Breaking Barriers with Buried Ducts
In a 2013 cost study commissioned by Leading Builders of America, it was estimated that as much as $1,700 in construction costs can be saved by using a performance path instead of the prescriptive table in the International Energy Conservation Code (IECC). Compliance with the energy code was driven primarily by a prescriptive method until recently. The performance path method uses energy modeling to demonstrate that the building as a whole uses energy equal to or less than the energy used by a home built to the prescriptive path. Thus the performance path allows builders the flexibility to trade out many of the high-cost assemblies required in the prescriptive path, delivering a home that is at least as efficient.
Building strong, resilient homes is still a priority for builders of high-quality homes. By using the performance path to meet energy requirements, builders can maintain their preferred building specifications for structural design, combined with energy-neutral alternatives for energy performance. A few examples include attic insulation, tight air sealing, efficient wall construction, insulated foundation walls, and energy-efficient HVAC systems. This article focuses on the HVAC option of burying ducts, and on the effectiveness of this option when combined with radiant-barrier roof sheathing.
Understanding New IECC Provisions for HVAC Duct Design
The 2018 International Residential Code (IRC) and the 2018 IECC add new provisions for HVAC duct design within ventilated attic spaces. These provisions will increase the options for builders who are working to meet the requirements of the energy code using the performance path or Energy Rating Index (ERI) method of compliance. Under the first provision, when ducts in ventilated attic spaces are “deeply buried” they are considered to be insulated to an effective R-25 value for energy-modeling purposes (2018 IECC, Buried Ducts R403.3.8). Under the second provision, the ducts within a ventilated attic space can be modeled as being in conditioned space, provided additional criteria are met (2018 IECC, Ducts Considered in Conditioned Spaces R403.3.7).
Ducts Buried in Insulation
To meet the 2018 IRC and IECC installation criteria for ducts buried in insulation, heating and cooling system ducts within ventilated attics must be installed in accordance with one of three methods, depending on specific climate zone requirements: partially buried ducts, deeply buried ducts, or ducts considered to be in conditioned space. Figure 1 illustrates the minimum R-value of attic insulation required for ducts partially embedded in attic insulation, ducts resting on top of truss or rafter chords, and ducts resting between truss chords.
With buried (or hanging) ducts, it is important to wrap a vapor-impermeable membrane around the duct insulation to reduce the formation of condensation on the surface of the duct. Reducing condensation potential in attics with buried ducts is discussed further later in the article.
Ducts that are “deeply buried” in attic insulation (Figure 2) can be considered to be insulated to an effective R-25 value for building energy simulation purposes provided the following criteria are met:
- The duct is located directly on the ceiling or within 5.5 inches of the ceiling.
- The total sum of insulation above and below the ductwork must be a minimum of R-19, excluding the R-value of the duct insulation.
- If the ductwork is installed directly on top of the ceiling, a minimum R-19 insulation is required only on top of the ducts.
- If the ductwork is installed above the ceiling joist, the minimum insulation below the duct must be R-8, which will most commonly be met by either R11 loose-fill insulation or R-13 batt insulation.
- The sides of the duct are surrounded with ceiling insulation of at least a R-30 value.
- The duct is covered on top with at least 3.5 inches of insulation.
Factoring for Conditioned Space
Under the second provision, ducts in an unconditioned attic can be considered within conditioned space (Figure 3) for building energy-modeling purposes when the following criteria are met:
- The air handler is located inside conditioned space.
- Duct tightness requirement is 1.5 CFM25/100ft2 of conditioned floor area served by the duct system, which is approximately three times the tightness of the base level requirement.
- The insulation value above the ducts must be at least the prescriptive ceiling insulation value less the R-value of the duct installation.
When incorporating these buried-duct options, it is important that they be installed so as to avoid condensation problems. While all applicable IRC duct requirements must be followed, making sure the vapor retarder on the duct insulation remains intact, that the duct tightness limits are not exceeded, and that consistent minimum insulation levels around ducts are maintained will help minimize condensation potential with buried ducts.
