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Musings of an Energy Nerd

Keeping Ducts Indoors

Five ways to bring ducts inside a home’s conditioned space

Image 1 of 4
One way to install ducts indoors is to build an air-sealed duct soffit that lowers the ceiling above a hallway.
Image Credit: Image #1: California Energy Commission
One way to install ducts indoors is to build an air-sealed duct soffit that lowers the ceiling above a hallway.
Image Credit: Image #1: California Energy Commission
If you order properly sized open-web floor trusses, your HVAC contractor should have enough room to install ductwork between floors.
Image Credit: Image #2: Whatley & Sons -
Most truss manufacturers can supply roof trusses with a central recess designed to accommodate a duct chase.
Image Credit: Image #3: Steven Winter Associates
Here are two more truss designs that can accommodate ducts above the finished ceiling. Before any ducts are installed, the chase needs to be lined with an airtight barrier (usually gypsum drywall).
Image Credit: Image #4: California Energy Commission

If you live in New England, you know that furnaces are installed in basements. But any New Englander who moves to Oregon soon learns that furnaces are installed in garages. And anyone who retires to Texas discovers that furnaces are installed in unconditioned attics.

Of course, there are many other examples of similar regional differences in construction practices. But this is one regional difference that matters. New Englanders have it right: furnaces and ductwork belong inside a home’s conditioned space, not in the great outdoors.

If you build in a region where ducts are usually installed in unconditioned attics or ventilated crawl spaces, it’s time to get with the program and learn how to bring your ducts indoors, where they belong.

The problem isn’t trivial

Unconditioned attics, vented crawl spaces, and garages are cold in the winter and hot in the summer. Since most duct systems are leaky and poorly insulated, duct systems installed outside a home’s conditioned envelope waste tremendous amounts of energy.

According to Dave Roberts and Jon Winkler, engineers at the National Renewable Energy Laboratories, ducts in unconditioned attics waste about 20% of the output of a furnace or air conditioner. These researchers report that during peak conditions, the losses are even greater. Roberts and Winkler wrote that in Houston, Phoenix, and Las Vegas, “The average DSE [distribution system efficiency] for the three locations on the design day, which would be considered the day of the season when cooling demand is highest, is 72%. This means that on the hottest day of the summer, 28% of the air-conditioner output is ultimately lost.”

These calculations assume that attic duct connections are all intact. However, as any home inspector knows, attic ducts are often crushed, ripped, or completely disconnected. Since homeowners rarely visit all the nooks and crannies of their attics, these problems can remain uncorrected for years.

Where does the furnace go?

It’s no longer acceptable to locate a furnace or air handler in the garage or an unconditioned attic. So where should the furnace go?

If you want to install your furnace inside your home’s conditioned envelope, it can be located in a basement, in a sealed crawl space, in an unvented conditioned attic, or in a centrally located mechanical room.

If you’re building a house with a basement, you’re all set.

A crawl space isn’t a great place for a furnace, because the location is awkward for anyone who has to change a filter or perform maintenance.

A conditioned attic will work, but access is to an attic is almost as awkward as to a crawl space.

Many sources advise that a furnace or air handler can be installed in a closet — and that’s true. However, every time I see a furnace or air handler crammed into a closet, I cringe. In general, the equipment barely fits. Because the space is so tight, filter changes are usually awkward, and any alterations to the equipment are almost impossible. And if the homeowner later decides to install an HRV or an electronic air cleaner, where is the new equipment supposed to go?

A furnace doesn’t belong in a closet; it belongs in a mechanical room. You can put the mechanical room anywhere you want — on the first floor or second floor — but make it big. Calculate the area you think you need, and then double it.

Ideally, the mechanical room should be in a central location so that duct runs are short. Since the furnace is inside the home, it should be a sealed-combustion unit. Choose one that’s quiet, and install a solid-core door on the room to reduce sound transfer.

Where do the ducts go?

The location of your ducts is partly determined by the location of your furnace or air handler.

There are five basic approaches. Ducts can be installed:

  • In a basement or a sealed crawl space.
  • In an unvented conditioned attic.
  • In open-web floor trusses.
  • In soffits (or dropped ceilings).
  • In a chase (an “inverted soffit”) designed into special roof trusses.

Putting ducts in a basement or sealed crawl space

Builders in the Northeast have been locating ducts in basements for decades. As long as the basement has insulated walls, this is an excellent approach.

What’s not to like? The ducts are inside the home’s conditioned space. In a basement, your HVAC installer can stand up straight and see what he or she is doing.

In a crawl space, however, the quality of the installation may drop, since your HVAC installer will probably be eager to finish the job and crawl out of there.

Putting ducts in an unvented conditioned attic

If you are used to installing ducts in attics, you can keep doing it that way — as long as you insulate the sloped roof instead of the attic floor.

For more information on this option, see Creating a Conditioned Attic.

Putting ducts in open-web floor trusses

If you are building a two-story house, it often makes sense to locate ducts in open-web floor trusses.

