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Green Building Blog

Keeping Ducts Where They Belong

With slab-on-grade construction growing in popularity, it's increasingly difficult to determine how to run ducts for heating and cooling

Ducts running through unconditioned attics or crawlspaces can create problems in all climates. Undesirable positive or negative pressurization, moisture and mold issues, and increased energy costs are often the result. In my work as an energy auditor, I’ve conducted blower door tests on many new homes that have duct work located outside the building envelope, many with poor test results.

As a matter of fact, of all the new homes I have tested that have failed code-required blower door tests, all had ducts in unconditioned attics or crawl spaces. With so many new slab-on-grade homes being built today, lots of designers and builders are trying to figure out: where is the best place to put the duct work? Before we get to that question, let’s look at some of what the 2018 International Residential Code (IRC) says about ducts outside of the building envelope.

What the IRC has to say about ducts

N1103.3 Ducts. Ducts and air handlers shall be in accordance with Sections N1103.3.1 through N1103.3.8.

N1103.3.1 Insulation (Prescriptive). Supply and return ducts in attics shall be insulated to an R-value of not less than R-8 for ducts 3 in. in diameter and larger and not less than R-6 for ducts smaller than 3 in. in diameter. Supply and return ducts in other portions of the building shall be insulated to not less than R-6 for ducts 3 inches in diameter and to not less than R4.2 for ducts smaller than 3 inches in diameter.

Exception: Ducts or portions thereof located completely inside the building thermal envelope.

N1103.2.2 Sealing (Mandatory). Ducts, air handlers, and filter boxes shall be sealed. Joints and seams shall comply with section M1601.4.1 of this code.

N1103.3.2.1 Sealed air handler. Air handlers shall have a manufacturer’s designation for an air leakage of not greater than 2% of the design air-flow rate when tested in accordance with ASHRAE 193.

N1103.3.3 Duct testing (Mandatory). Ducts shall be pressure tested to determine air leakage by one of the following methods:

1. Rough-in test: Total leakage shall be measured with a pressure differential of 0.1 inch w.g. (25 Pa) across the system, including the manufacturer’s air handler enclosure. Registers shall be taped or otherwise sealed during the test.

2. Postconstruction test: Total leakage shall be measured with a pressure differential of 0.1 inch w.g. (25 Pa) across the entire system, including the manufacturer’s air handler enclosure. Registers shall be taped or otherwise sealed during the test.


1. A duct air leakage test shall not be required where the ducts and air handlers are located entirely within the building thermal envelope.

2. A duct air leakage test shall not be required for ducts serving heat or energy recovery ventilation that are not integrated with ducts serving heating or cooling systems.

A written report of the result of the test shall be signed by the party conducting the test and provided to the building official.

N1103.3.4 Duct leakage (Prescriptive). The total leakage of the ducts, where measured in accordance with Section R403.3.3, shall be as follows:

1. Rough in test: The total leakage shall be less than or equal to 4 cubic feet per minute (113.3 L/min) per 100 square feet (9.29 m2) of conditioned floor area where the air handler is installed at the time of the test. Where the air handler is not installed at the time of the test, the total leakage shall be less than or equal to 3 cubic feet per minute (85 L/min) per 100 square feet (9.29 m2) of conditioned floor area.

2. Post construction test: Total leakage shall be less than or equal to 4 cubic feet per minute (113.3/L/min) per 100 square feet (9.29 m2) of conditioned floor area.

Interpretation and opinion on the code requirements

So, according to the IRC, any duct work that leaves the conditioned space of the home needs to be insulated and air sealed and the duct work must be tested to assure there is very little air leakage. Ducts can be installed correctly, but it seems that only happens in areas where inspections are required and duct blaster testing happens. It’s the wild west outside of code enforcement areas.

In my opinion, ducts shouldn’t be in unconditioned spaces in the cold climate where I live (and probably not in any climate). With duct temps close to 120°F, and vented attics close to outdoor temperatures, often there’s more than a 100°F temperature difference. We had -40°F actual air temperature during the polar vortex of 2019.

Another issue is the penetration, through the air barrier, by the ducts. Who is going to make sure these holes are sealed? It could fall on the HVAC contractor, the insulator, drywaller, or the general contractor, but it is just as likely that no one takes responsibility to seal the penetrations.

As you can probably tell, I may be a bit skewed in my opinions because I see mostly homes with problems. I’m sure there are some homes that work, but I don’t get called to audit them.

If the duct work remains inside the conditioned envelope, no insulating or testing is required by the IRC, though air sealing is still needed. And insulating ducts that remain inside the conditioned envelope may still be a good idea, especially if you live in a hot climate or mixed climate where cold duct work can become a dehumidifier and sweat.

One code I find interesting is the requirement for testing HRV and ERV ducting when these appliances are connected to the heating or cooling system. If these supply and exhaust ducts leave the conditioned envelope, they are now subject to the same code requirements as heating and cooling ducts. When the HRV and/or ERV have separate, dedicated duct systems, they do not require an airtightness test, even if installed outside the building envelope.  At least that’s my interpretation of the code.

