Why ducts in the attic need insulation and how much

Bare ducts in a hot attic absorb heat before it reaches your rooms. Adding hvac duct insulation helps hold the conditioned air at the right temperature from the furnace to the ceiling vent. You have to seal the metal joints first and wrap them with the right material.

Where ducts usually run and why that matters

A forced-air heating and cooling system relies on a network of metal or plastic tubes. The air handler pushes conditioned air into a main supply plenum, which is a large metal box sitting directly on top of the furnace. From here, the air moves into a rectangular metal trunk line. Smaller round branch ducts tap into this trunk to feed individual rooms.

Builders often route this system through the attic or the crawl space. This keeps the bulky metal pipes out of your living space and saves interior square footage. The downside is that these areas are unconditioned spaces, so they run hotter in summer and colder in winter than the living space. Understanding how a house is framed helps you see exactly how these duct runs weave through the joists and trusses.

A bare metal pipe in a hot attic can pick up heat through its thin wall, so the air at the bedroom vent may be warmer than intended. In winter the reverse can happen, and heat can escape the pipe before it warms the house.

Energy Star advises insulating all ducts you can reach, such as those in attics, crawlspaces, unfinished basements and garages. Before you insulate a crawl space or seal an attic floor, you should deal with the bare ductwork passing through it.

How much air leaky ducts can lose

Thermal loss through bare metal is only half the problem. Air leaks can be as big a problem. Every elbow, tee and reducer is a seam where air can escape.

Energy Star says about 20 to 30 percent of the air moving through a typical duct system is lost to leaks, holes and poor connections.

Energy Star says duct leaks raise utility bills and make a home harder to keep comfortable. Conditioned air lost into an attic is energy you paid for.

Leaky ducts can also leave some rooms hard to heat or cool, which Energy Star lists as a sign of poor ducts.

Seal first with mastic or metal tape

You might want to buy hvac duct insulation wrap and cover the bare pipes right away. Do not do this. You have to stop the air leaks first.

Wrapping a leaky duct leaves the leaks in place, so the air keeps escaping under the wrap. Sealing first also makes the insulation easier to put on.

Energy Star says to seal duct leaks with mastic sealant or metal tape and never with duct tape, which does not last.

Mastic is a thick paste brushed over seams and joints that dries into a flexible coating. Follow the product label for how thick to apply it.

Metal tape is foil backed tape that suits flat seams on straight metal trunks. Press it down firmly and smooth out air bubbles.

Wipe the metal clean with a damp rag before applying mastic or tape. Dust prevents a good mechanical seal. Focus on the connections near the air handler first. Energy Star says to seal connections at vents and registers too, where leaks are common.

How much insulation attic ducts need

Insulation limits heat transfer. The building industry measures this resistance using an R-value. A higher number means a greater resistance to heat flow.

The DOE Building America Solutions Center lists a prescriptive minimum of R8 insulation for supply ducts in an unconditioned attic.

Supply ducts are the pipes blowing conditioned air from the furnace to your rooms. They need the most protection. Return ducts pull air from the house back to the furnace. Return lines matter too, and the DOE page lists the R-8 figure for supply ducts only.

R8 duct insulation usually comes in long rolls. One side is bare fiberglass. The other side has a plastic or foil vapor barrier. The facing is there to limit moisture, so follow the manufacturer's instructions.

You can buy this material at a home center, so read the package to confirm the R-value. The DOE page lists R-8 as the prescriptive minimum, and that is a model energy code path, so a lower rating may not meet your local rules. Before you insulate attic ductwork, check your local requirements. Adding or replacing a long duct run can be one of the home projects that need a building permit from your city building department.

Wrapping metal ducts and checking flex duct

Insulation for heating ducts requires careful handling and proper tools. You need a utility knife with a sharp blade, a tape measure and a marker. Wear long sleeves, gloves and a dust mask. Fiberglass can irritate the skin and lungs, so follow the manufacturer's safety instructions.

Measure the circumference of the round duct with your tape measure. Cut a length of wrap to match this measurement and add enough extra material to create a secure overlap. Wrap the fiberglass side directly against the bare metal. The shiny foil side faces out into the attic.

Overlap the seam along the top of the duct. Seal the entire length of the seam with metal foil tape.

Do not pull the wrap tight. Compressing the blanket can lower its R-value, so wrap it just snug enough to stay in place.

Secure the wrap around the pipe at regular intervals. You can use large plastic zip ties or thin metal wire. Do not pull the ties tight enough to crush the insulation.

Many homes use flexible ductwork for the branch lines instead of rigid metal. Flex duct consists of a wire helix inside a plastic liner. This is wrapped in a fiberglass blanket and an outer foil jacket.

Look at the jacket on your existing flex ducts, where the manufacturer prints the R-value directly on the plastic. If the printed value is below R-8, ask a licensed HVAC contractor whether to replace the duct, since the DOE page lists R-8 as the prescriptive minimum for supply ducts.

When to call a licensed HVAC contractor

Homeowners can tape accessible seams and wrap exposed metal trunks in an open attic. Other situations require a licensed HVAC contractor.

Some ducts run inside narrow chases, behind finished walls or under heavy attic flooring. You cannot reach them to seal or insulate the joints. A professional can handle this.

Many HVAC contractors also repair ductwork, according to Energy Star, and they have tools for sealing ducts you cannot reach.

Flex duct can be crushed if stepped on, and crushed lines can restrict airflow. Energy Star also lists tangled or kinked flexible ducts as a sign of poor ducts, and a contractor can advise on replacement.

A licensed pro also measures the airflow to ensure the whole system is balanced. If some rooms stay hard to heat or cool, ask a licensed HVAC contractor to find the cause.

Major changes to duct size or layout should be designed by a licensed HVAC contractor and may need a permit from your building department.