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Community and Q&A

Rockwool for Crawlspace Insulation

JWWine | Posted in Green Building Techniques on

I’m planning to insulate my 1905 bungalow on the Central Coast of California (Zone 3). At this point for cost purpose I’m going to limit the insulation to the attic and crawlspace. A green builder in town recommended blowing in cellulose in the ceiling and then to use Rockwool and Mento for air ceiling in the crawlspace. Another builder in town said the Rockwool would end up hanging down by force of gravity in a couple years in the crawlspace. Is this true? He’s advocating for R-19 fiberglass.


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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Jeremy,

    According to Martin Holladay, a crawlspace is not the place for Rockwool bacause of the air-permeance of mineral wool. He says the material can’t prevent humid interior air from contacting the cold crawlspace walls, the likely result being moisture accumulation and mold. Fiberglass batts present problems too. Rigid foam would be a better option in the crawlspace. Read why here: What’s the Best Way to Insulate Crawlspace Walls?

    1. JWWine | | #2

      Thanks, Kiley. I think humidity is one reason why the green builder in town had recommend Rockwool. My town is very dry year-round. It rarely goes higher than 20% relative humidity, except when it rains, of course.

  2. Expert Member
    BILL WICHERS | | #3

    If you're putting the rock wool in the ceiling of a crawl space, it will stay in place much better than fiberglass. I wouldn't use it on the walls though -- I'd use rigid foam for that, preferably polyiso.

    I wouldn't use fiberglass anywhere in a crawlspace.


    1. JWWine | | #5

      Thanks Bill! What's the objection to fiberglass? Would you put something down on the bare ground?

      1. Expert Member
        BILL WICHERS | | #7

        I like mineral wool better, and it tends to hold in place better since it's more rigid compared to fiberglass. Fiberglass will work if you install it correctly though -- but remember that the facer on faced batts installed under the floor over a crawlspace should be on the TOP (warm) side. I see it installed upside down all the time.

        If you want to insulate the bare ground, you should be using some type of rigid board type insulation. My preference would be EPS here, or rigid mineral wool (Comfortboard) if you're feeling like spending a lot of money. XPS would be next best. Polyiso isn't an option on the ground due to moisture absorption issues.


  3. dan_saa | | #4

    If you are not concerned about an air barrier, then you can use either. Support insulation with staves, wire, or netting.

    1. JWWine | | #6

      Hi Daniel,
      I'm actually thinking that it'd be a good idea to have an air barrier, maybe both on the bare ground itself and up the walls? What do you think?


      1. dan_saa | | #8


        I think a vapor barrier on the ground only makes sense if you are doing an encapsulated crawlspace which has lots more work involved for ventilation, insulation, emergency overflow drainage if a pipe leaks, etc. For mild central coast climate the air barrier may not be worth it. (My work is in LA where air barriers seem to be unknown.) For a typical ventilated crawlspace insulate the floor assembly per the Building America example. If your builder has plans for Mento and Rockwool under the floor framing and the price is good, I'm sure they have a method to keep the insulation in place and will give you better performance than fiberglass insulation.


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