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Community and Q&A

Mineral Wool vs. Cellulose Insulation

GibsonGuy | Posted in Energy Efficiency and Durability on

I am building a house in CZ6.  I have an r 70 roof system and walls with 4” of recycled polyiso sheets.  The sheets are 2” thick with staggered seams that are foamed. We installed Enerlux triple glazed windows.  The house is wrapped with Resisto Red Zone, a self adhered membrane.

The house is framed 2 x 6, 24” o.c. and I have not decided what type of insulation to use.  I carefully blew in cellulose into the parallel roof trusses, but I was told the free rental machine cannot dense pack.  The highly recommended cellulose professional in my area stated that the 24” o.c.  stud bays are a PIA too fill as they install they apply netting after they blow in the walls. Therefore, I’m considering mineral wall.  The only Rockwool that is available in my area is 16” and for 4” stud bays and I initially had hesitated to use mineral wool because of wiring, plumbing..,

My only experience with mineral wool was doing a basement ceiling where we used it for sound proofing.  On that job that supplier ran out of Rockwool and we had to switch to another brand.  I remember feeling that the other brand was inferior in that it didn’t hold together as well.  This was about four years ago.

So I’m looking for advice on cellulose vs. rockwool and possibly going with high density fiberglass. Thank you in advance.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    With your 4" of polyiso exterior insulation, the walls are already insulated above code. Anything more is a bonus. I would use standard density unfaced FG batts. They are easy to get in 24"x6" sizes, inexpensive and simple to install well. The difference in insulation value between these and dense-pack cellulose or rockwool won't justify any price difference. I tend to like Knauff Ecobatt as it contains more recycled content and less formaldehyde adhesive that others.

  2. GibsonGuy | | #2

    Good info, thank you.

  3. user-5946022 | | #3

    Spray applied damp cellulose
    Should cost very similar to fiberglass

  4. user-7124595 | | #4

    Give some thought to strapping the interior walls horizontally on 1'-0" centers. I have used this method over the years when dense packing. Let the sub put up the netting first, then give you a day or two to strap before they come back. Insulator and drywall sub will love that the 3/4" space allows them to pack solid and not worry about bulging cellulose, and you will get a first class install

  5. GBA Editor
    Kiley Jacques | | #5

    "Cellulose is far greener." You might want to read this Q&A discussion
    comparing mineral wool to cellulose. As I am sure you realize, there are a number variables to consider, and your ultimate selection will depend on your priorities.

  6. Expert Member
    BILL WICHERS | | #6

    Mineral wool is easier to install than fiberglass in most cases since it self supports better. All you have to do to go around wiring and plumbing (ideally you won't have any plumbing in your exterior walls though) is to split the batt, then install one part behind and one in front of the wire or pipe. Electrical boxes are just a notch you cut in the side of the batt, remembering to put a little bit behind the box too. It goes reasonably quick. There is a bread knife that I have linked to on GBA before that I use myself that does a great job cutting these batts.

    What I've done in the past when I have oddball width stud bays is to run the batts "sideways", horizontally instead of vertically. This makes for a lot more cutting, but you can get whatever width you need. I've found that the mineral wool typically stays in place even if I have two pieces next to each other in a stud bay.

    Rockwool tends to be tops in quality in my experience. I've used a lot of Owens Corning too, which works fine but is dustier. I haven't had any problems with either brand in terms of the material holding together though. When I last checked, Rockwool is currently cheaper than Owens Corning, which is the opposite of what it was a few years ago.


  7. GibsonGuy | | #7

    To all of the above posts: Thank you for your feedback, I have a lot to think about. I'm not a big fan of FB. I am a non builder, but have done most everything myself and for the most part willing to do labor intensive work. i.e. Recycled 1 1/2" polyiso for site built baffles which leaves a 2" continuous air flow. On the interior face, I use 3/4" foil faced polyiso with foil tape and 2 x 4's for furring strips. I feel I have excellent primary and secondary air sealing for the ceiling. Labor in my area has skyrocketed and getting subs in all facets it difficult.

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