duct insulation

Duct insulation is important whenever the ducts are installed in unconditioned spaces. Be aware, however: insulation doesn't stop air leaks. A clear proof is in the insulation applied to ducts without previous or proper air sealing: the insulation is often dirty, in a clear sign that the ducts are leaky and that insulation doesn't prevent air leakage...

When to insulate

If the ducts are installed inside conditioned living space (or in semi-conditioned spaces) of the house, they will need no insulation or will only demand a minimal insulation. Take into account your climate, and the likely impact of heating and cooling losses (due to the lack of insulation) to decide whether to insulate your ducts or not.

Before insulating

Before insulating, you should seal your ducts… Begin with those 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

Broad view about duct insulation

You should insulate the entire ductwork (boots and plenums included) in unconditioned spaces. 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.

Continuous application

The duct insulation should not involve gaps or voids. That is particularly important in connections and at duct boots, elbows and plenums.

Overlapping

The duct joints where the sections of insulation meet, should overlap by at least 2 inches. 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, to avoid reducing its performance. The attachments should not compress the insulation more than 20%, except in bends, where a higher compression is natural.

Use of vapor barriers

In climates where condensation is a problem, if you are 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.

This solution will avoid moisture problems, and ensures a higher efficiency of the insulation.

Duct terminations

In flexible duct boots, collars and other terminations pay attention to some crucial details. The insulation jackets should be properly pulled over the insulation and secured and sealed properly to the fitting.

Insulation levels

Contractors use to apply 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.