If you go this route, remember:

  • It’s easy for duct leaks to pressurize the joist bays, so be sure that your rim joist is carefully air sealed to keep conditioned air indoors.
  • You’ll need to order deep joists — in most cases, your joists will be 16 inches deep. When designing your stairs, be sure to account for these deeper joists.
  • If you are building a tight, well insulated house with high-performance windows, supply registers can be located near the center of the home instead of at the home’s perimeter. That keeps duct runs short.
  • Installing ducts in open-web floor trusses won’t work without close communication and coordination between all the trades. If the HVAC contractor runs the ducts without proper coordination, the plumber will scream, “Where am I supposed to run my drains?” and the electrician will scream, “Where do I put the can lights?”

Putting ducts in soffits or dropped ceilings

In a single-story home, it often makes sense to install ducts in soffits or a dropped ceiling. This works especially well in a home with a central hallway flanked by bedrooms on both sides. In a house with a more complicated floor plan, duct soffits can be built along the top of any wall. Ideally, the home will be designed with 9-foot ceilings, so that there’s plenty of room to drop the ceiling height where necessary.

For this approach to work, it’s essential to complete installation of the soffit’s air barrier before the ducts are installed. If the drywall crew can’t come to the site twice, the soffit “ceiling” will probably be sealed by the framing crew, perhaps with OSB and caulk (or better still, plywood and high-quality tape). If this step of the job is done poorly, however, the house will have a major air leak. Of course, any leaks in the supply ductwork will pressurize the soffit, greatly increasing the air leakage rate.

The GBA detail library includes a detail drawing of a duct soffit.

Putting ducts in a chase built into a special roof truss

It’s possible to order a roof truss designed to accommodate a duct chase near the attic floor. Some truss manufacturers refer to this type of truss as a “plenum truss.” When these trusses are used, the resulting chase has many names, including “raised HVAC coffer” and “inverted soffit.”

If you’re building a house with 8-foot ceilings, these special roof trusses make more sense than site-built soffits. Like a site-built soffit, a chase-within-a-roof-truss must be carefully air sealed before the ducts are installed, which means that the drywall hangers and tapers have to come twice, or your framing crew has to be trained to do the required air sealing.

These roof trusses obviously create a bump in the attic floor, complicating the work of the insulation crew. If the chase has vertical walls, be sure that the vertical insulation is protected by an attic-side air barrier.

Remember, you still need to seal duct seams

In years past, it was common to read that “as long as ducts are installed inside a home’s conditioned envelope, you don’t have to seal the duct seams.”

These days, however, most energy consultants insist that HVAC contractors seal duct seams with mastic, even when all the ducts are inside. There are at least two good reasons for this practice:

  • If ductwork is leaky, remote registers may get weak air flow, leading to comfort complaints.
  • Duct leaks in soffits or joist bays can pressurize the soffit or the joist space, forcing conditioned air outdoors through cracks in the rim-joist area.

For more information on duct sealing, see Sealing Ducts and Duct Leakage Testing.

There’s no free lunch

Let’s say that you have always installed ducts in unconditioned attics. What would happen if you were to move the ductwork inside the home’s conditioned space?

  • It may be possible to downsize the air conditioner and furnace.
  • Room-to-room temperature differences will probably be reduced, improving occupant comfort.
  • Homeowners will see a significant drop in their energy bills.

However, it’s important to be realistic. Bringing the ducts inside the conditioned envelope is usually a headache for the builder, and the necessary details raise costs. Open-web trusses will probably cost more than I-joists, and deep joists may require a longer stairway.

For a successful job, the general contractor will need to budget time for facilitating coordination between all of the trades, including the framers, HVAC contractor, plumber, electrician, and drywallers.

More Information

“Building with Ducts in Conditioned Spaces”

California Energy Commission: “Home Builders Guide to Ducts in Conditioned Space”

Home Energy: “Moving Ducts Inside”

“An Innovative Approach to Reducing Duct Heat Gains for a Production Builder in a Hot and Humid Climate”

Ducts in the Attic? What Were They Thinking?


  1. User avater GBA Editor
    Martin Holladay | | #1

    Reponse to James
    Thanks for your feedback.

    My article mentions most of your points, including the drawbacks of any system that requires two visits by the drywall contractor, and the fact that duct soffits are easy to install anywhere in a home with 9-foot ceilings.

    I'm glad to hear that your HVAC contractors do quality work in crawl spaces.

    I disagree with one of your points, however: "If the ductwork is a first-class sealed installation it would not be a problem to omit the air-seal above." A leaky soffit may not reduce the performance of the HVAC system -- assuming the ducts are sealed perfectly -- but a leaky soffit will still degrade the performance of the home's thermal envelope.

