Air Ducts: What You Need To Know For Maintenance, Repair And Insulation

High Utility Bills? Stuffy Rooms? Dust?

It could be your ducts.

In houses with forced-air heating and cooling systems, ducts are used to distribute conditioned air throughout the house. Your home’s duct system is a branching network of tubes in the walls, floors, and ceilings; it carries the air from your home’s furnace and central air conditioner to each room. Ducts are made of sheet metal, fiberglass, or other materials. In a typical house, however, about 20 to 30% of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts, according to the Department of Energy. The result is higher utility bills and difficulty keeping the house comfortable, no matter how the thermostat is set.

Evaluating Your Ducts

Some ducts, depending on the material and the location, are more vulnerable than others when it comes to air loss, but any duct has the potential to leak. With age, house settling, rust from moisture, animal intrusion and a variety of other hazards, your ducts may obtain anything from pinholes to gaping voids, loose and leaking connections between pieces or even potentially missing ducts. Your joists or wall studs may actually form the “duct” run if a portion of your HVAC ductwork is missing – or the air may simply pour out to the exterior. Because ducts are often concealed in walls, ceiling, attics, and basements, identifying problems and repairing them can be difficult. When inspecting your ducts, consider 3 things: location, material and insulation.


Traditionally, to preserve headroom and square footage, ductwork is run through unconditioned (neither heated nor cooled) locations that the homeowners are not using, like basements, crawlspaces, and even garages. The greater the temperature extreme between the air inside the duct and the air surrounding the ductwork, the greater your need for duct insulation. Uninsulated ducts can also accumulate condensation, which eventually turns to rust. Once the ducts start rusting, you face having to install new ducts.


You’ve most like seen those bulky, gray, box-shaped ducts so common everywhere. They are made of metal, and metal conducts heat. Sometime metal ducts are lined with a duct liner, a 1-inch fiberglass board that insulated the interior of the duct. If your ducts already have liner, then you don’t need to insulate the exterior (lucky you!). There are also duct products like duct board and flex duct that have insulation built in.


To insulate the exterior, you have a choice between sleeve-style insulation and blanket wraps, which literally wrap around the ductwork. There are many different types of insulation, and new and innovative products arise continually. When evaluating any type of duct insulation, make note of the installation method, the drawbacks and advantages, as well as the insulation power, known as the R-value. For example, to install sleeves, you must either disassemble the ductwork and slide it on before reassembling the ducts or slit it and wrap it around like the blanket-style material – as many homeowners find themselves doing. So, in the end, it’s probably best to buy the blanket insulation in the first place.

Simple Steps to Improving Duct Performance

Some homeowners choose to take on duct sealing and insulation as a do-it-yourself project. Easily accessible small holes or disconnected duct work can be simple (but messy!) repairs for the confident DIYer. Do what you’re comfortable with. Beyond that, a qualified professional should always perform changes and repairs to a duct system.

Properly Seal Ducts, Registers and Grills

Most ducts come loose near junction points like registers and grills. Start with dry ducts. Not necessarily clean ducts (no need to bust out the bleach), but running a damp rag over the area you will be working on will make sure the material bonds correctly. Seal all the duct joints, holes, and connections near the furnace. Seal the duct-to-register connection as well. Wherever there’s a joint or intrusion, use sealant. Keep in mind that it is better to over-seal than to have a remaining leak. Duct mastic is the preferred material for sealing ductwork seams and joints. Never use duct tape. It doesn’t last.

Insulate Ducts in Unfinished Areas

Uninsulated metal allow the unrestricted transfer of heat. When ducts run through unconditioned areas, especially during the summer and winter when the temperature difference is the greatest, your HVAC system can kick into overdrive attempting to make up the difference. When selecting your duct insulation, use the R-value you require to determine what product – or combination of products – you need. Use more than one layer if a single layer won’t give you the value you desire.

Think about Air Flow

Aside from sealing your ducts, the simplest and most effective means of maintaining your air distribution system is to assure that furniture and other objects are not blocking the airflow through your registers, and to vacuum the registers to remove any dust buildup. Look for and straighten kinks in flexible ductwork that maybe restricting air flow from a room.

The Benefits

A duct system that is well-designed and properly sealed can make your home more comfortable, energy efficient, and safer. It all comes down to money. Who doesn’t want more money? To have more, you either have to earn more or spend less. Here’s your opportunity: The better insulated your ducts, the more money you will keep in your pocket, guaranteed.

Important Notes

  • Duct mastic is messy and may not come out of your clothes. Do not attempt to do any of this work wearing clothes that you really care about and don’t want to ruin.
  • Remember that insulating ducts in the basement will make the basement colder. If both the ducts and the basement walls are not insulated, consider insulating both. Water pipes and drains in unconditioned spaces could freeze and burst if the heat ducts are fully insulated be-cause there would be no heat source to prevent the space from freezing in cold weather.
  • Be sure a well-sealed vapor barrier exists on the outside of the insulation on cooling ducts to prevent moisture condensation.
  • During normal operation, gas appliances such as water heaters, clothes dryers, and furnaces release combustion gases (like carbon monoxide) through their ventilation systems. Leaky ductwork in your heating and cooling system may cause “backdrafting,” where cold air pushes these gases back into the living space. Sealing leaks can minimize this risk. Consider a carbon monoxide detector for added protection.


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