If you must have a ducted system

What are our options to keep duct work inside the conditioned space? Well, one option is to eliminate the duct work all together. Hot water in-floor heating systems and hot water or electric baseboard heat are all popular choices, but they can’t supply cooling nor address dehumidification.

Ductless minisplit air source heat pumps are also a good option, but some people don’t like the looks of having a fan unit mounted to the wall or the idea of having to leave doors open for consistent temperatures throughout the home. There is also a cost to installing two completely different systems, one to provide heating and the other for cooling, which would be needed in some climates.

Sometimes ducted systems are the best fit.

There are a few ducted choices that can be effective. I’ve seen sealed polyvinyl coated steel (PVS) duct work placed under a slab. These systems work but can be more expensive to install. The connections are made below grade; all seams need to be both airtight and watertight.

Changing the attic from unconditioned to conditioned space is common in hot and some mixed climates. This typically requires insulating the roof deck, either between the roof framing or above the roof sheathing. This will allow enough area to place duct work and HVAC equipment in the conditioned attic space. This approach is rare in the cold climate I live in.

In a home with a flat ceiling, a plenum truss can be used to create a chase for ducts. The home’s air barrier follows along the bottom of the trusses and the attic floor is insulated, keeping ducts within the conditioned space. Click to enlarge.

False ceilings and soffits can hide duct work and still allow it to remain inside the conditioned space. This is a great option but it requires detailed planning for the location of the ducts. I recently worked on a project where plenum trusses were used. This notch in the roof trusses as shown in the photo to the left, allowed for a flat ceiling framed in after the installation of the ducts. All ducts in these special trusses remained inside the building envelope. This service cavity also provided a chase for electricians and plumbers to keep much of their cables and pipes inside the home—a design feature I will be using again.

When designing heating and cooling systems,  the best practice is to utilize HVAC design services to calculate heating loads (Manual J heating and cooling load calculations), select equipment based on performance criteria (Manual S), and design the duct layout so the proper amount of heat and air flow is reaching each room (Manual D).

Rules of thumb will not work with most new homes constructed today. As an example, a recent well insulated and air sealed build that I was involved in Northern Minnesota was just under 2000 square feet. Had there not been a heat load calculation completed, the smallest readily available forced air furnace would have been installed, which is 60,000 BTU. A third-party heat loss calculation showed we needed only 18,868 BTU. The heating system would have been three times the required size.

As for the builder trying to plan duct work locations for a soffit or plenum truss design, Manual D is a piece of critical information. The size and location of both the supply and return air will be needed to plan how to keep all the ducts inside the building envelope. Working with the installing HVAC contractor during the design process will be required.

Problems with ducts outside the envelope 

By requiring insulation, air sealing, and testing ducts when they leave the conditioned space of the home, building codes are discouraging the placement of duct work in these locations—an all to common problem I see during energy audits. Here are a few photos that I’ve taken over the past couple years where duct work is located in an unconditioned space. They will clearly illustrate some of the problems:

This photo was taken in a slab-on-grade home built in 2005. I did not conduct a blower door test on this home. The purple areas are cold air entering the home from the attic space. This is the location of two return air ducts. I believe the wall cavity is being used as the return air path, a practice no longer allowed by code. The reason for my visit to this home was high heating costs and uncomfortable temperatures in areas of the home during the heating season.
This photo was again taken in a slab-on-grad home, built in the late 1970s. Taken during a blower door test, the yellow area to the right is the location of the supply air duct which was operating right before the blower door test began. The dark purple area where the register vent is located is substantially colder than the duct location suggesting colder temperatures at the vent. The duct work is not air sealed and during the blower door test, I was able to draw the cold attic air through the duct work where is enters the home at the vent, resulting in the cold vent area.
This photo was taken at the same time as the previous picture. This is the roof right above the duct work. This home had severe ice build up along with ice dams. Water from the ice dams had entered the home in several areas. Air temperature was 20°F at the time of the photo. The heat and air leaks from the duct work was heating the roof enough to melt snow and cause the ice damage. Notice the temperature of the ridge vent, 50°F with an outdoor temperature of 20°F.
This picture was taken in a crawlspace that didn’t know if it was conditioned or unconditioned; there was no clear boundary. This was an old seasonal cabin that had been converted to a year-round rental home which had blower door test  results of 15ACH50. Very leaky. The pipe moving into the ground is the main sewer line, which was partially disconnected and dumping sewage on the ground. None of the duct work was sealed. Not only was this home hard to heat, it also had an odor.
This, and the next photo, where both taken at a new home. Duct work in the attic along with several unsealed plumbing and electrical penetrations through the top plate and into the unconditioned attic are causing the cold air to move down though the wall during a blower door test. The two square spots on the right-hand side of the photo are a pair of drywall patches that were drying. This home failed the blower door test.
Another photo of the home that failed its blower door test, showing a supply air vent from an unconditioned attic space.

This is just a small sampling of what I see when duct work is installed in unconditioned spaces. It requires some planning but the best practice is to keep heating and cooling equipment and ducts inside the home.

