GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Uninsulated Return Ducts in Crawlspace

sagarun | Posted in Energy Efficiency and Durability on

I have two return ducts in the crawlspace. They are huge and none of them is insulated. Do i need to insulate them? Will improve the efficiency of the system? Is this something can be done DIY? I do have some R4 insulation, i was going to tape them around the ducts. The box is uninsulated as well, it will be hard to insulate. I did see some small nail holes on the ducts and some part of the duct is very close to the ground on top of rocks.

It is a vented crawlspace. I am in northern California not sure what zone it is in.  The crawlspace is not encapsulated.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Kiley Jacques | | #1

    I will pose a few questions to give yours a bump. What is your climate zone? Is the crawlspace within the conditioned envelope? Is it vented?

    1. sagarun | | #2

      Thank you. It is a vented crawlspace. I am in northern California not sure what zone it is in. The crawlspace is not encapsulated.

      1. Expert Member
        Dana Dorsett | | #4

        >"It is a vented crawlspace. I am in northern California not sure what zone it is in. The crawlspace is not encapsulated."

        Are the ducts well sealed with duct mastic on EVERY seam and joint?

        Are the register boots air sealed well to the subfloor?

        The location "northern California" covers climate zones from 3C @ 4C to 4B & 5B, and the location matters in terms of what the best or most cost effective solution would be.

        Find yourself on this map:

        Since it's probably impractical to retrofit insulate the ducts adequately, it's likely to be easier to put down a heavy ground vapor barrier (10 mil polyethylene or EPDM) sealed to the foundation wall, seal up the vents then air seal & insulate the crawlspace walls.

        In zone 3 (A, B, or C) an inch of closed cell spray foam or 3/4" of foil faced polyiso foam board (R5) would be enough to meet the IRC code prescriptive.

        In zone 4B it would take 1.5"-2" (R10 minimum) of polyiso or closed cell spray polyurethane.

        In 4C @ 5B it would take 2.5-3".

        See TABLE N1102.1.2:

        Verifying the duct return paths is also a good idea. With unbalanced ducts with inadequate returns the air handler would pressurized doored-off rooms relative to the outdoors when the doors are closed, forcing some of the return air path to be "The Great Outdoors". It also amplifies the (energy and indoor air quality) negative effects of duct leakage even when the leakage is all inside the nominal pressure & insulation boundary of the house.

        Are the SUPPLY ducts all fully inside the pressure & insulation boundary of the house?

  2. Brian Wiley | | #3

    As Kiley mentioned, knowing the climate zone (which you can look up by zip code) would be helpful. Pictures would be helpful as well if you happen to have some.

    In any case, sealing the ducts with mastic if they haven’t been already would be a great first step. It’s diy-friendly, albeit messy; don’t be tempted to use tape. If your crawlspace is anything like mine was, the dust would render tape ineffective in truly sealing things up.

    After that’s done, any insulation should help, but if you’re going to be doing the labor you may want to bump it up to something higher than r-4 where you can. This article by Dr. Bailes gives a good overview of the increased loads when you decrease the amount of insulation.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |