Energy efficiency measures, such as cool roofs, radiant barriers, interior radiative control coatings, and buried ducts are increasing in popularity and are promoted by energy codes because of their energy-saving potential. However, these strategies can also pose moisture risks in attics by lowering surface temperatures and increasing condensation potential and moisture accumulation. Of particular concern in hot-humid climates is dripping condensation on cold air-conditioning ductwork in the summer—commonly referred to as duct “sweating”—which threatens the attic floor with conditions conducive to mold growth and rot. One strategy to mitigate these moisture issues is to wrap ductwork in thicker duct-wrap insulation with an integrated exterior vapor barrier, but thick duct wrap can be difficult to come by, expensive, and unwieldy to work with.
This study explores an alternative strategy of reducing moisture issues while embracing energy efficiency by using unvented attics with vapor diffusion ports and buried ductwork in hot-humid climates. Vapor diffusion ports have been studied so far in a wide range of U.S. climates, mostly in the context of conditioned attics. In this study, the strategy is implemented in the novel context of hot-humid climates with ductwork sitting atop blown-in attic floor insulation in unconditioned attics. Using a combination of field experiments and hygrothermal modeling, the findings of this project indicate that an unvented attic with vapor diffusion ports and buried ducts may be a key part of a successful low-cost method for reducing the attic moisture load by venting excess moisture out of the attic, keeping duct-jacket surfaces above dew point temperature, and keeping the roof deck safe from winter moisture accumulation.
The study monitored occupied experimental homes in DeBary, Florida (International Energy Conservation Code [IECC] Climate Zone 2A), to evaluate in-situ hygrothermal performance at the roof deck and attic floor over the course of nine months (including summer, fall, and winter). The homes were all new construction production homes in the same residential development. The five construction variations in the monitored homes were:
1. Baseline construction: Unconditioned, vented attic; ducts hung and wrapped in typical R-8 duct insulation
2. Buried ducts: Unconditioned, vented attic; R-8 ducting buried in minimum R-11 attic insulation
3. Diffusion port: Unconditioned, unvented attic with vapor diffusion ports replacing off-ridge vents; ducts hung and wrapped in typical R-8 duct insulation
4. Diffusion port + radiant barrier: Unconditioned, unvented attic with vapor diffusion ports replacing off-ridge vents and a radiant barrier draped at the roof deck; ducts hung and wrapped in typical R-8 insulation
5. Diffusion port + radiant barrier + buried ducts: Unconditioned, unvented attic with vapor diffusion ports replacing off-ridge vents and a radiant barrier draped at the roof deck; R-8 ducting buried in minimum R-11 attic insulation.
Although all five attics were monitored for ductwork condensation and surrounding conditions, the baseline (#1) and diffusion port + radiant barrier + buried duct (#5) cases were instrumented in greater detail. The configurations were monitored for moisture performance of the attic floor and roof deck in order to assess the potential for diffusion ports to address the hygrothermally stressful conditions posed by the combined energy efficiency measures. A one-dimensional hygrothermal model was developed to represent the diffusion port + radiant barrier + buried duct case and was later implemented to explore what-if scenarios involving more hygrothermally stressful conditions, including cool roofs, site shading, cooler climate, lower occupant set points, and higher occupant moisture generation.
Both measured and modeled cases were assessed using industry-standard criteria for mold, rot, and corrosion risk. Field observations showed no signs of sustained mold, rot, or corrosion risk in any of the experimental homes, although the weather was generally warm during the measurement period compared to historical averages. Visual observations during decommissioning confirmed the absence of moisture issues, including no indication of mold, rot, corrosion, or stains from puddling below ductwork. No evidence of condensation dripping from ductwork was present, but the authors note that future field experiments should instrument both the top and bottom of the exterior duct jacket at various locations along the duct runs to better detect presence of condensation, because relative humidity (RH) can be higher above the ducts than it is below them.
The hygrothermal models were then created to “stress test” the diffusion port + radiant barrier + buried duct home for potential moisture risk under more hygrothermally stressful conditions. The modeled results demonstrated high sensitivity to occupancy conditions, indicating that low temperature set points can be highly impactful to mold index and corrosion at both the roof deck and attic floor. The stress test case indicated that, although observed conditions did not prompt moisture concerns, the diffusion-port strategy should not be widely recommended as the sole method to mitigate attic moisture issues in hot-humid climates without further study.
Additional findings of this study include that unvented attics may boast improved extreme-weather resistance, as demonstrated by their reduced humidity levels during and directly after Hurricane Ian and Tropical Storm Nicole. Additionally, unlike in unvented-attic studies in conditioned attics where the highest mold risk is found toward the roof ridge, the roof deck in these unconditioned attics had higher mold risk lower down the roof slope. Several factors may influence this finding, but of particular note is the presence of a suspended radiant barrier, whose implementation may influence stratification trends and cause convective looping. Additional study is required to investigate the specific influence of radiant barriers on the effectiveness of vapor diffusion ports, as well as sensitivity to other construction strategies (such as the introduction of small amounts of conditioned air to the attic, different configurations of diffusion ports, and variations in attic airtightness). After further tuning the hygrothermal models, additional sensitivity studies beyond this preliminary modeling effort could also be performed to assess the impacts of climate, construction materials, roof reflectivity, and occupancy variation.
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