Sweating Duct

Have you ever walked outside on a hot summer day with a nice cool drink in your hand?  You set the drink on your patio table and get the grill fired up.  The next thing you know the outside of the drink is covered with condensation.  This is simple science and can be translated into home building science too!

If you have ever seen watermarks on your ceiling or elsewhere and wondered if you have a roof leak or plumbing leak, that might be true……or it may be false!

Sweating Science

It’s very common for your HVAC ducting to sweat or condensate in the summer during times when the air conditioning runs for extended periods of time.  Somehow warm/humid air is hitting the cold metal and condensing.  Then, this water drips onto the floor, or onto your ceilings and walls. Scientifically what is happening is the air is hitting dew point at the metal ducting. For every degree in Fahrenheit the temperature of the air drops, the humidity of the air rises approximately 2.5%!  BAM!  Dew point and condensation happens.  This causes drywall damage, water marks, and possibly can create mold if it stays wet long enough.

Duct Wraps

If the HVAC duct work that is sweating is outside your building envelope such as running through a crawl space, attic, or a knee-wall, then the duct work needs to be properly insulated.  We need to separate the hot air from touching the cold metal.  Typically this can be done by a homeowner by purchasing “duct wrap” which now is an R-8 metal faced insulation. This is sold in rolls of 50 feet by 4 feet in width. It can be installed by the homeowner by cutting it to size, wrapping it around the duct work, and taping it with an overlap so that the duct work is nicely sealed and insulated.

Sometimes this can’t be done very well as the duct work may be laying on ceiling joists, or hanging from joists, too.  In this case, it may be hard for a homeowner to perform this work.  A home performance contractor can remove the existing duct wrap and completely encapsulate this duct work with a closed cell spray foam insulation.

Duct Diagnostics

If the HVAC duct work that is sweating is inside the building envelope, then there is an air leak in the building that is allowing the warm/humid air into the home and allowing it to touch the cold duct work, thus creating the condensation.  When this scenario occurs, about the only thing a homeowner can do is contact a home performance professional to diagnose where these air leaks are coming from and have them sealed up properly. Diagnostic tools such as blower doors, thermal cameras, and smoke pencils are used to find these leaks.

This condensation can also occur inside the home on water lines, metal framing, or any other cold surface.


Comfenergy, LLC
100 Glenn Drive
Suite #A1
Sterling, Virginia 20164

jdobson@comfenergy.com | (800) 604-2252 | www.comfenergy.com

The premier Home Performance Contracting company in Northern VA.  Comfenergy exists to solve comfort and energy issues in residential homes. Solving hot/cold spots, drafty areas, mold/mildew issues, high energy bills, dusting too often, cold floors, rooms over garages, allergy issues, crawl spaces.

Justin Dobson Comfenergy, LLC

About Justin Dobson Comfenergy, LLC

Co-Owner| jdobson@comfenergy.com | Justin Dobson has held every position in an HVAC company, including owner for over 25 years. He started Comfenergy, LLC in 2010 which is a Home Performance Contracting Company. He started Comfenergy to help provide residential homeowners total comfort and energy solutions. He understands HVAC, ducting, and the building envelop all play an integral role in total comfort and energy efficiency. He is a life long resident of Northern VA, and was recently awarded the "Entrepreneur of the Year 2015" *Located in Loudoun County.