One other point to mention is that ducts to be used for moving cool air should have a vapor barrier on the outside of the insulation so that moisture will not migrate to the cold surface and condense. Before wrapping up this topic, let's take up again the issue of dirty ducts. You may have been called by a firm offering to vacuum your ducts. There are several things you should be aware of in making a decision on cleaning your ducts. First, you should know that if a duct system is properly sealed along the supply and return runs of the ductwork, there is little likelihood that significant amounts of dust, dirt, and biological material will accumulate in the ducts.
Ducts get dirty when holes or gaps in the ductwork permit foreign matter to enter the air ducts.
That being the case, it may be wise to investigate and confront the source of this problem rather than repeatedly cleaning your ducts. Second, you should be very careful about permitting contractors to use biocides or other chemicals in your ducts. If there is a problem of mold or vermin in your ducts, injecting harmful chemicals could affect you and your family as harshly as it does the creatures in your ducts. Finally, you should know that the EPA does not recommend that air ducts be cleaned except on an as-needed basis. This is because of a continuing lack of evidence showing a health benefit of duct cleaning in most cases.