Duct surfaces often get infested with dirt and debris that come with the air passing through it. The dirt along with other contaminants generally sip in through the leaks and holes often found in ducts. The contaminants also make their way into your rooms with the air. To prevent this there is a special category of material that is available in the market called encapsulants or sealants. The surfaces are coated with these by spraying them in the ducts just like biocides. As a result the dust particles that pass through the ducts stick to the surface and do not blow into the rooms. This is especially true for duct boards made of fiberglass or lined with them. After sometime of usage small chips of fiberglass starts coming out which reaches your livings space with the air. Exposure to them is harmful for the health. Applying encapsulants to such duct boards often prevent chipping. Some of these products are also resistant to molds. Molds cannot grow on surfaces coated with sealants.

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.