However there are certain flip-sides of applying encapsulates. According to laboratory finding applying encapsulates to duct boards made of fiberglass or lined with fiberglass will reduce its acoustical (noise) and fire retarding characteristics. It can also make the manufacturer’s warranty on the duct board void.
The risks of health hazards due to sealants are yet to be known. Organizations concerned with duct cleaning, including EPA, NADCA, NAIMA and the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) do not commend routine use of sealants to encapsulate contaminants in any type of duct.
Sealants are often used to repair damages in fiberglass ducts or ducts that have sustained fire damages. However remember sealants should never be sprayed on wet ducks or duct lining infected with molds. They are to be used only after the duct surface has been spruced of dirt and debris and are dry.