This webinar, co-hosted by Pro-R Duct, addressed the long-standing technical challenges associated with outdoor ductwork installations, particularly on rooftops, and presented a prefabricated duct solution designed to mitigate common failure modes related to moisture, leakage, insulation degradation, and structural durability. The discussion focused on why traditional approaches to outdoor duct construction often underperform and how a phenolic-based, double-wall duct system is engineered to improve long-term performance.
Outdoor ductwork is frequently required when rooftop units cannot be located directly above vertical shafts or when multiple rooftop units serve a shared shaft. In these cases, ductwork may need to run significant distances across the roof. The most common industry approach uses standard galvanized sheet metal duct, which is then field-wrapped with fiberglass insulation and an external jacket. This approach introduces several technical vulnerabilities:
- Moisture intrusion: Fiberglass insulation is open-cell and readily absorbs water.
- Through penetrations: Screws, fasteners, and hangers penetrate the duct jacket, creating direct pathways for water.
- Sealant degradation: Mastics and sealants used to weatherproof joints degrade over time due to UV exposure, thermal cycling, and weather.
- Insulation performance loss: Once fiberglass insulation becomes wet, it can lose approximately 85% of its thermal effectiveness, reducing an expected R‑12 system to an effective R‑2 or R‑3.
- Permanent compression: Even after drying, wet fiberglass remains compressed, resulting in permanently reduced R‑value.
- Indoor air quality (IAQ) risk: Trapped moisture can lead to mold and mildew growth, a critical concern for schools, healthcare facilities, and other sensitive occupancies.
These issues lead to both energy loss and long-term maintenance problems, often within only a few years of installation.
The Pro‑R duct system is based on phenolic insulation panels manufactured by Kingspan, which form the structural and thermal core of the duct.
Key material characteristics include:
- 90% closed-cell structure, significantly limiting moisture absorption
- Water absorption of approximately 0.14% after 24 hours, even when exposed
- Fiber-free construction, reducing IAQ concerns
- High density and rigidity, contributing to structural stability
- 1¾-inch panel thickness achieving approximately R‑12 thermal resistance
- Compliance with UL 181 duct construction standards
- 25/50 flame and smoke rating, meeting common building code requirements
Unlike fiberglass, phenolic insulation maintains its thermal performance even after exposure to moisture and does not support mold growth.
The presenter showcased a defining technical feature of the Pro‑R system is its double-duct (duct-within-a-duct) construction. The phenolic panel forms the inner duct wall, which is then fully encapsulated by an external aluminum jacket.
The outer jacket consists of:
- 0.040-inch aluminum, equivalent to approximately 18-gauge aluminum
- Continuous wrap around the insulated core
- No exposed insulation or vapor barrier
This dual-layer construction provides both mechanical strength and environmental protection, differentiating it from single-wall phenolic duct systems. The double-wall design allows the duct system to withstand static pressures up to approximately 10 inches w.g., making it suitable for larger air distribution systems with higher pressure requirements.
Internally, the duct includes structural reinforcement that allows it to support significant loads, withstand foot traffic during installation and maintenance, and maintain shape and joint integrity under pressure. Despite its strength, the system remains lighter than comparable galvanized steel duct, improving rooftop handling and reducing labor requirements during installation.
A critical technical advantage of the Pro‑R system is its joint construction method. Instead of screws, flanges, or external fasteners, sections are joined using a Pittsburgh-style seam, which forms a pocket-and-lock connection between sections.
This approach provides several performance benefits:
- No through penetrations in the duct wall
- No reliance on field-applied sealants or mastics
- Reduced long-term degradation risk
- Consistent joint geometry from prefabrication
As a result, the system achieves less than 1% air leakage, significantly outperforming standard SMACNA-allowed leakage rates. For comparison, traditional duct standards permit up to 6% leakage at 1 inch w.g. per 100 square feet of duct—levels that are particularly problematic for outdoor applications.
Low leakage also limits moisture intrusion, even during wind-driven rain events.
Roofing and Drainage Considerations
Because of its low leakage rate and lack of penetrations, the Pro‑R duct system does not require a sloped roof or weather hood. Traditional outdoor duct installations often rely on sloped covers to prevent standing water, but these covers add cost, weight, and complexity. In this system, standing water does not readily enter the duct, there are no sealants to fail over time, and long-term weather resistance is inherent to the construction
The webinar highlighted performance in hail-prone environments, specifically in Colorado. Testing and demonstrations showed that: the aluminum jacket may dent under impact; structural integrity and insulation performance remain intact; and the duct is not punctured or breached under typical hail or impact conditions. In contrast, traditional insulated duct systems can suffer catastrophic failure during hailstorms, leading to exposed insulation and rapid water intrusion.