Indoor air quality refers to the atmosphere within and around buildings and structures as it relates to the health and comfort of its occupants. It can be affected negatively by microbial contaminants, such as mold and bacteria.

The HVAC system is designed to respond to environmental changes in a high-performing building. Additionally, the design and construction stages of a building’s HVAC system directly affect IAQ, specifically the installation of ductwork insulation. Properly installed ductwork insulation ensures adequate IAQ for the health and comfort of the building occupants.

Different types of HVAC systems require different ductwork insulation, which is available in different sizes, shapes and configurations for ease of application. The optimal type of ductwork depends upon the job it will do.

Duct wrap insulation offers thermal properties that reduce unwanted heat loss or gain and condensation during system operation. This blanket-type insulation is used to insulate rectangular and spiral duct. It is used as external insulation on commercial HVAC ducts and controls condensation. Duct wrap is lightweight, flexible and easily conforms to flat or irregular surfaces, and its roll-style qualities allow for faster installation at lower labor costs.

Duct board insulation is made out of fiberglass and a thin layer of foil on the exterior. It is primarily constructed from two layers to ensure efficient flow and protection from outside elements. A versatile part of an HVAC system because it is lightweight and simple to assemble, duct board is popular among HVAC construction workers, installers and insulators. Noise control is an additional benefit from this type of duct insulation because the fiberglass layer muffles the loud sound of the HVAC system blowing warm or cool air. 

Common installation issues

The key to optimum performance of a building enclosure and HVAC system is the proper installation of ductwork insulation. Too often, ducts are not properly insulated or lack insulation all together. These issues cause leaks, condensation on surrounding materials, increased energy costs and health concerns for building occupants.

Leaks occur between sections of duct insulation when the insulation isn’t overlapped or sealed properly, or when pressure on the side of the ducts results in a breach. The leaks pull air out of the ducts or from outside, reducing the effectiveness of the HVAC system. Excessive air movement significantly reduces the thermal integrity and performance of the building envelope and is a major contributor to energy consumption.

Condensation on ductwork is a serious problem and causes various issues, such as leaking ceilings and mold infestation. Condensation is caused by inadequate insulation on ductwork, improper sealing of the ductwork or too much moisture in the air. Ductwork insulation itself retards condensation, and if the insulation is installed properly, it prevents condensation from forming on the ducts. When insulation is installed improperly or there is a lack of insulation, warm air permeates the ductwork, causing condensation and microbial growth or mold on adjacent surfaces. The mold and other allergens then blow throughout the building, causing health concerns for the building occupants.

Climate and geographic location play a large role in the level of condensation that can form on ducts and ductwork. In humid climates, ducts are prone to moisture condensation. If the insulation doesn’t cover the entire surface of the ducts, including the connections, condensation can occur. If duct seams are not properly sealed, condensation can form due to the surface temperature being cooler than the warm, humid outside air. A lack of insulation in certain climates slows the HVAC system’s ability to condition or heat the air. If a duct passes through a spot close to an uninsulated outside wall, it may warm or cool the surface at the wrong time of the year. That will temper the heat or the coolness of the air inside the ducts, causing the HVAC system to work harder and increasing energy costs.


Duct insulation and sealing, especially insulated supply ducts delivering conditioned air within a building, saves energy. The intent of energy efficiency codes, as it relates to duct insulation and sealing, is to keep mechanically warmed or cooled air as close to a constant, desired temperature as possible and prevent it from escaping the duct system. 

The U.S. Department of Energy mandates energy efficiency codes for duct insulation and sealing in commercial buildings. One code requires that all supply and return air ducts be insulated with a minimum of R-5 insulation when located in unconditioned spaces and a minimum of R-8 insulation when located outside the building. When located within a building assembly, the duct should be separated from the building exterior or unconditioned spaces by a minimum of R-8 insulation.

When insulating HVAC ductwork, the Energy Department requires foil-faced fiberglass insulation with an R-6 or higher R-value and a metallic foil duct tape to seal and hold the insulation in place. Proper installation requires sealed joints, seams and penetrations for assured condensation control. Duct wrap should be installed facing the outside to obtain the specified R-value with a maximum of 25 percent compression. All insulation joints should be butted firmly together and ensure the seam of the vapor retarder is overlapped by a minimum of two inches. The vapor retarder’s performance relies on all penetrations, joints, seams and damage to the facing to be sealed with a foil-scrim-kraft (FSK), polypropylene-scrim-kraft (PSK), foil tape or glass fabric and mastic prior to system startup.

Regarding rectangular ducts more than 24 inches wide, the insulation should be secured to the bottom side of the duct with mechanical fasteners spaced on 18-inch centers to reduce sagging. It is important to avoid overcompressing the insulation with the retaining washer. It isn’t necessary or recommended to join duct wrap to duct surfaces with adhesive.

Duct board insulation is approved for operating temperatures from zero to 450°F at a maximum recommendation thickness of 4 inches. Application of duct board insulation should be installed on dry, clean surfaces. Metal ducts must be sealed before application, and it is recommended to pre-score rigid insulation board where necessary to conform to curved surfaces. All insulation joints must be firmly butted together, and the insulation should be secured with mechanical fasteners or banded together. To assure a firm fit and maintain thermal performance of the duct board, minimum compression should be applied. Vapor retarders should overlap a minimum of 2 inches at all seams and be sealed with the appropriate pressure-sensitive tape or mastic. Where vapor retarder performance is necessary, all penetrations and facing damage should be repaired with tapes or mastic with a minimum of 2 inches overlap. 


In commercial settings, regular inspection of ductwork and its insulation is recommended by professional mechanical insulation contractors specializing in the HVAC market for the facility owner. A facility owner can perform the initial inspections visually by looking for water damage, mold growth, tears or rips, sagging or other deterioration. The facility owner may also notice leaking or poorly performing air ducts, wet ceiling tiles, increased energy bills, or spaces that are difficult to heat or cool. 

However, a facility owner should hire a professional mechanical insulation contractor to perform a full inspection. During the full inspection, the contractor will verify that the supply and return ducts have been insulated to the specified R-value in accordance with the approved mechanical plans and that the ducts have been sealed in accordance with approved design document code requirements. Verifying all seams and connections for the entire duct system in a commercial building can be tedious and time-consuming. The inspection typically involves spot checks to confirm that the seams and connections are sealed properly. Testing the duct system for air tightness provides the final confirmation of proper duct sealing.

As the demand for sustainable high-performance buildings continues to rise, building owners and developers should demonstrate a commitment to occupant health and sustainability through good design, smart construction processes and specifying low-emitting products. Properly installed duct insulation is a critical step in protecting a building from heat loss or gain, moisture intrusion and mold contamination, providing healthy air quality.

It is imperative that ductwork fabrication and installation contractors are knowledgeable about the types of ductwork insulation, follow recommended application instructions, and meet all codes and requirements for commercial buildings. Not only is it vital to insulate the ductwork properly, but also having a professional mechanical insulation contractor who knows the proper, efficient techniques for installing and replacing insulation is key for optimal performance of a commercial building’s HVAC system.