Double Wall Pre-Insulated Spiral Duct offers exceptional thermal and noise control. It is the most reliable, durable and permanent method of insulating spiral pipe. It is comprised of two concentric spiral pipes of different diameters with the intervening space filled with fiber glass insulation. Dual wall insulated spiral duct is virtually airtight and the insulation is protected from the four most common types of failure -: job site damage, erosion due to high pressure, turbulent airflows and condensation. Since the insulation is factory installed, time- consuming and labor-intensive installation at the job site is eliminated.