How much heat am I losing through leaky ducts?

Energy Star puts it at up to 20% of the air your system conditions. How much of that applies to your house depends on how much duct runs through unheated space and how many joints have failed — which is answerable by looking rather than by guessing.

Is duct tape the right thing to seal ducts with?

No, despite the name. Cloth duct tape dries out, the adhesive lets go and it falls off the joint within a few years, which is why so many joints are open in the first place. Mastic and foil tape are what stay stuck.

Do you clean ducts as well as seal them?

No. Cleaning and sealing are different services with different purposes, and only sealing is offered here. Sealing stops conditioned air escaping into places you are not trying to heat; it is not a cleaning and it is not an air quality treatment.

One room is always cold. Is it the ducts?

It is one of three usual answers, with a closed damper and not enough insulation in that room. A branch that has come apart in a crawl space delivers nothing at all to the room above it, so hold a hand at the register and check air is moving rather than that the vent is open. Rule all three out before anybody talks about a bigger furnace.

Should the ducts be insulated as well as sealed?

It depends where they run. Sealing stops air escaping; insulating stops heat leaving air that stays in the duct. Through a warm basement, sealing is most of the win. Through an unheated crawl space, garage or attic, a perfectly sealed duct still gives its heat to that space along the whole run, and skipping the insulation is the common shortcut.

Can you seal ducts inside my walls and ceilings?

Not without opening them up, and that is said before the job rather than discovered during it. Exposed basement ductwork is straightforward. Duct buried in finished walls and ceilings mostly is not, and how much of a given house is reachable is the main thing that decides the scope.

A Fifth of Your Furnace Heat May Never Reach a Room

The fault nobody suspects

Energy Star puts the loss through cracks, gaps and holes in ductwork at up to 20% of the air a system conditions. A fifth of what you pay to heat, delivered to a basement, a crawl space or the inside of a wall instead of the room it was meant for.

What makes it hard to find is that nothing looks broken. The furnace runs. The vents blow. There is no alarming noise and no visible damage. The only symptoms are a room at the end of a run that never quite gets warm and a gas bill that seems high for the size of the house — and both of those get blamed on the furnace, on the windows, or on the house just being old.

And the ducts are in the basement, which is the one part of a house nobody looks at on purpose.

Cloth duct tape is usually the villain

Despite the name, cloth duct tape is close to the worst thing you can seal a duct with. The adhesive dries out, the backing goes brittle, it loses its grip on the metal, and within a few years it is hanging off the joint it was supposed to seal.

This is why so many ducts are open at the joints: somebody did seal them, correctly as they understood it, with the product named after the job. Then it failed quietly, in a basement, years later.

What stays stuck is mastic — a thick paste brushed over the joint that cures and stays flexible — and foil tape, which is metal-backed and rated for the purpose. If you look at your own ductwork and see silver cloth tape that has gone hard and curled at the edges, you are looking at the most common failed seal in American houses.

Four places it actually leaks

The joints, where one section of duct meets the next. Many were never sealed at all beyond being pushed together and screwed.
The boots — the metal box where the duct meets the hole in your floor or ceiling and the register screws on. A boot that was never sealed to the floor leaks conditioned air straight into the floor cavity, which is to say into the wall.
The plenum, the large box directly off the furnace, where the pressure is highest and so the loss per square inch of gap is worst.
Branches that have come apart entirely. This happens more than people expect, usually where a flexible duct was pushed onto a collar and never strapped. It then delivers nothing at all to the room above it.

The room that is never warm

There are three usual answers and it is worth ruling them out in order, because the first two are cheap:

A damper that is closed or a register that is blocked by furniture. Free to check.
A duct branch that has come apart or is leaking most of its air before it arrives. Go to the room, hold a hand at the register with the furnace running, and establish whether air is actually moving rather than whether the vent is merely open. A disconnected branch gives you nothing.
Not enough insulation in that particular room — an exterior corner room over a garage or a crawl space is a different thermal problem from the rest of the house.

All three are worth checking before anybody suggests a bigger furnace. Oversizing equipment to overcome a duct leak is an expensive way to lose the same fifth of the air faster.

Sealing the duct and insulating it are two different things

Worth separating, because a job that does one and skips the other leaves money on the floor.

Sealing stops air escaping from the duct. Insulating stops heat leaving air that stays inside the duct.

If your ducts run through a basement you keep reasonably warm, sealing is most of the win. If they run through an unheated crawl space, an unheated garage or a vented attic, a perfectly sealed duct still hands its heat to that space the whole length of the run — and you are paying to warm a crawl space very efficiently. In those houses the duct wants sealing and then insulating, and skipping the second half is the common shortcut.

What this is not

Not duct cleaning. Different service, different purpose, and not offered here. Sealing stops conditioned air escaping into places you are not trying to heat. It is not a cleaning and it is not an air quality treatment, and anyone selling it as one is selling you something else.
Not equipment work. Furnaces, heat pumps and air conditioning are licensed mechanical work and are not taken on. Sealing ductwork is not the same as working on the machine, and where the fault turns out to be the machine it goes to a licensed heating contractor.
Not duct design. Replacing or re-routing a system, adding returns, re-sizing runs — that is design work and it is referred out.
Not duct inside finished walls and ceilings. It cannot be sealed without opening them up. That is said before the job rather than discovered halfway through it, and it is the main thing that decides how much of a given house is reachable at all.

This is a new service: the records carry one job that touched ductwork and no duct sealing jobs. That is stated here for the same reason it is stated on the service page.

Four things you can check today

Go to the room that is never warm and confirm air is actually blowing, not just that the register is open.
Look at the exposed duct in the basement for silver cloth tape that has gone brittle or come away. Most common failed seal there is.
Follow the runs and see whether any of them disappear into a crawl space, an unheated garage or an attic. That is where insulation matters as much as sealing.
Hold a hand near the joints while the furnace runs. A joint you can feel blowing is a joint that has been blowing all winter.

Book it

Book a free quote · call 517-505-7135 · or read more about duct sealing.
The same argument for the house itself: air sealing. Want the loss measured rather than estimated? Home energy assessment and a blower door test. Who pays: energy rebates and grants.
Serving Lansing, East Lansing, Okemos, Haslett, Holt and the rest of Greater Lansing.