  2. User avater
    James Morgan | | #2

    Good post
    This is an important reminder about the significance of strategic planning for distribution ductwork. I have a concern though about the various modified truss options shown here - creating a proper air seal above the ductwork will require two visits by the sheetrock crew, before and after HVAC rough-in, and I fear many contractors will not bother. If the ductwork is a first-class sealed installation it would not be a problem to omit the air-seal above, but as that requires exacting workmanship and good supervision it's not exactly a fool-proof strategy. And if you were to be going to that trouble wouldn't it be simpler just to run the ducts through the attic and heap plenty of insulation on top?
    I'd also point out that trussed joists are not necessary to use the space between floors for ductwork if a home is thoughtfully designed. In two-story homes we commonly use 9' as the ceiling height in the first floor main rooms on the S side and use a dropped ceiling through ancillary spaces: mud room, bathroom, office etc arranged along the length of the N side. Then we have plenty of room to run a rigid E-W box lateral and bring flex duct up into the standard 2 x 10 joist depth to distribute N & S as needed. This is of course an extended form of the hallway detail shown above: hallways seldom run the full length of the house, but the lateral needs to do so. In a two-story home this solution is neat, tidy, presents no air barrier issues, and the ceiling height change can be used to accommodate a dropped beam where needed for structural support and add architectural definition in open plan areas.
    Regarding crawl spaces: yes we do a lot of retrofit work where cramped working conditions are an impediment to first-class workmanship, but in new construction we can normally manage a mechanical area in the crawl space which has good access and is at least 40" high, and often much higher. Our HVAC techs experience little difficulty achieving a good standard of work in these spaces. And as return air filters are normally located in the conditioned space above, routine maintenance access by the homeowner is not necessary.

  3. User avater
    James Morgan | | #3

    Sorry Martin
    Somehow I missed your section on dropped soffits. Clearly I need to read more carefully before rushing to post. Need more coffee.
    Regarding leaky soffits - wouldn't the air barrier be continuous at the ceiling below the ducts?

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to James
    Q. "Wouldn't the air barrier be continuous at the ceiling below the ducts?"

    A. Well, that depends on two factors: whether the builder even knows where his air barrier is, and (assuming he knows where it is) where he plans to put it.

    I certainly don't recommend that any builder plan to install his air barrier at the bottom of a duct soffit. That's done in far too many cases, and the result is that the ducts are no longer indoors -- they are partially outdoors.

    My recommendation is that the air barrier be continuous with the top of the soffit, not the bottom of the soffit.

  5. User avater
    James Morgan | | #5

    Cross purposes
    I was thinking of the ceiling below the modified trusses, not below the dropped soffits where I entirely agree the air barrier belongs at the ceiling plane. In two-story construction a first floor ceiling chase can deliver both up and down, if needed, without introducing any air barrier complications. In single-story construction I try to avoid dropped soffits though for this very reason. If the ductwork absolutely has to go overhead, I'd sooner use a standard truss, allowing a simpler airtight sheetrock installation, and then ensure that the ductwork installation is well sealed to the air barrier at the register and riser penetrations and then well covered with insulation. I'd trust our HVAC techs to do a good job on a demanding installation, the sheetrock crews not so much.

  6. Troy Farwell | | #6

    Make sure the aesthetics are right

    I bought a two story house with ducting in soffits on the ceiling of the lower floor (daylight basement in Oregon), with a High-eff heat pump in a mechanical closet. I love the efficiency and simple design of the short branches within the conditioned space. What I did not like was the dated, clunky look.

    During our remodel of the basement, I worked the layout so the ducting is basically invisible. Along one main run, I thickened up the entire wall with built-in book cases under the soffit. On the other run, which was just hanging from the ceiling, dividing a large bonus room, I put end walls on it to make it a thick wall with a large pass-through, again with built-ins. In the bedrooms we had the single run soffits, which I built out into full tray ceilings.

    The point of my response: with a bit of thoughtful design, the best ducting options can fit into almost any house without poor aesthetics.

  7. Ben McCann | | #7

    Furnace location
    You say "It’s no longer acceptable to locate a furnace or air handler in the garage or an unconditioned attic." Why is that?

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Ben McCann
    Perhaps that sentence could have been better phrased. In reality, it has never been a good idea to locate a furnace or air handler in a garage or an unconditioned attic. The only difference between 20 years ago and today is that most builders should know better by now.

    In most areas of the country, installing a furnace or air handler in a garage or an unconditioned attic is still legal. Just because the practice isn't forbidden by most building codes, however, doesn't mean it's a good idea.

    The reason that building scientists call this practice unacceptable is because of the energy losses associated with the practice. During the winter, a furnace or air handler located in an unconditioned attic or garage loses tremendous amounts of wasted heat; during the summer, the heat flows the other way. Heat from a hot attic raises the temperature of the cool air in the air handler. Moreover, almost all ducts and plenums leak, and this duct leakage compounds the problem.

    There is a further danger when a furnace or air handler is located in a garage: leaks in the supply plenum and supply duct pull contaminated garage air (with a mixture of car exhaust and fumes from paint cans and pesticide containers) into the house.

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