Adding Radiant-Barrier Roof Sheathing
Radiant-barrier sheathing panels have surfaces that reduce infrared radiation exchange in attics. While the 2018 IRC/IECC buried-duct provisions do not account for the use of a radiant barrier on the underside of the roof deck, it is important to consider how the addition of a radiant barrier affects the energy performance of the structure.
Radiant barriers reduce the infrared radiation between the roof deck and the attic under any conditions. They may be of more importance during the hours when solar radiation is hitting the roof, but they work all the time to reduce the infrared exchange.
For example, when the sun heats the roof, the roof assembly absorbs the solar energy and heat (see Figure 4). Once the temperature of the roof assembly exceeds the ambient temperature within the attic, heat from the roof assembly is transferred into the attic. A radiant barrier on the underside of the roof deck reduces the radiative heat transfer (long-wave radiation) from the underside of the roof assembly to the other surfaces in the attic.
Another example is when the ventilated-attic ambient air is cooler than the air inside the building envelope. Heat from the conditioned space then transfers through the ceiling assembly into the attic.
A radiant barrier installed on the underside of the roof deck reflects radiative heat emitted by the ceiling back toward the interior, as shown in Figure 5.
Understanding the ERI/HERS Index
The use of buried ducts and radiant-barrier roof sheathing affects how a building will be modeled within the energy modeling software used to determine compliance with the ERI Compliance Alternative option or the IRC and IECC Simulated Performance Alternative option. The HERS index is the existing compliant ERI method and is nationally recognized for inspecting and calculating a home’s energy performance. (See “ERI Score.”)
The ERI score is defined as a numerical score where 100 is equivalent to the 2006 IECC and 0 is equivalent to a net zero energy home. Each integer value on the scale represents a 1% change in the total energy use of the rated design relative to the total energy use of the ERI reference design.
Radiant-barrier roof sheathing panels are most effective in warm climates, but these can also be used in cold climates. For climates where heating is the dominant consideration, the energy savings are less than in warmer, cooling-dominated climates. Proper attic ventilation is also required to ensure that the condensation potential on the underside of the roof sheathing is reduced. Radiant-barrier sheathing is not recommended for climate zones 5 and higher.
The advantages of buried ducts and radiant-barrier sheathing are noted Table 1 as reductions in the total ERI score.
Table 1. Estimated Impacts in the Energy Rating Index Score
|IECC climate zone||2||3||4||5||6||7|
|Radiant barrier roof sheathing1||−3||−3||−2||NR||NR||NR|
|Deeply buried ducts 1,2||−2||−2||−1||−1||−1||−1|
|Ducts considered in conditioned space 1,2,3||−7||−7||−6||−7||−7||−7|
Reduce Condensation Potential with Ventilation
Adequate attic ventilation and properly installed duct vapor retarders are important to achieve optimum performance when radiant barriers and buried ducts are used. In a properly ventilated attic, the attic air is similar to the exterior ambient air. When radiant-barrier roof sheathing is combined with deeply buried ducts installed in warm climates, or when the direction of heat transfer is from the roof assembly into the attic, it is especially important that adequate attic ventilation be provided to remove excess water vapor from the attic air. (See “Condensation.”)
Condensation on a surface occurs when the temperature of a surface is at or below the dew point temperature of a parcel of air. Warm air can hold more water vapor than cold air.
Because the attic floor temperature is reduced by the use of radiant-barrier sheathing, the condensation potential on the surface of the duct increases. This is one of the reasons why wrapping a vapor-impermeable membrane around the duct insulation is so important.
The 2018 IECC delivers more flexibility for meeting the performance pathways of the energy code, including partially buried ducts, deeply buried ducts, and ducts to be considered in conditioned space. These options can reduce energy use while containing construction costs. The addition of radiant-barrier panels reduces radiant-heat gain resulting in cooler homes, lower utility bills, improved energy efficiency, and a more comfortable indoor environment.
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