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Do I need a vapor barrier with loose-fill cellulose insulation in my attic?

rjl1 | Posted in General Questions on

I am replacing insulation in my attic, in a 100-year old house.  The old insulation was removed because of nesting of squirrels and because it was many decades old.  The attic has no soffit vents.
I have no background in construction and am getting very different proposals and answers from insulation contractors, so I’m hoping GBA can help.
1) If I use cellulose insulation, is a vapor barrier needed?
2) I have seen no evidence of moisture problems in the attic (though I haven’t monitored carefully for it).  How important is air-sealing around lighting fixtures? How important is a cover for a hatch that uses a drop-down stairway?
3) There is an air handler in the attic.  The ductwork is insulated, but a contractor proposes that the duct system be encapsulated with  polyurethane foam?  How important is that?
4) If loose-fill insulation is installed, how will electricians and other service people access, e.g., wiring, that could need repair in the future?

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  1. user-2310254 | | #1

    Please tell us where you are located. Climate zone matters.

    1. In most climates, you want to avoid vapor barriers. In some climates, you need a vapor retarder. For more info, see

    2. Air sealing is critical. For more info, see

    3. Ducts in the attic are common but not really recommended. Consider converting your attic to a conditioned space if possible. For more info on conditioning the attic, see For more info on ducts in a ventilated attic, see

    4. If you have old wiring (knob and tube, for example), it shouldn't be buried. For more info, see

  2. rjl1 | | #2

    Thanks for the quick response and for pointing me to these articles. I've read them, but a lot of the content involves construction expertise that I don't have.
    Regarding question #1, I am located in climate zone 5. I did some more reading on the web and the Cellulose Insulation Manufacturers Association recommends against vapor barriers when using cellulose insulation (Cellulose Insulation Manufacturers Association ( I read the GBA article about vapor retarders but was unable to determine whether I need a vapor retarder. (The attic is unvented and unconditioned.)
    Re #3, I'm not in a position to condition the attic--it's shared in a side-by-side duplex and thus decisions about spending are shared. What is a second best solution? Does encapsulating the duct work with polyurethane foam make sense?
    A contractor has recommended NOT burying the duct work in cellulose fill because the potential need for access for servicing. Instead he proposes using bats next to the air handler and duct work. I read the GBA article on Burying Ducts in Attic Insulation, but didn't see anything that mentioned service access.
    Re #4, I don't have knob and tube wiring and I've read the article on insulating with cellulose in attics with electrical wires. My question is, I think, very basic. How are electrical or other functions/features that may need servicing in the future located or accessed if they are buried in cellulose fill? In related vein, is it typical to build some sort of dam to contain the cellulose fill? I've gotten different responses from various contractors..
    Thanks again for the guidance.

  3. Expert Member
    BILL WICHERS | | #3

    Just because you don’t have soffit vents doesn’t mean your attic isn’t vented. Do you have cable vents? Roof vents? A ridge vent (this one really needs soffit vents to work properly though)? If you have a vented attic, a vapor barrier or retarder is not normally needed on the ceiling under the attic, but air sealing is still very important.

    I wouldn’t worry about burying the ductwork in insulation being a problem for future service work. The same goes for wiring. Neither of those is likely to have much in the way of service work done, and you can always dig them out if needed. I’ve never cell seen ducts encapsulated with spray foam the way I think you’ve been advised. Usually you either bury ducts in blown insulation, use the “sock” type wrap/tube insulation, or both.

    I would NOT bury an air handler in loose fill (blown) insulation. This device DOES need to be accessible for service work.

    Converting the attic to a conditioned attic has many advantages if you have mechanicals (ducts, air handlers) up there, but it’s a big project.


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