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Blown cellulose or blown mineral wool?

dug8498 | Posted in General Questions on

I’m in the process of adding some more insulation to my attic. It’s about 700 sq ft, 2×6 construction with 2 layers of fiberglass batts. I live in Zone 5-6, southern NH. So far I’ve lifted up all batts and have sealed up holes around wiring and fixtures with great stuff foam. I’ve also installed vent extenders over every soffit opening.  As far as I can tell my attic is very dry and well ventilated. I saw no evidence of water damage when up there. I was planning on blowing in cellulose insulation with my buddy and then I got carried away reading negative things about this material. Now I am concerned that maybe this isn’t a good choice for blowing in on top of existing batts as I’ve read concerning things about moisture absorption etc.. Is this a warranted concern or not?

I’ve also looked into blown mineral wool but it’s about twice as expensive as the blown cellulose. I’m also not sure that I can use the same rental blowing machines to blow mineral wool which would be an issue.

Any advice would be greatly appreciated. I’m willing to spend more money if it’s worth it, but I also know that I may be over complicating a simple thing here.

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  1. Expert Member
    Dana Dorsett | | #1

    Cellulose is fine, and somewhat more air-retardent than blown rock wool. If you specify "borate only, sulfate-free" cellulose it won't stink or become corrosive when wet the way cellulose treated with ammonium sulfate as a fire retardent can be.

    Borate fire retardents are even toxic to the gut flora of wood boring insects needed to digest the wood, mitigating against potential insect damage.

    Rock wool is completely fireproof and won't melt even if the house burns down (the fiberglass can and will melt in a house fire) but I personally wouldn't pay extra for rock wool in an open-blown attic application. YMMV. It takes a LOT of sustained heat to light off cellulose and keep it going.

  2. charlie_sullivan | | #2

    Cellulose great for that application: cheap, easy to install, and problem free. If you have a roof leak, your insulation will get wet, but otherwise I can't see it having moisture problems. If you point us to what you read, we might be better able to debunk it.

  3. dug8498 | | #3

    Thanks Dana and Charlie,

    I can go back and find those articles later! I'm unfortunately not near a computer right now. They were mainly subjective experience. I had also read people posting concerns about "what if the roof leaks" etc.. I had also read that some people felt that this material will degrade too quickly compared to some other options.

    Anyway, I feel better at this point. I respect what you both have to say and will proceed with this option as it is substantially cheaper and way easier for me to acquire the materials. I was either going to purchase a pallet of this:

    or this as there's a place nearby that has it in stock:

    It seems like either of these would work based on your recommendations of being borate treated.

    Last question if youd dont mind: I have a single batt layer running in between the 2x6's across the attic floor. those all seem to be in good shape. Then I have another layer running perpendicularly to the 2x6's. Most of that is in pretty good shape. Should I keep that second layer or remove it? If i keep it, it would bring blown insulation pretty close to the roof on the edges I think as my attic is not very tall. I am also concerned about navigating the attic when I blow in the cellulose if that second layer of fiberglass is on top and blocking my view of the 2x6's. I haven't fallen through the ceiling yet and really dont want to!

    1. Trevor_Lambert | | #4

      I don't think there's much (if any) performance benefit to removing the batts. As for not being able to navigate the attic without seeing the 2x6's, that's a legitimate concern, but only you can decide how critical it is for you.

      1. dug8498 | | #5

        Thanks Trevor. I think I am going to remove the second layer as some of it looks like it has been chewed up by critters and I am worried about navigating the attic when blowing in the cellulose in. I also think only having one layer of the fiberlass will make the attic substantially easier to access if I ever need to in the future.

        I can still get the attic to R60 with the pallet of cellulose I've purchased

  4. Expert Member
    Peter Engle | | #6

    You could just remove the fiberglass in a strip along the center and install a catwalk over it. You can blow the cellulose over the catwalk as you go.

    Or, build an elevated catwalk and blow cellulose under it and everywhere else, leaving the catwalk for future access.

    If you haven't done so already, make sure to insulate and air seal the attic access. An uninsulated access hatch defeats the purpose of just about everything you're doing.

    1. dug8498 | | #7

      Peter good ideas thank you! Its a little tricky to put much up there as the hatch is so small, but I could definitely bring some 1-2 ft width boards up there and nail them in.

      Yes the previous owner of the hatch put a section of foam board insulation about 6-8 inches thick on the backside. I just need to put some foam stripping around the trim which the hatch rests on so that the connection is nice and snug/airtight .

      Any other suggestions would be greatly appreciated !

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