Construction Design-Keeping Ducts Where They Belong

Ductwork in unconditioned attics or crawl spaces can create problems in all climates. Undesirable positive or negative house pressures, moisture and mold issues and increased energy costs are often the result. In my work as an energy auditor, I’ve conducted blower door testing on many new homes that have ductwork 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 ductwork in unconditioned attics or crawl spaces.  With so many new homes slab on grade homes being built today, lots of designers and builders are trying to figure out: where is the best place to put ductwork?

Before we get to that question, lets look at what code has to say about ductwork outside the building envelope, code from the 2018 IRC.

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 inches in diameter and larger and not less than R-6 for ducts smaller than 3 inches 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 percent 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 2018 International Residential Code, any ductwork that leaves the conditioned space of the home needs to be insulated and air sealed and the ductwork 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’s job is it to bake sure these holes are sealed?  It could fall on the HVAC contractor, the insulator, the drywaller or the general contractor, but it’s just as likely that no one takes responsibility to seal the penetration.

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 ductwork 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 also be a good idea, especially if you live in a hot or mixed climate where cold ductwork can become a dehumidifier and sweat.

One code I find interesting is the requirement for testing HRV or 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 separated, dedicated duct systems, they do not require an airtightness test, even if installed outside the building envelope.  At least that’s my interpretation of that code.

If you must have a ducted system

What are our options to keep ductwork inside the conditioned space? Well, one option is to eliminate the ductwork 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 or address dehumidification.

There is also a cost to installing two completely different systems to provide both heating and cooling, which would be needed in cold climates.

Ductless mini-split 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.

Sometimes ducted systems are the best fit.

There are a few ducted choices that can be effective. I’ve seen sealed PVS or Polyvinyl Coated Steel ductwork 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 air and watertight.

Changing the attic space from unconditioned to conditioned 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 ductwork and HVAC equipment in this conditioned attic space. This approach is rare in the cold climate I live in.

False ceilings and soffits can hide ductwork and still allow it to remain inside the conditioned space. A great option that will require detailed planning for the location of the ducts. I recently worked on a project where a plenum truss was used. This notch in the truss allowed for a flat ceiling framed in after the installation of the ducts. All ducts in these special trusses remain inside the building envelope. This service cavity also provided a chase for electricians and plumbers to keep much of their wires and pipes inside the home. A design feature I will be using again.

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 the ducts within the conditioned space.

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 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 18,868 BTU. The heating system would have been three times the required size.

As for the builder trying to plan ductwork 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 insulated, air sealed and testing of ducts when they leave the conditioned space of the home, building codes are discouraging the placement of ductwork in these locations – an all to common problem I see during energy audits. Here are a few photos I’ve taken over the past couple years where ductwork is located in an unconditioned attic space.  They 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 1970’s. 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 ductwork is not air sealed and during the blower door test, I was able to draw the cold attic air through the ductwork 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 ductwork. This home had severe ice build up along with ice dams. Water from the ice dams had entered the home is several areas. Air temperature was 20°F at the time of the photo. The heat and air leaks from the ductwork 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. No clear boundary. This was an old seasonal cabin that had been converted to a year-round rental home which had a blower door test of 15 ACH50, 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 ductwork 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. Ductwork 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 a blower door test. Supply air vent from an unconditioned attic space.

Just a small sampling of what I see when ductwork 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.

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