Skip to main content

Insulating Ductwork With Spray Foam Insulation

I am currently a licensed contractor performing remodeling services for residential and commercial customers in Wisconsin.

Uninsulated ductwork in an unconditioned crawlspace lacks efficiency.

Uninsulated ductwork in an unconditioned crawlspace lacks efficiency.

How to Improve Furnace and AC Efficiency

Many people spend a great deal of money buying new furnaces and air conditioners that have 98% efficiency ratings. That is great! I am a big advocate for energy efficiency. However, how efficient is a furnace or central air unit if the ductwork is full of leaks?

Seal Your Ducts!

Metal is not easy to work with. It can only be bent in certain ways and pieces of metal are put together to form a piece of ductwork. The ductwork usually does not fit perfectly together and thus there are leaks. These leaks are responsible for about 20% of the loss of efficiency of a heating or cooling system.

So how do you fix it? Simple. Seal the ductwork.

Ductwork insulated and air sealing with closed cell polyurethane spray foam insulation

Ductwork insulated and air sealing with closed cell polyurethane spray foam insulation

Closed-Cell Spray Foam Insulation

This is the only foam that is rigid enough to handle the constant vibrations and metal expansion and contraction that happen when the ductwork heats or cools a space.

Tips for Spray Foam Ductwork

  • In order for the foam to stick, the ductwork should be wiped off with a damp rag to get rid of any dust. This will also get rid of the oil used to manufacture the duct if it is galvanized metal.
  • Spray the seams first to ensure that all of the air leaks have been sealed. Then the system can be tested simply by running the fan.
  • After the seams are sealed, apply a flash coat of approximately 3/4 of an inch to prevent heat loss in the wintertime.

This is a job that you can probably do yourself with a portable spray foam kit. These cost about $400 for 200 board feet. After you seal the ductwork, you should have more than enough left over to do the box sills, which would add to your efficiency.

Another Way to Seal Ductwork

The other method of sealing ductwork is using a product called duct mastic. It is a plaster-like material that you apply with your hand.

How to Use Duct Mastic

  1. Simply put on a rubber glove, dip your hand in the container, and wipe over the leaky areas.
  2. Allow 2–4 hours to dry and test for leaks.

This is for sealing only and costs roughly $60.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2009 Energy Guild


John on August 12, 2020:

Would using spray foam keep the duct from getting condensation on it

James Waszak, VA Master licensed hvac&r technician on October 31, 2018:

Excellent article. Definitely a good product.

However, I disagree with the statement "Sealing the return duct work will not make any difference in efficiency", as it is written.

When return ducts are in the conditioned space, it’s true. It then minutely effects efficiency. Here in coastal VA/NC, that is rarely the case. Less than one percent of homes have basments. Same with FL, coastal GA, SC, etc... Most often ahu/furnaces and the duct work is all in the attic or crawlspace. In these common conditions, An uninsulated return duct will transfer a considerable amount of heat in and out of the home. (respectively per heat or cool mode) Greatly effecting efficiency.

Energy Guild (author) from Ripon, WI on April 24, 2013:

Hey Adam,

The sound attenuation you are asking about depends on the size of the cavity. This refers to the distance between the walls, the length of the chase, and the ability to access these areas.

Adam on April 12, 2013:

Looking to spray foam in an existing wall. The home is 2 years old. In the winter the duct makes all kinds of noises due to the expansion of the metal. It's a vertical duct between 2 studs. If I filled it full would it expand enough to dampen the sound or eliminate it all together?

Craig on May 22, 2012:

I agree that if the ducts are in conditioned space, there is not as much importance of sealing due to lower temperature differences. Isn't it amazing that 1000's and 1000's of homes were built with air handlers in the garage during the construction boom in Arizona? I will admit that usually the returns are not open in anyway to the garage - they are encased in drywall so the likelihood of pulling CO is remote. The big problem is being open to the interior walls which in turn are connected with the attic.

Energy Guild on May 15, 2012:

Hey Craig,

I apologize. You are talking about air handling systems in Southern climates or climates where cooling rahter than heating is the main concern. This type of system is not typical in Northern states where the majority of the ductwork is most often present in conditioned space (typically a basement or crawlspace). For these types of systems, there is significantly less overall leakage and none of the ductwork is exposed to temperatures that are much warmer or cooler than the conditioned space. In many homes pre 1960, the return ducts are not ductwork at all but simply empty interior wall cavities that have been blocked with wood and sheet metal fastened to the floor joists where it is dumped via what it more commonly seen as return ductwork near the furnace (commonly called a floor return). The second part of that is that most homes in Northern climates that are pre 1950's are multi-story homes so there is a large amount of ductwork that simply cannot be sealed unless you are going to do a lot of wall demolition so I would argue that sealing the return ductwork in these types of systems would see the 15% efficiency drop that you see with your attic air handling systems. That and the fact that most of your air leakage is around the platform return. The other part of that is that warm air and cold air travel much differently through ductwork. So much so that ducts are sized much larger in northern climates as opposed to southern climates to allow for proper air flow between winter and summer months because there is a large emphasis put on heating rather than cooling but the air handler is utilizing the same set of ductwork, central A/C systems often under perform especially in retrofit applications. One thing I will say that stands for any climate is that a return should not be located in a garage even if it is located in a closet because of the possibility of carbon monoxide fumes being pulled into the system and distributed throughout the house.

