The Lambro FMD3-425 is a 4-inch insulated flexible air duct designed for low-volume airflow applications, including bathroom exhaust venting and small residential branch lines. Stretching a full 25 feet, this flexible ducting allows installers to navigate complex structural paths and tight joist spaces effortlessly. The internal insulation layer maintains thermal performance, keeping exhausted or supplied air at a stable temperature during distribution.
This duct features an aluminized vapor barrier sleeve that provides excellent resistance against humidity and external moisture buildup. The barrier prevents surface sweating, which shields nearby insulation, drywall, and timber framing from potential moisture-related deterioration. This level of protection is highly advantageous for installations traversing cold attics or unheated crawlspaces where warm exhaust air could cause condensation.
Prior to beginning your installation, ensure that your exhaust fans or inline branch fittings are fully compatible with a 4-inch round connection. The lightweight and flexible construction minimizes setup time and tool requirements, offering an efficient alternative to rigid piping. This insulated duct product represents a reliable, cost-effective choice for modern residential air distribution and spot ventilation improvements.









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