How to Seal Heating Ducts in a Mobile Home to Save Money

Sealing heating and cooling ducts in an older mobile home with forced-air heating systems is one of the best DIY home improvement projects you can do because it’s fairly easy, doesn’t cost a lot of money, and has tremendous benefits. We’ll give you easy step-by-step instructions on how to seal heating ducts in your mobile home, why it’s so important, and other ideas that can save you money on your heating and cooling costs.

Forced-air Heating Systems 

Forced-air heating systems are very popular in manufactured homes. They work by forcing air over a hot metal heat exchanger and then blowing the warmed air through your heating ducts, or supply ducts, to each register in every room of the home.

Most mobile homes will have their duct work in the flooring but there are some, especially in the South, that will have their ducts in the ceiling. We are focusing on the duct located in the floor.

What are Heating Ducts?

Heating ducts, or just ducts or duct work, is the galvanized sheet metal formed in a rectangle under your home. Short pieces are snapped together to create a long tunnel that carries heated air (or cooled) throughout your home to warm it. It’s the veins of your heating system basically.

Ducts are usually made of very thin, pliable galvanized sheet metal. It’s lightweight and easy to form so it is perfect for mobile and manufactured homes. However, the qualities that make sheet metal so perfect also cause complications: it dents and bends easily.

Watch this video to see how sheet metal becomes heating ducts:

Why You Should Seal (and Insulate) Heating Ducts

Sealing the duct-work under your a mobile home is a fairly straight forward project. Holes and cracks in your underfloor venting increases drafts, moisture, dust, pollen, and noise. Sealing those holes and cracks and adding additional insulation around the duct-work will improve your indoor air quality and reduce your utility bills.

We recently wrote about the great benefits of adding rigid foam insulation under your mobile home. You may want to consider sealing your heating and cooling ducts, or vents, that run under the home either before or during an insulation installation.

Together, the two projects can greatly improve your home’s energy efficiency.

Read how one couple greatly improved their mobile home by installing rigid foam insulation under the flooring.

Improve Energy Efficiency, Reduce Utility Bills

HUD’s Saving Money by Saving Energy book states the following about duct leaks:

Leaky ducts are common in older manufactured homes and can dramatically increase heating and cooling bills. It is not uncommon for an older duct system to lose 20% of the heated or chilled air to the outside.

If your annual heating and cooling bill is $2,000 and your ducts are leaky, you could be spending $400 every year to heat and cool the outdoors.

Mobile Home Energy explains:

“….duct systems however have not evolved. Down draft furnaces, coupled to under-floor trunk lines and risers is still the standard installation method and the workmanship is often inferior.   The techniques for duct sealing described below are relevant even in newer manufactured homes.

Sealing the ductwork may reduce heating bills by 15-17%.

Even newer manufactured homes can benefit from sealing ducts. They can become loosened or damaged by transporting or settling. You can learn more about the benefits of sealing your duct here.

Materials You Need to Seal Heating Ducts

You only need a few basic materials to seal your mobile home heating ducts. You will have most of the items and what you have to buy is affordable and easy to find in home improvement stores.

  • Mastic (water based, in a bucket)
  • Paint Brush (1.5″ or so)
  • Utility Knife and/or scissors
  • Metal Duct Tape – foil or cloth backed tape that works with mastic
  • Old clothes  (mastic is messy)
  • Gloves

Seal heating ducts-materials needed to seal heating and cooling ducts


What is Mastic? 

Mastic is the gooey grey stuff in buckets.

It’s a great product for sealing heating and cooling ducts because it brushes on and stays flexible. The water-based mastic is easy to clean up and doesn’t create volatile fumes so it is ideal for sealing duct work.

Look for mastics with a low flame spread and smoke-developed rating. For most jobs, builders will want to make sure that the mastic they use is UL 181-rated. UL 181-rated mastic is proven to be more flexible, adhere better to ducts, and last longer than other mastics. (Source: HGTV)

Mastic is preferred over duct tape, or foil tape, because it is non-hardening and doesn’t break down as quickly as tape. Its gooey properties make it easy to get the product into the cracks and holes.

