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Community and Q&A

Fiberglass vs. Rockwool Batts

mpsterner | Posted in Energy Efficiency and Durability on

Is there any advantage to spending more on Rockwool batts if I already have thick exterior insulation?

Zone 7 Northern Wisconsin new home build…

I have 2×6 walls with 5″ of Rockwool comfortboard exterior insulation, rainscreen strapping, cladding.

I am running way over budget on my project and wondering if I can opt for fiberglass batts in the 2×6 wall or if there is really any advantage to Rockwool batts or dense-pack cellulose given that I have a 5″ blanket of exterior insulation?

I understand there are benefits to Rockwool and dense-pack from a windwashing perspective and in terms of getting a tight-fitting batt, but isn’t all that pretty marginal once I have a solid surround of exterior insulation?

Wondering if it is still worth the extra cost or if this is a good place to save some money?

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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Michael,

    There are some arguments in favor of the extra cost made in this Q&A thread in response to a similar question—the main being Rockwool’s superior energy efficiency, fire-resistance, and promise of thermal comfort. There is also plenty of support for cost-saving measures. Personally, I favor this opinion: “If the savings of going to r21 fiberglass allow a budget for better air sealing measures, take that option every time.”

  2. Expert Member
    BILL WICHERS | | #2

    The main advantage to mineral wool with a DIY install is that it's easier to install well, since it is more dense and tends to hold its shape. Mineral wool is better for fire resistance too, but to really take advantage of that you need to do more than just install mineral wool so I wouldn't consider the fire resistance a "primary" penefit.

    In terms of plain on' insulating performance, all else being equal, R value is R value. R15 worth of fiberglass installed well will perform the same as R15 worth of mineral wool installed well. The "installed well" part is the key. If you're willing to spend the time to do a good job of installing the stuff, I think using fiberglass is a good way to save some money. Using high-density fiberglass batts will help a little (a bit more R value, and a bit easier to work with), but they may cost enough more to offer little savings over mineral wool. If there were no savings, I'd just go with mineral wool.

    Personally I prefer to work with mineral wool so I pay the extra money to get it instead of fiberglass for walls. For attic use, I don't think it matters as much. I have a small project right now in my own home where I'm using fiberglass batts and rolls to better insulate a small part of attic floor over a room I've been renovating. Every "open the wall" project I've done has been done with mineral wool though.


  3. mpsterner | | #3

    Thank you both! I do appreciate and understand the stated advantages of mineral wool. I also like it better.

    I think the primary difference here is that I already have a highly detailed and continuous air barrier and 5” of rigid mineral wool on the outside of the entire building. So, is a “great installation” still even all that important? There will be nearly no thermal bridging, any minor gaps will be covered by my R20 of exterior insulation, etc.

    I don’t even have any electrical or services in my 2x6 wall because all of that is in an interior service cavity in board of a second interior air and vapor barrier.

    It seems like performance difference would be negligible all things considered and if I go with mineral wool it should be because I like installing that better.

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