Ducts running through unconditioned areas - attics, basements, garages... - should be properly insulated to prevent energy loss.
Keep in mind: duct insulation doesn't stop air leaks. Do not insulate your ducts without air sealing them first.
Always begin with the ducts running through unconditioned spaces: attic, unheated basements and crawlspaces... Only then you should insulate other parts of your ductwork.
Insulation should involve the entire ducts, boots and plenums included.
Typical installation involves wrapping the ducts with duct fiberglass insulation or another equivalent material with an insulation R-value of at least 6* (prefer R-8 or more**).
* Metric System; R-1; ** Metric System: R-1.5
Pay attention to thickness. In some cases, a vapor retarder facing the outer side of ducts can be advantageous.
Pay special attention to duct connections and boots, elbows and plenums; the covering should not involve gaps and voids; they can compromise the effectiveness of the insulation. Duct insulation may provide huge energy savings in harsh climates, whenever ducts are installed in unconditioned spaces.
In conditioned spaces, insulation may not be advantageous enough.
Consider an Insulation R-value of 6, preferably 8, for duct systems in unconditioned spaces. The 2000 International Energy Conservation Code establishes the standards for duct insulation.
Overlap the insulation seems: tape the seams with vinyl tape or aluminum foil; the end of each piece of tape should point downward.
The insulation should be firmly secured to the ducts. It should not depend entirely on tape to be hold in place; secure it with wire, plastic cable ties, twine...
The attachments should not compress the insulation more than 20%, except on the bends, where higher compression is inevitable. Excessive compression will cause under-performance.
Use of vapor barriers
In climates where condensation is a problem, if you have metal ducts and cooling equipment, consider the installation of a vapor barrier all over the insulation layer, without tears, voids and gaps. You may use foil-faced fiberglass.
Professional vs. DIY Insulation
Ducts are often installed in locations that are hard to access; in this case, only experienced professionals have the means and the know-how to ensure a quality result and an appropriate selection of materials.
Also do not forget that the insulation of ducts in basements and crawl spaces will make them colder, which may cause water pipes and drains to freeze, in harsh-cold climates.