Mastic is a permanently flexible, indoor and outdoor rated, gap filling sealant that sticks to sheet metal, foil faced insulation, wood and most materials. Because it’s a gooey (like peanut butter) brush-on product, mastic conforms to any shape and contour. For long term duct sealing mastic is superior to metal foil HVAC tape. (Source:

Relevant: Sealing Ducts: What’s Better, Tape or Mastic?

But What About Using Duct Tape?

Duct tape has the words duct and tape so it seems perfect to tape ducts, right? Technically, it can be used to seal ducts but it isn’t going to last.

Duct tape is just a temporary fix to seal ductwork.  When it first goes on, if it is applied meticulously and thoroughly, it will seal the leaks.  Then, it will deteriorate over time.  Duct tape does not expand and contract with heat.  As the ductwork expands and contracts with the flow of heat, the duct tape will be repeatedly stretched and relaxed until it literally falls apart.  In the process, the tape and its adhesive will dry out and the tape will lose its air seal. (Source: St. Louis Energy Saving)

Related: Duct Tape Isn’t For Ducts

You will still need duct tape when sealing your ducts so keep it close.

Consider Code and Regulations 

Before you start on any DIY home improvement project it is important to consider your local codes and regulations. While sealing your heating and cooling vents is fairly easy and usually doesn’t require any structural modifications, your local and state laws may require inspections.

Learn About Mobile Home Duct Systems

One advantage of manufactured homes (among the many) is the simple placement of their heating and cooling ducts.

Single wides that have lengthwise joists have ducts running right down the middle of the home, directly under the flooring.

Single wides with crosswise joists have ducts running down the middle of the home but the ducts are usually under the joists, not directly under the floor.

Both setups reduce the corner connections needed and allows the forced air to better reach every room.

Double wides are essentially two single wides joined together so their duct work will usually run down the middle on each side, depending on the joists. Here’s a diagram that I found in Saving Money by Saving Energy:

Seal heating ducts-floor duct location in double wide

Pay Special Attention to Crossover Ducts 

Since double wides have two rows a crossover duct, or branch duct, is used to connect them. This duct will need special attention to seal and insulate because it loses a lot of warmed or cooled air.

Dealing with the Underbelly

You will need to get above your plastic belly wrap, or underbelly, to reach the ducts.

Cutting a straight line directly under the duct will work and you can use belly wrap tape to tape it back. There’s a couple of videos on the end of our article about installing foam insulation under your mobile home that will show you how to tape the plastic.

Click here to learn how to tape your underbelly together. 

Where to Seal Heating Ducts

You’ll want to pay special attention to particular locations.

  • The trunk line itself should be taped at the corners.
  • The end of the duct work , technically called the termination cap, is an abrupt cutoff that forces the air to go straight up. This sudden change will cause air loss. You’ll want to tape all edges and add ample insulation on all 3 sides.
  •  The boot is an arm that opens the main underfloor vent up to each floor vent location. You’ll want to use the tape (or mastic) to seal all edges and angles.
  • The crossover vent on double wides is an area that you will want to be especially attentive too. It forces the warm air to make a corner turn. The smallest crack can significantly reduce the warm airs ability to go where it needs to go.

Below you can see the most common areas that need resealed in an older duct system:

Seal heating ducts-common duct work leak locations

How to Seal Heating Ducts

You don’t have to be HVAC expert to inspect and seal your mobile home’s heating and cooling duct work. It’s a fairly simple system and DIY project.  Honestly, your biggest issue will probably be crawling under the home!

Follow these easy steps for sealing heating ducts:


Once you get to the ducts you’ll want to inspect it thoroughly. Look for large dents, cracks, or blockages. If there are large cracks or holes you will need to give those areas special attention:

For Large Holes and Cracks: 

Tape over large holes and cracks to reinforce the duct then apply mastic over the tape.

Clean the Surface

Clean the joints on the sheet metal using a regular household cleaner and rag. Mastic bonds are better on clean surfaces.

Apply Mastic

Use your paintbrush or fingers to apply mastic to the joints or small cracks and holes. Don’t be stingy – goop it on and then spread it out. You’ll want a couple of inches of mastic on both sides of every joint. Using your fingers may work better since you can scoop the goop. However, remember that sheet metal has sharp edges so you need to wear protective gloves.

