Piping: While not a cause of high humidity, poorly insulated cold fluid piping throughout your building can allow condensation to form and then drip onto finished materials, causing damage and localized high humidity.
    • Water Based Cleaning: Recently had your carpets or floors cleaned? It is important to communicate to staff or outside contractors to make sure the area is well-ventilated and aggressively dehumidified, allowing the spaces to dry completely before returning systems to normal operation. Consider dry-cleaning options or flooring solutions that do not retain moisture.

Now that you have identified your likely cuplrits, stay tuned for the next blog post on Implications of Corrective Actions. And as always, we are here, eager to learn more about your situation and help you to improve your buildings!

Authored by:
Stuart Heisey, PE Photo

Stuart Heisey, PE


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Categories: Buildings & Campus

Tagged: Mechanical  |  Building Humidity

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Great Article, thank you for posting .
Posted by Matt Delaney on Thursday, October 12, 2023, 8:39 AM