Though I live and work in a very cold climate, the problems I see when ducts are installed in unconditioned spaces isn’t unique to my climate. Fixing leaky ducts and duct insulation issues becomes much more difficult and expensive after construction is complete. Designers, builders and HVAC contractors all need to give some thought to how these systems will be installed before construction begins.

-Randy Williams has worked as an electrician and builder and currently works as an energy auditor and performs blower door tests for code compliance. You can find him on Instagram @northernbuiltpro and on his blog Photos courtesy of the author.


  1. JC72 | | #1

    Slab-on-grade w/ducts in a vented attic has been the standard in the SE US for decades. Especially with production builders.

    Of course this is done for cost reasons because:.

    - HVAC within the building envelope takes up interior space and there's the noise issue. For example imagine trying to design a system for a 2-3 story townhome with an open floor plan.

    - Production builders like to offer options with regards to room configurations. HVAC inside the envelope adds additional complexity and that means higher costs.

    -Expensive solution: Spray foam. I believe there is one production builder who figured out a cost effective way to use ocSPF underneath the roof deck. This allows them to retain the flexibility with regards to differing floor-plans and standardizes the install for the HVAC contractor.

    Now if we could only get them to stop using ductboard. ;)

    1. D_Barnes | | #11

      Meritage Homes in CA and AZ is doing ocSPF under roof decks and walls, they also blower door test every home they build. They also installed a nice duct system in all the houses in my subdivision, no ductboard! BUT... they installed 3.5 ton A/C and 88kbtu furnace on my house while I modeled it at only 17,600 btu heat load, 15,400 btu cooling load.

  2. johns3km | | #2

    I walked through a new construction home (rebuilt on existing foundation) outside of Boston, and the air handler/furnace/water heater was in the garage. This mind you was a $1.6m home, with a single 100% below grade garage door with finished basement. I'm sure the walls are all insulated minus a leaky garage door, but having your HVAC for your home shared with your car exhaust seems insane to me.

    1. Northernbuilt | | #3

      One of the new homes I tested that failed the code compliant blower door test failed for that reason. Ductwork was in the attic, I believe air sealed and insulated with closed cell spray foam but the door to the room where the HVAC equipment was installed, inside the garage, was a standard interior door. I was pulling air from the garage, through the forced air furnace and into the home.

      1. johns3km | | #4

        I found a pic on the listing. Lots of air to pull through those doors.

      2. PBP1 | | #7

        Low delta P heat pump air handler, well sealed, in garage seems OK, as long as all penetrations are well sealed. The negative pressure is quite low (and if w/short run) and supply is positive pressure, so seems like, while not optimal, can be doable. Most modern cars, or how about electric cars, reduce or eliminate many risks. Blower door is not representative of any normal operating condition. Just wanted to edit to say, great article and thanks for posting.

        1. johns3km | | #8

          I’m guessing this was a traditional air handler and not heat pump system.

          1. PBP1 | | #10

            Mitsubishi SEZ, horizontal ducted air handler (0.2 in W.G.) connected to air-to-air heat pump (outside), with about 2' long duct to intake, which I assume is where the risk of entraining garage space air would be - supply side leakage, if any, would likely leak from air handler/supply duct to garage space. Garage is sprayfoamed (walls/ceiling w/living space above) and has an insulated panel garage door with minimal leakage at edges (nothing more than about 1/4 inch near bottom corners). All ducts insulated and SEZ insulated to some extent too. House is well below 3 ACH50.

        2. Northernbuilt | | #9

          I have blower door tested several homes with utility closets in attached, heated garages. Most have exterior doors leading from the garage into the utility space. All but a couple use gas boilers for in-floor hot water heating systems. They also contain an HRV. The ducts for the HRV end up in the vented attic. I'm sure there is some leakage around the penetrations through the air barrier and maybe a little in the duct connections, but I have not had a home with this configuration fail a blower door test. The home in comment #3 had a forced air system in a utility closet located in the garage with poorly sealing interior doors. If my memory is correct, this slab on grade home tested at over 4 ACH50. I agree, utility closets in garages that are well air sealed can work. That being said, any home that I have design input will have the HVAC systems inside the home.

  3. charlie_sullivan | | #5

    Plenum trusses are a great solution. Another good one is hydronic with cooling through the use of mini-slim fan-coil units like those available from Chiltrix. They are smaller and slimmer than minisplit heads and thus avoid the aesthetic objection, and in heating-dominating climates you can supplement with a few small panel radiators and/or floor or ceiling radiant to provide silent heat everywhere.

    It seems like the only disadvantage of that approach is a lack of contractors who are adequately familiar with it, and the small selection of high-performance residential-scale hydronic chillers.

  4. D_Barnes | | #6

    Here in hot, dry California, slab on grade is the standard for homes built the last 40 years, along with ducts and usually air handlers in the attic. Luckily it’s possible (Dry climate) and not too difficult to deep bury the duct system in R50-R60 blown in insulation. If this happens with a new HVAC you can usually downsize by 1 Ton, and hopefully remove the furnace and switch to a central heat pump while you’re at it. SMUD has a great rebate program promoting this. My new Meritage Home does the ocSPF at the roof deck and walls, creating a conditioned attic and a tight home, 600 [email protected] for 2790 sq ft home.

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