Sealing the Boot:

In the image above, you’ll see the boot. The Boot is part that connects your register/vent in your floor to the main truck line. You want the entire area sealed so that no cold air from under your home can mix with the heated air. Hear’s what it should look like:

Seal heating ducts-sealing the boot - register in your home

Inspect the interior of the duct and remove all objects that have fallen into the duct.  Scrape and vacuum out the duct, then scrub it down thoroughly with rubbing alcohol and a rag.  Use a good respirator when using the alcohol.  Washing down the dirty metal insures that the sealing materials will adhere to the duct.

When the metal is clean and dry, tape all seams and joints with approved foil faced butyl tape.  Do not use “duct tape” which is an inferior product and will fail over time.  Tape all sides of the boot up and over the edge of the subfloor.  Staple that tape to the subfloor.  Then, use approved mastic to coat all seams and edges of the tape.  Let the mastic dry and replace the register cover.  If the register has movable dampers, remove them with a plier or replace the register with an open cover. (Source: Mobile Home Energy)

Sealing the Crossover Vent:

The flexible duct between the two trunks on a double wide needs to be carefully sealed. You’ll want to seal the two connection points as well as the duct itself. The image below shows how to seal the connection points – you can reinforce with tape too.

Seal heating ducts-mastic seal of duct work

Sealing the End of the Ducts:

The abrupt end of the main trunk lines causes issues in forced air heating systems. You’ll want to add fiberglass (or foam) insulation to decrease air loss.

….roll up a 12 inch length of fiberglass batt and place in a kitchen sized garbage bag.  Stuff this “poly bag” down the register boot and into the main trunk line just past the boot.  Then use your butyl tape to create an airtight barrier.  Coat this new duct barrier with mastic. (Source)

Related: Duct Sealing Techniques

Solve Condensation Issues By Insulating Ducts

In addition to sealing your duct work, you may want to wrap them with insulation. If you see water pooling at the bottom of a vent you likely have condensation due to poor duct insulation.

The warm air heats the sheet metal duct up. If the exterior of the duct is surrounded by cold air you will get condensation. Condensation can affect the warm air that you paid good money to get.

Mobile Home Repair (a very informative blog) shows a great image and describes how to insulate your ducts:

The picture shows an ideal situation where you have access completely around the duct work. This is not always the case. You can however wrap all of the duct work that you can and then staple the insulation to the floor or joists in a cradling fashion.Seal heating ducts-how to insulate heating ducts

Source: Mobile Home Repair

Helpful How-to Videos About Sealing Heating Ducts

This next video is very informative. It isn’t specifically for mobile homes but our ducts are made from the same material so it’s all relevant.

Other Ways to Reduce Utility Costs

  • Insulate around your water heater
  • Use a programmable thermostat
  • Replace your furnace filters regularly

 Thank you so much for reading Mobile Home Living!
If you have sealed your heating ducts please share your experience in the comments below!




  1. EESI: Energy Use in Mobile Homes (PDF)
    A Challenge for Housing, Energy, and Climate Policy June 2009

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  1. C.E. Smith says

    We purchased a 1993 double-wide manufactured unit in 2017. Taking off the registers, we see that the ductwork appears to have a yellow insulation all along its insides. Is this common? Seems it would HOLD dust and other yuk. When the furnace begins to come on in earnest in cold weather, we have more dust than I have ever seen. We change the furnace filter at the beginning of each month. Thanks for any insight…

    1. Crystal Adkins says

      Hi C.E.,

      I’ve never seen anything other than silver ducting used and I imagine that could be very bad for your health. Even if the material is made of Rockwool it still can’t be good for you if you’re breathing it in all night. I’d be replacing that quickly!

      1. norman r. says

        The yellow insulation is glued inside almost all duct work by the Heating & Ventilation contractors as a equirement when ducts are exposed to extreme cold or where the heat/cold that radiats is not appreciated. Could be moving cold a/c air in the in summer. As far as I know it’s safe. You probably need your duct work cleaned out by a vaccum system of some sort. Call air duct cleaners. They call our home every week from Pakistan and now you need them. Laugh

  2. Art Houghton says

    Thanks Crystal, since I am doing a whole house remodel I am going to be sealing all of the ductwork. I cannot believe how many gaps/openings there are in this system starting at the furnace location. Hoping for big energy savings. Once I seal the system I will put down the new t&g flooring. Thanks again

  3. Ron says

    Excell;ent Article !