Water intrusion into the insulation of HVAC systems can have severely deleterious effects on the entire system.

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Moisture in insulation will not only reduce the thermal properties of the insulation reducing energy efficiency it may also lead to bigger issues such as mold growth, corrosion under insulation (CUI) and breakdown of the insulation itself.  Over time this can lead to the need for expensive repairs, remediation or potentially replacement of the HVAC duct system.  Therefore, it is critical to keep water out of HVAC insulation systems to protect and prolong its service life.Water may enter the insulation either as a liquid or potentially as water vapor when the HVAC system is operated below ambient temperatures.  In many HVAC systems, particularly those in southern climates, the HVAC system is operated primarily for cooling which means there is a continuous vapor drive into the external insulation.  Once that insulation becomes wet it will not have a chance to dry back out.

To prevent water and water vapor from entering cold insulation systems it is critical to ensure the insulation is properly flashed and sealed against moisture and that the external insulation is protected with a water vapor retarder on its outer surface.

ASTM C755, Standard Practice for Selection of Water Vapor Retarders for Thermal Insulation, provides guidance and considerations for the selection of vapor retarder systems including those for HVAC duct insulation.  In Table 2 of ASTM C755 maximum permeance requirements for vapor retarders are provided depending on the insulation type, insulation application and operating temperatures of the system.  For HVAC Duct systems the required vapor retarder permeance values for systems operating between ambient and 40oF and 39oF and below are a maximum of 1.0 and 0.02 perms, respectively for low permeability insulations and maximum 0.03 and 0.02 perms respectively for insulations with permeability greater than 4.0 perm-in such as fiberglass insulation.

Low permeance vapor retarder coatings and mastics are ideally suited to contribute to or create the vapor retarder system for the external HVAC insulation.  As liquid products they fully wet out the insulation surface creating a monolithic surface coating free of seams, gaps and voids where water and water vapor may enter.

It is common for many external HVAC insulations to be covered with an aluminum foil laminate jacket such as an ASJ, FSK or other similar types to provide the primary vapor retarder protection.  This may be over fiber glass board or duct wrap or other rigid foam insulations including polystyrene, polyisocyanurate or rubber foam insulations.  While the aluminum foil laminate may provide good vapor retarder protection of the insulation it does have its limitations.  First the aluminum foil is typically of a thin gauge allowing it to be easily punctured or damaged negating its vapor retarder properties.  Second, the insulation is often mechanically fastened to the duct using stick pins that result in puncturing of the foil laminate breaching the vapor retarder.  Third, all foil laminate jacketing will require it to have breaks in the foil at insulation seams and at terminations where moisture may enter, and finally irregular surfaces such as elbows, T’s, trunks and other fittings can be difficult to seal with sheet jacketing requiring cutting and overlapping to fit properly.  This can lead to potential for voids and wrinkles in the jacket where moisture and water vapor may enter.

Vapor retarder coatings can be particularly useful when used in conjunction with aluminum foil laminate jackets to provide a robust and long performing vapor retarder system for the HVAC insulation.  In areas such as mechanical rooms, mezzanines, and areas of open ceilings where the foil laminate may be prone to puncture or damage, vapor retarder coatings applied with mesh reinforcements over the entire foil surface can protect the foil laminate from breaks in the vapor retarder.  Vapor retarder coatings applied over elbows, T’s, terminations, and other irregular areas provide a seamless system free of wrinkles or voids leaving potential for moisture intrusion.   Vapor retarder coatings may also be used over seams in the foil laminate facings and to seal punctures from stick pins or around protrusions through the foil laminates.  Pressure sensitive foil tapes sometimes used over seams and punctures can provide a continued vapor retarder, but they can often delaminate over time, opening up entry points for moisture and water vapor.  Vapor retarder coatings do not delaminate over time and can seal down tapes or replace them with permanent vapor retarder protection.


Breach of the vapor retarder in foil

Figure 1:  Breach of the vapor retarder in foil laminate jacketed fiberglass duct insulation due to physical damage.  Moisture will freely enter the insulation system.   

Foil laminate jacketed fiberglass duct insulation protected with a vapor retarder coating.

Figure 2:  Foil laminate jacketed fiberglass duct insulation protected with a vapor retarder coating.  The tough, reinforced coating prevents damage to the foil system maintaining the vapor retarder properties.

Taped seams and punctures in foil laminate faced duct insulation

Figure 3:  Taped seams and punctures in foil laminate faced duct insulation.  Note the peeling tape leading to loss of the vapor retarder protection at the seams

Figure 4:  Vapor retarder coating sealed seams and stick pin punctures over foil laminate faced HVAC duct wrap insulation. Delamination will not occur preventing any loss of vapor retarder protection..


Section 7.2.2 of ASTM C755 specifically discusses the use of mastics (ie coatings) as vapor retarders and provides guidance for the coatings when used alone as the sole cladding material or in conjunction with a separate very low permeance vapor retarder membrane.

When considering the best design and application of the vapor retarder system for HVAC duct insulation it is important to consider the long-term performance of the system, particularly in those regions where the HVAC duct is primarily used for cooling. Properly selected coatings, with low water vapor permeance values in accordance with the ASTM C755 standard, play an important role in maintaining the HVAC systems operating performance over time. By protecting against moisture intrusion coatings protect the performance of the vapor retarder system ensuring the HVAC system will continue to operate at its intended efficiency without the issues created by unwanted moisture and thereby extending the life of the system and avoiding costly repairs.