Grilles, registers and diffusers: what they are and how to prevent condensation on them

I ran across the term “diffuser” while researching another topic and they were new to me in terms of ventilation.  When and where should they be used?  What is the benefit of a diffuser?  To start, let’s take a look at three similar products that have some overlapping characteristics and uses: grilles, registers and diffusers.   (source:

Grilles can be used on the intake (return) to HVAC systems, as well as the distribution.  They have fixed bars or louvers that don’t direct air as much as keep large objects from entering the ductwork.  They have no moving or adjustable parts.  A grille can also be placed in a door or wall to allow air movement between the two spaces. 

Registers are most likely what we grew up with in our homes: they are grilles with adjustable dampers.  The dampers can change airflow direction somewhat, or shut it off completely.  The typical rectangular register with flow damper is very common as a supply vent cover but is never used as a return (intake vent).

HVAC design can be somewhat complex, although many residential installers have simplified the systems to fit more homes with standard products.  If you really want the best performance out of your AC system, it’s time to look at diffusers.

When you need to distribute air evenly around a room, diffusers are the right choice, as the louvers and dampers are multi-directional.   Diffusers come in a multitude of shapes and designs to accommodate and hide ventilation outlets.  For example, InViAir has many different styles and sizes to accommodate design taste, lower noise (no one likes to hear the rush of air coming out of registers in a quiet room), and minimize condensation.  Diffusers can also be combined with lights so that they accomplish two functions in one unit, decluttering a space visually.

One problem home and business owners can encounter is water dripping from a vent and discoloring the surfaces below, and the ceiling itself.  There are many reasons a vent terminal can drip water, and it happens most often in the summer season.  Condensation is not just a problem of water dripping onto the floor or other surfaces; if the condensation continues, it can be a prime habitat for mold.  The condensation usually occurs because of one of two conditions: either the air in the room is too humid, or the supply vent is too cold. (

Here’s how to find out what is happening: check the room humidity with a humidity sensor like these that shows Temperature and Relative Humidity.  Then, go to a dew point calculator and input the room temp and relative humidity to get the dew point.  This is the dew point of the room after air has mixed in it.  Now, bring the sensor to the vent itself (carefully use a ladder if necessary.  Read the temperature of the air coming out of the vent.  If the temperature of the air coming out of the vent is at or below the dewpoint, the condensation is occurring because the cold air is hitting the warm, moist air of the room and water vapor from the room is condensing on the cold metal.  

There are two main solutions to prevent the condensation: reduce the relative humidity (RH) of the room (house) so that the dewpoint associated with the temp and RH is below the incoming air temperature, or increase the incoming air temperature.   Check out our post on a healthy home inventory regarding sealing the indirect leaks, direct leaks, attics, fireplaces and crawl spaces, before you go looking for smaller leaks such as doors and windows. 

How can you increase air temperature coming into the room?  According to, the two main causes of excessively cold vents are low refrigerant level and low air flow through the system.  An HVAC tech will need to check the refrigerant level, but here are the low air flow problems you can check:

  • a dirty filter

  • a filter that is too restrictive (higher MERV) than the system was designed for

  • Blocking of return or supply vents by furniture, rugs, or other items

You can also ask the HVAC company to check that the cooling coil is not dirty, or that the ducts themselves are the proper size (I found out that the return air ducts in my home were too small for the system).  Here are some other remedies you can do yourself if you are not allergic to dust or fiberglass:

  • If the duct above the vent has insufficient insulation or gaps in the insulation, then water vapor from the hot, humid attic air can condense on the duct and drip down into the room or on the ceiling around the vent.  You’ll need to go into the attic and bring some insulation, foil tape and knife or scissors with you.  Replace and seal all insulation gaps on the ductwork, all the way down to the ceiling (make sure the boot is properly sealed to the duct first; see the next point). 

  • There are two components to the vent terminal: the boot (the part above the ceiling that connects to the duct) and the vent terminal, which is the grille, register or diffuser below the ceiling.  If these are not properly connected/insulated, then you can have condensation.  

    • If the leak is between the boot and the duct, it should be sealed with a mastic sealer. If it’s between the register and the drywall, you can seal the leak using caulk. If you can’t find the leak, call a professional air conditioning company for help. (source:

  • Condensation can occur on vent terminals when cold air coming out of the duct chills the metal of the terminal, and water vapor from the warm, humid air in the space condenses on the cold metal.  You can exchange a metal grille, register or diffuser with a plastic or composite one.  These diffusers by InViAir are made with a proprietary composite material that prevent condensation. 

It may take some time to prevent those pesky drips, but it’s worth it to avoid the mold!

Photo by Will Francis on Unsplash