Duct sealing

I mentioned in the last post that we sealed duct seams and connections. The subject of airtight duct work is rarely mentioned outside the green building community, but can be rather important.

The basics

When ducts get assembled, seams and connections are typically secured with small sheet metal screws. The screws don’t make airtight connections, though. Quite the opposite. These connections freely leak air, which can lead to substantial energy losses in the case of a forced air or air conditioning system.

If the ducts are used for a ventilation system, as in my case, leaking ducts can cause pressure imbalances and prevent sufficient fresh air delivery to the point of supply, rendering the ventilation system and needed air exchange ineffective. In other words, I would use electricity, for which I have to pay, to move air around in unexpected places and without getting the desired results.

The problems don’t stop here. Exhaust air that is returned from wet rooms such as a bathroom is often laden with moisture. Leaking duct work allows that moisture to escape back into the building where it can cause significant problems, such as condensation leading to mold growth.

In recognition of these issues, the 2006 International Residential Code (section section N1103.2.2) and the 2004 International Energy Conservation Code (section 403.2.2) require duct work to be substantially airtight or sealed, respectively.

The problem is that these code requirements are rarely, if ever, enforced, unless we are talking about a green building project.

What can I use to seal my ducts?

The first thing that comes to mind is, of course, duct tape. BEWARE! The pervasive duct tape, the silver stuff that is used to literally fix everything, is actually not suitable for ducts! I don’t know how it got this name.

Some research brought me back to GreenBuildingAdvisor.com, one of my web resources. Here I found a very informative blog post that covered the subject of suitable materials for sealing ducts.

I learned that real duct tape must be UL listed. But I also had learned from Mariusz, my plumbing and heating contractor, that even the real duct tape is likely to come loose at some point, because it so difficult and time consuming to clean all surfaces first.

The blog post corroborated Mariusz comment, and pointed to duct mastic as an alternative. Putting my sixth sense to this, I decided that duct mastic would be the more reliable and longer lasting option to seal my ducts and keep them airtight.

Putting things together

The application of the mastic is very simple. I used a paint brush to put a coat of up to 1/16 inch over the seams and joints.


Most elbows and reducers are not made out of one piece but rather several sections. The connections of those sections are not airtight either and it is a good idea to apply a coat of mastic here too.

ventilation-07 ventilation-08

As you can tell from the pictures, these ducts with the mastic are not the most attractive things in the world. That doesn’t matter in the storage rooms or corridor.

For the living space, we switched to spiral ducts, which are much nicer to look at. These ducts come pre-assembled, are much sturdier and more expensive than regular ducts.

But, they don’t require any mastic or tape for sealing!

ventilation-09 ventilation-10

ventilation-11 ventilation-12

The ducts get connected with couplings that have neoprene gaskets on either end. This allows for a very easy, presentable and airtight connection.

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.