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Duct Systems

Duct Insulation

The 2018 IRC/IECC requires R-8 on supply ducts and R-6 on return ducts in attics. Ducts in other unconditioned spaces must be insulated to R-6, except those 3" and smaller, which can be insulated to R-4.2.

Duct Sealing

The 2018 IRC/IECC requires that all ducts, air handlers, and filter boxes be sealed. It also requires than any panned returns be sealed (see sidebar).

Sealing must follow the details in the 2018 IRC, which allows the use of mastics, liquid sealants, and tapes.  It also allows the use of air-impermeable spray foam products as both duct insulation and sealant.

Duct Mastic Photo

The 2018 IRC/IECC requires that air handlers be designated for leakage of no more than 2% of the design air flow rate (per ASHRAE 193).

Duct Leakage Testing

If any ducts are outside the thermal envelope, a duct blaster test must be conducted to determine the Total Leakage.

Duct Blaster Photo

The 2018 IRC/IECC specifies different leakage limits depending on the test conditions.  In all cases the limits are based on the conditioned floor area (CFA) of the house:

Total Leakage Limits Table

(Note:  these limits only apply to projects following a prescriptive compliance pathway.  Projects following a performance or ERI pathway are allowed to trade off higher duct leakage for improved performance in other areas.)

For ducts to all be considered "within the building thermal envelope", they must be on the interior side of both the insulation and air barrier. Therefore, if any ducts are located in attics, vented crawlspaces, or exterior wall cavities, the system must be tested and meet the leakage requirements.


Panned Returns

The 2018 IRC/IECC does not allow building cavities to be used as ducts.
The PA Labor and Industry Rules and Regulations, however, include an amendment that adopts the 2009 IRC/IECC provisions that state "building framing cavities shall not be used as SUPPLY ducts".
Therefore, in PA, building cavities are allowed to be used as return air pathways.
 Panned Return Graphic
This allowance should be used with caution by builders. If any ducts are outside the conditioned space, the total system leakage must tested. Panned returns are likely to cause test failures. Remediation may be more expensive than installing hard-ducted returns would have been.

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