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I invite you to explore my YouTube channel, where you'll see first-hand how insulation looks in dark and mysterious attics, crawl spaces, and basements scattered across Maryland. Witness first-hand how we tackle the unique challenges found in these hidden spaces, and gain valuable knowledge on how to conquer your own house.

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Eric Gans
Certified Energy Auditor


Residential Comfort & Energy Efficiency

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I invite you to explore my YouTube channel, where you'll see first-hand some of the issues we are out there fixing.

Friday, October 30 2020
How to Seal HVAC Ducts - Learn the Tricks to the Trade


Watch this Ridiculous Duct Leak

Then Learn How to Seal Yours!


When it comes to improving home comfort and energy efficiency it is amazing how little attention is paid to sealing HVAC ducts.  Properly sealing leaky HVAC ducts should be a high priority for energy efficiency.

According to the EPA and other studies, the HVAC ducts in your home can be leaky enough to account for up to as much as 30% of a household's total energy loss. 

So, what can you do?

Three Not So Obvious Reasons Leaky Ducts are an Issue

1. Like Blowing Up a Balloon - Increase Air Leakage to the Outside
Leaky ducts create pressure dynamics that can have negative effects on energy efficiency.  If too much air is leaking out in any particular area, similar to blowing up a balloon, pressure forces some of the conditioned air out through cracks in the building shell.  In fact, pressures are often great enough to double or triple the air leakage compared to when the HVAC ducts system is off.

2. Ever Wonder Why One Room is Always Cozy & One is Always Uncomfortable?
If HVAC ducts located inside the home are leaky, another issue that stems is pressure imbalances within the duct loop.  Although not as costly as ducts leaking in an attic or crawl space, they can make a living space super uncomfortable and used less often as a result.  The bright room that was once the reason you bought the house is now a no man's land.

3. Leaky Ducts Can Lead to Moisture, Mold & Drywall Issues
When HVAC ducts run through an attic or crawl space that is open to the outside, it is important to focus on the ducts in order to prevent moisture and mold due to condensation.  If the duct is extremely leaky during summer humidity the cool, conditioned air will interact with the warm humid air - all above your head in the attic - and create a mold issue if not resolved.  This can also happen in a basement ceiling.

What's the Solution?  A Three Pronged Approach

1. Seal air leakage around the building shell.  Don't get overwhelmed and feel as though you need to seal every single duct in your home.  You cannot seal every single leak in your home's building shell. But, a combination of sealing the accessible building shell areas and the accessible HVAC ducts, the needle can move towards "perfection".

2. Closely target any area that has been a problem and seal HVAC ducts.  For example, if there is a ceiling in the basement that had some dampness due to duct condensation, it might be a worthwhile investment to treat those specific areas.  Your local certified energy auditor can advise you on this.

3. Seal the accessible HVAC ducts in the proper way. 

Prescribed Method for How to Properly Seal HVAC Ducts

*This method and it's precise instruction was prescribed using the Standard Work Specifications Field Guide for Single-Family Homes created by Maryland Department of Housing and Community Development

Desired Outcome:  Sealed HVAC Ducts and plenums to prevent leakage. 


Less than 1/4":  Seams, cracks, joints, holes, & penetrations will be sealed using fiberglass mesh & mastic.  

*Mastic alone will be acceptable for holes less than ¼" that are more than 10' from air handler. 

Between ¼" and ¾": Seams, cracks, joints, holes, and penetrations will be sealed in two stages:

• They will be backed using temporary tape (e.g., foil tape) as a support prior to sealing

• They will be sealed using fiberglass mesh and mastic


Eliminate air leakage into or out of ducts and plenums

Ensure adhesion of primary seal (mastic and fiberglass mesh) to the duct

Reinforce seal

Support fiberglass mesh and mastic during curing


How to Seal HVAC Ducts - See Below

how to seal hvac ducts

A duct that has not been sealed.

Add fiberglass tape to seal HVAC ducts.

Add Mastic to properly seal HVAC ducts.




Written by Eric Gans
I have over 1000 energy audits under my belt in Maryland.  I like to take my personal experiences with each of my audit customers and try to get the things that concern them out into the world so others can make good home improvement decisions - in the right order - according to their needs.

Have you sealed your own accessible HVAC ducts?  If so, we want to hear about your experience sealing HVAC ducts in the comments below!


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Insulation Video Library

Insulation Video Library

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    Our mission is to help Maryland homeowners create a more comfortable home and save energy. We achieve this by applying building science principles and modern air sealing and insulating techniques.

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