Green Building Library
Weatherization Tips

How to Insulate Metal Ducts

  • Insulate all heating and cooling ducts in attics, crawlspaces, garages and other unheated spaces. This means supply ducts, return ducts, furnace plenums and register boots.  Ducts inside heated spaces don't need to be insulated.
  • Before insulating ducts:
    • Support and repair any ducts that sag or have become disconnected.
    • Seal all air leaks with duct mastic.
  • In attics and under floors, ducts may be covered with enough cavity insulation that the ducts don't need more.

Duct Wrap

Duct wrap comes in several thicknesses, and it may have a vinyl or foil facing.  Because it comes in wide rolls, it can quickly be cut to the proper width.  Duct wrap is fast and easy to install.

  • Use unfaced insulation in crawlspaces and attics.
  • Use faced insulation in areas, such as basements and garages, where people will routinely be exposed to it.  Also use vinyl-faced insulation on ducts that will carry air cooled by an air-conditioner or heat pump.  Be sure that the facing has the proper flame spread rating.


Step 1

Measure Duct

To measure the width to cut, wrap twine 1 1/2 times around the duct.  Tie knots to mark the width. .


Step 2

Cut insulation

Use the twine to measure the correct width on the roll. Mark the spot with a pen or utility knife. To hold the roll together, tie twine around the roll on the leftover side of the cut mark. Cut the insulation with a hand saw.


Step 3

Wrap duct
Start at the registers and work back toward the trunk.  Lay the insulation along the duct.  Wrap one end around the duct or the register boot and tie it in place.


Step 4

Secure insulation

Secure the insulation with twine or wire every 18 to 24 inches.  Don't compress the insulation more than 1/2 inch.  Tie the twine each time, instead of wrapping it in a spiral around the insulation.  Slice the insulation to fit around each duct support wire or strap.  Repair the slit with the tape.


Spiral Wrap

In some cases, it may be easier to wrap the insulation around the duct in a spiral.  This may be true for register boots and small ducts.  Overlap each turn by 3 inches.  Secure with twine or wire to keep seams closed but do not compress it more than 1/2 inch. Tie with twine every 18 to 24 inches.


Ducts in Floor Cavities

Ducts in Floor Cavities

Ducts that run between floor joists are easy to insulate.  Place insulation on both sides of the duct.  Then attach a separate insulation blanket to the bottom of the floor joists with twine or lath.  Insulation should touch the duct, leaving no air gaps.

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© Copyright 1993 Iris Communications, Inc.


Click any image below to view details.

Measure Duct

Step 1: Measure


Cut insulation

Step 2: Cut


Wrap duct

Step 3: Wrap


Secure insulation

Step 4: Secure