duct insulation

Wherever heating ducts run through unconditioned spaces - attics, basements, garages... - they can waste as much or more heat or cold than that they receive. Insulation is, for this reason, crucial. But be aware: insulation doesn't stop air leaks...

Insulation and air sealing

Do not apply insulation without a prior and proper air sealing of the ducts. Insulation around ducts is often dirty, in a clear sign that the ducts are leaky (and proving that insulation doesn't prevent air leakage).

When to insulate

Duct insulation may provide huge energy savings in harsh climates, whenever ducts are installed in unconditioned spaces. In other cases, insulation may not be truly advantageous. Duct Sealing and Insulation

Do not forget to properly seal the ducts, before applying any insulation to them.

Always begin with the ducts running through unconditioned spaces: the attic, unheated basements and crawlspaces... Then, if you climate is harsh, consider also sealing the ducts in cooled and heated living spaces…

See: Ducts sealing
Image credit: DOE

Complete coverage

Insulation shouldn't be partial. In unconditioned spaces, they should involve the entire ductwork, boots and plenums included.

Wrapping the ducts

The commonest solution involves wrapping the ducts with flexible and rigid fiberglass insulation (duct insulation should have an insulation R-value of at least 6). In some cases, a vapor retarder facing the outer side of ducts is also necessary.

Pay specially attention to the connections and to the duct boots, elbows and plenums; wrapping should be properly done, without gaps or voids; they would compromise the effectiveness of the insulation.

Duct Joints Overlapping

The duct joints should overlap by at least 2 inches at the meeting of insulation sections. These joints should also be tightly sealed.

Securing insulation to the ducts

The insulation should be firmly secured to the ducts.

See: Duct fasteners

Don’t compress the insulation materials

When securing the insulation, do not compress it too much. The attachments should not compress the insulation more than 20%, except in bends, where a higher compression is inevitable. Excessive compression are a cause of reduced performance.

Use of vapor barriers

In climates where condensation is a problem, if using metal ducts and cooling equipment, consider the installation of a vapor barrier all over the insulation layer, without tears, voids and gaps. Foil-faced fiberglass is a common solution in these cases.

The vapor barrier will avoid moisture problems and will ensure a more effective insulation.

Duct terminations

When applying flexible duct boots, collars and other terminations, pay attention to the insulation jackets. They should be properly pulled over the insulation and secured and sealed properly to the fitting.

Insulation levels

Consider an insulation R-value of 6 for duct systems in unconditioned spaces (or a little more for variable-speed air handling equipment). The 2000 International Energy Conservation Code establishes the standards for duct insulation.