What is Sweating?
Sweating is moisture that forms on the faces of lay-in diffusers and T-bar grids adjacent to those diffusers, where it collects/drips into the areas below.

Why is it a Problem?
While sweating may be a simple annoyance, it can also be dangerous. Either way, it is a symptom of a larger problem with the HVAC system, the building construction, or both. Dripping moisture is an unpleasant occurrence; with smooth flooring, it can create areas that are slipping hazards. Over time, the accumulation of moisture will contribute to the rusting of the diffuser and damage to the surrounding ceiling system.

Why Does Sweating Occur?
The sweating phenomenon happens mostly in southern states, coastal locations, or any other location with high levels of humidity. However, humidity is usually just a contributing factor in sweating. Sweating is most common in locations where there is significant traffic between the occupied air-conditioned space and the outside environment where hot/humid air is allowed to get into the air-conditioned room. In those situations, the air-conditioning system runs for longer periods of time. It chills the metal diffuser and tries to maintain a comfortable inside temperature.

Sweating occurs when the warm, moist room air contacts the chilled diffuser face while the supply air temperature is below the dew point temperature.

Sweating occurs when the warm, moist room air contacts the chilled diffuser face while the supply air temperature is below the dew point temperature. Dew point temperature is the temperature at which water vapor condenses into water. The problem is that the supply air temperature is lower than it should be for the current conditions of temperature and humidity.

How do you Address the Issue?
Simple items to check that can contribute to sweating are:
• Is your air-conditioning unit cycling? If so, switch to a constant running option.
• Are your returns undersized, restricting return airflow?
• Are the filters dirty?
• Are the coils on the VAV box dirty?

If none of the quick fixes resolve your sweating problem, then there is likely a larger system issue and you will need to look deeper into your system setup or the building itself.
 
Since sweating happens when the difference between the room air and the supply-air-chilled diffuser face is below the dew point temperature, you can decrease the difference between the room temperature and the supply air temperature (ΔT). This can be done by adjusting your thermostat to a higher temperature. Doing this, while still meeting the space’s load requirements, will require an increase to the CFM’s into the occupied space.
 

An easier method of preventing sweating is to install an insulation blanket on the backside of the diffuser.

An easier method of preventing sweating is to install an insulation blanket on the backside of the diffuser. This will insulate the diffuser face from the warmer air in the attic space and prevent the face from collecting moisture, thus, decreasing the (ΔT). With the temperature differential minimized, sweating will decrease or be eliminated.
  
Another way to avoid sweating in a room using HVAC equipment is to always make the occupied spaces have a positive pressure with respect to the outside areas. Beware that in new construction, you could have some sweating collecting in diffusers and other ceiling equipment because it takes time for the humidity to settle into normal levels.  

Sweating is only a symptom of a larger problem in the HVAC system. Since sweating manifests first in diffusers, it may be easily perceived as a problem with the diffuser itself. It is important to diagnose and treat the system issues that lead to sweating. In doing so, you will avoid costs due to renovations needed because of rusting and water damage in the ceiling system. After correcting these problems, your HVAC system will run more efficiently.

Information provided by Titus. Direct questions to Titus Communications (communications@titus-hvac.com) and/or Titus’s Senior Application Engineer José Palma (jpalma@titus-hvac.com).