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Community and Q&A

Double fiberglass vs. mineral wool

bobbyarndt | Posted in Energy Efficiency and Durability on

having a hard time making a choice on cavity insulation for my 2×6 walls.

I really want to go with Mineral Wool due to the increase in r-value, quality of the product, and ease of getting a class 1 install but at about twice the price of fiberglass I am having a hard time justifying it. (1.01/sqft vs .46/Swift)

For twice the price, wouldn’t it make more sense just double up the fiberglass in the cavity?  It seems to me by doubling up, even with compression, you would be gaining more than R-3 and could gain the other benifits of a more dense cavity insulation?

am I crazy for thinking this is a better bang for my buck that filling the cavity with mineral wool batts?

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  1. user-6863358 | | #1

    With 2 x 6 wall cavities to fill, both rockwool and fiberglass batts are able to deliver the same R value in that cavity depth, but your location and the stud spacing may dictate the availability of one or the other.

    Each of those products is available with a range of R values to fit 2 x 6 depths without compression. Each has a maximum R value batt of R23 in their published data. The R23 product from fiberglass producers is considered a high density batt product and fabricates (therefore installs) much more easily than do light density fiberglass products from a finished quality perspective. So, using a high density R23 fiberglass product would address your concerns and avoids having to "double up". I have used each product many times. With cost always being a factor, just make sure that you are accessing all available information to assist with your decision.

    1. Expert Member
      Dana Dorsett | | #3

      >"So, using a high density R23 fiberglass product ..."

      I've never seen a fiberglass R23 (lots of R21s though). Who makes it?

  2. Expert Member
    Dana Dorsett | | #2

    What R-value are you looking at for the fiberglass?

    R19 fiberglass only performs at R18 when compressed to 5.5".

    R20 fiberglass performs at R20.

    R21 high density fiberglass performs at R21, and has roughly the same air-retardency of R23 rock wool in a 2x6 cavity.

    It''s fine to compress R19s down to 2.75", and each would yield something just shy of R11, for a total of R21-R22. But a pair of compressed R13s would also deliver R21-R22, and might be even cheaper. (An R19 batt is really just a "fluffed up" R13- they're the same weight per square foot.)

    I'm not totally sure what to expect out of mid-density R20s compressed to 2.75", but it might be R12-ish. An R15 compressed into a 2.5" deep 2x3 cavity yields R11, so at 1/4" thicker it would be R12-ish. See:

  3. bobbyarndt | | #4

    Im in Minnesota so code here is r-21 and I was thinking about doubling that up but with your info it seems like it wouldn’t be worth the effort for maybe a few additional r value.

    If R-21 fiberglass has similar physical & insulation properties of r-23 mineral wool, how does anyone justify the price difference of mineral wool? Paying twice as much to be mold resistant?

    1. Expert Member
      Dana Dorsett | | #6

      >"If R-21 fiberglass has similar physical & insulation properties of r-23 mineral wool, how does anyone justify the price difference of mineral wool? Paying twice as much to be mold resistant?"

      The difference in FIRE resistance between fiberglass and rock wool is pretty significant. Some contractors in my area use it for insulating kitchens & boiler rooms even when the rest of the house is insulated with fiberglass.

      There is also a difference in the the amount of fiber shards that can potentially end up in the air. Building air-tight assemblies mitigates against chronic airborne glass particulates though, it's rarely a problem in tight houses.

      I thought MN code required R20 + R5 continous insulation (or R13 + R10 c.i.) , not R21, which is what it would be if following the IRC prescriptives from 2012 and later:

      But it looks like that's been relaxed to R20 in zone 6 (southern MN) and R21 in the zone 7 (northern):

      (That's substantially lower performance than the IRC calls out.)

  4. irene3 | | #5

    Mineral wool is much nicer to work with, and more fire resistant as well (can be used as a fire stop). It's more sound-deadening, if you care about that. Somewhat less attractive to rodents, supposedly, though I'm told they have been known to nest in it. When I was insulating the upper walls of my basement I was buying a pack at a time retail and the price difference didn't seem enough to worry about, but of course I was not doing a whole house.

  5. Expert Member
    BILL WICHERS | | #7

    If mineral wool is what you really want to use, and it’s twice the price of fiberglass, then it seems a little silly to spend twice as much on fiberglass... why not just buy the mineral wool you really want? You’ll get a little better insulating performance, MUCH better fire resistance, a little better sound deadening (mineral wool has more mass), less tendency for nesting critters to burrow in it (one of my big reasons for “upgrading” to mineral wool in my own house), and an easier time getting a good installation.

    I’d go with the mineral wool. You only get one chance at that. It will be a lot easier to install the mineral wool than it will be to install doubled fiberglass so don’t forget to think about your labor.


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