Craig on May 15, 2012:

I was referring to existing homes on a retrofit basis - specifically platform returns usually located beneath the air handler in an interior closet or the garage. Common setup from the 60's to present. The "open to the attic" part is unintentional. It is duct leakage. It can and should be sealed. It is legitimate design but will pull air from the walls if not sealed properly with mastic or lining wall cavities with duct board or drywall and sealing all seams and edges.

Return duct work leakage WILL effect the efficiency of the system in two ways. First, it pulls cavity air into the return stream - cavity air that can be at 0 degrees or 130degrees. Research shows a 15% return leak from the attic can decrease the effective capacity and efficiency of an air conditioner by 50% (see FSEC-PF-217-96). It is worse in cold weather if you have a heat pump, the cold air stream may kick in your electric heat strips.

The second effect of sealing your supply leakage and leaving your return leakage is that you can create a positive pressure in your home. If you deliver 100% of the supply air flow from the blower and return 90% back to the unit and 10% from your leakage source (attic), then you create a positive pressure in the home. This forces your expensive conditioned air out every penetration it can find.

I seal a lot of duct systems guided by a blower door. If there is a platform return, it often is the largest source of duct leakage and typically accounts for 25-50% of the total system leakage.

Energy Guild (author) from Ripon, WI on October 17, 2011:

Hey Craig,

First, return ductwork should not be open to the attic. If it is, then it is installed wrong and should be reconfigured to meet typical HVAC installation standards. What you are talking about is a return pull vent that was popular for a very short amount of time in the 1950's before return ductwork was really used because most furnaces at that point were direct vent instead of induced draft which is almost all that is installed in today's homes. Again, anything that goes to the attic and leads down to the furnace that is an open duct should simply be sealed closed.

Normal return ductwork is designed to take air into it to go back to the furnace and cycle it through the filtration system for redistribution throughout the home. If this ductwork leaks in a few areas, it will not affect the efficiency of the furnace or A/C. However, if the supply ducts leaks, the air that would typically blow into the rooms you are either trying to heat or cool will not get there as efficiently so sealing these ducts makes sense.

The best rule of thumb Craig is that conditioned air stays conditioned air unless you want to bring it into a home with an air recovery ventilator that controls the fresh air intake amount going into the home.

Craig on October 17, 2011:

"Sealing the return duct work will not make any difference in efficiency"

You should re-evaluate this comment. Just like supply ducts, return ducts are made of the same metal that is hard to work with. Even worse, returns are often routed through building cavities, such as a drywall box underneath your air handler(platform return). These platform returns worse case have OPEN wall cavities that run all the way to the attic, best case have spaces between bottom and top plates that lead to attic. A 15% return leak pulling 115 degree air from an attic will cut the SEER of your AC in half. Don't have numbers from the study for cold climate, but it will kill the efficiency of your furnace also(having to heat zero degree air instead of slightly under room temp.) Return air leaks will also cause pressurization of your home because the system is supplying more air to the home than what it is pulling out.(since it gets some of its air from the leak) This pushes conditioned air outside the envelope through all penetrations.

Energy Guild (author) from Ripon, WI on October 25, 2010:

Hey Joshua,

The air should NEVER be pulled from the attic or crawlspace in a return ductwork system. The return ductwork is simply pulling the air that is inside the house that was distributed through supply ductwork. If you are pulling air from an attic or crawlspace, your ductwork is not installed properly and should be reconfigured as you WILL be pulling air that is not meant to be distributed by your ductwork. If the ductwork is attached to an air recovery ventilation system, then there should be an exterior intake and an exterior exhaust vent. Under no circumstances should ductwork return ductwork terminated into an unconditioned space.

Joshua on October 24, 2010:

"Sealing the return duct work will not make any difference in efficiency"

I don't think this is true. If the air is pulled from the attic or crawlspace instead of your house enveloper not only you pull all that dust into your house you're pulling cold air from outside as well.

Keith Schroeder from Wisconsin on January 01, 2010:

I skipped the ductwork when insulating with closed cell spray foam. Instead, I taped the conections and wrapped the ducts in fiberglass duct insulation. I am interested how much added efficiency I would gain by changing to spray on closed cell foam on my ducts.