R-8 insulated flex duct: inner liner (metalized polyester), fiberglass insulation, vapor barrier outer jacket. Sizes: 4 in through 18 in diameter. R-value: 8.0 (meets IECC minimum for ducts in unconditioned spaces in most climate zones). Max length per run: 25 ft recommended (excessive length increases friction loss). Airflow capacity: 6 in = 100 CFM at 0.08 in w.c./100 ft, 8 in = 200 CFM, 10 in = 350 CFM, 12 in = 500 CFM. Installation: fully extended (no compression), supported at 5 ft intervals and within 3 ft of connections. Max 3 ft unsupported sag. Seal connections with mastic + zip ties (not tape alone). UL 181 Class 1 listed. Cost: $0.50-$2.00/ft. Disadvantages: higher friction than rigid, can sag/kink, noise at high velocity. CSI 23 31 13.