Consider a source duct from the furnace/AC. The ducts run in a vented crawlspace. The floors atop the crawlspace are a mix of hardwood and kitchen vinyl.

The symptoms are:

  • the longer run vents aren't blowing as hot as the short run vents. They've just been foil-taped to ensure no leakage.
  • the furnace runs longer to heat the house; the thermostat is in the room with the longest duct vents.

Does it make sense, or is it recommended to insulate these metal ducts? Currently considering these products:

  • Reflectix foil wrap - reflective bubble wrap. Claiming an R-value of 4. Unsure if this R-value is worth the time/cost!
  • fiberglass batt with foil backing

House is in a climate that sees -15 C through 35 C. That's 5 F through 95 F.

  • When should metal ducts in a crawlspace be insulated?
  • What products are best suited to insulate ducts?
  • What R-value insulation should be considered satisfactory?

1 Answer 1


Any time you have heating ductwork in an unheated area, it is very wise to insulate it. There are several products, but I tend to go with the high density, foil backed fiberglass wrap most of the time. The exact type differs depending on the shape of the ductwork. (round vs rectangular) I always seal any seams in the insulation with foil tape. To solve the uneven heating problem, you will need dampers in the shorter runs. By adjusting the dampers, you can balance the amount of hot air delivered to each register.

  • thanks! Any comments on the claimed R-value of the Reflectix product? 4 seems low to me. This duct system is of the round variety.
    – p.campbell
    Feb 16, 2011 at 16:20
  • 2
    This insulation is only about 2 inches thick, so R-4 is probably right. I'll check my HVAC references and post it as soon as I can. Feb 16, 2011 at 23:45

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