8.16   Duct Insulation

Insulate supply ducts that are installed in unconditioned areas outside the thermal boundary such as crawl spaces, attics, and attached garages with vinyl- or foil-faced duct insulation. Don’t insulate ducts that run through conditioned areas unless they cause overheating in winter or condensation in summer. Use these best practices for installing insulation.

üAlways perform necessary duct sealing before insulating ducts.

üDuct-insulation R-value must be ≥R-8.

üInsulation should cover all exposed supply ducts, with no significant areas of bare duct left uninsulated.

üInsulation’s compressed thickness must be more than 75% of its uncompressed thickness. Don’t compress duct insu­lation excessively at corner bends.

üFasten insulation using mechanical means such as stick pins, twine, staples, or plastic straps.

ü       Cover the insulation’s joints with tape to stop air convec­tion. However, tape often falls off if the installer expects tape to support the insulation’s weight.

ü       Install the duct insulation 3 inches away from heat-pro­ducing devices such as recessed light fixtures.

Important Note: Tape can be effective for covering joints in the insulation to prevent air convection, but the tape fails when expected to resist the force of the insulation’s compression or weight. Outward-clinch staples can help hold the insulation fac­ing and tape together.