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Community and Q&A

Weighing the Cost of Rockwool

wiscoguy | Posted in General Questions on

So I have roughly r14 on my exterior walls through exterior insulation. My plan was to use rockwool in the wall cavities.  I’ve always felt it’s a superior product for insulating but I’m wondering if at the added cost it’s really worth it if I’m already using exterior insulation  and a really good wrb. The piece is about 40% more in my area. It has a very modest gain in r value but I do like some of the other properties. Also add to the fact next to no one in my area wants to do it so that leaves me too which pushes my schedule back further.

Most of my quotes have been around 10k for fiberglass and 16k for rockwool.

The question is for wall cavity insulation is rockwool justifiable at a 50% or more markup. Especially with the added benefit of all the exterior insulation.

I’m having a hard time justifying the increase in cost any thoughts here are appreciated.

Thanks, Tom

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  1. Andrew_C | | #1

    Rockwool won’t get eaten by ants and it’s more resistant to fire than foam. More drying to the outside is rarely a bad thing. For one time cost, this is a place to spend, imo.
    Note that with rigid foam insulation the drainage plane is usually on the outside of the foam. With rockwool, the drainage plane is normally at the outside of the sheathing. This may influence window placement (innies vs outies) and flashing, so switching from one exterior insulation to another is not a no-brainer. There are important details that need to be thought through in advance.

    1. wiscoguy | | #3

      I was talking about in the walls rather than outie insulation. Sorry I will fix the main post.

      1. Andrew_C | | #8

        Sorry, the title threw me off and I didn't read carefully enough to question it.

  2. JC72 | | #2

    Depends on the quality of the installer. If you can hit grade 1 install with fiberglass batts then pass on the rockwool.

  3. Expert Member
    Michael Maines | | #4

    With enough exterior insulation to prevent condensation, and an airtight layer at the sheathing, and a reasonably airtight layer at the interior, I don't think the choice of insulation makes a big difference. I usually go with cellulose for its low-carbon aspect, but with mineral wool and fiberglass having similar levels of embodied carbon, I don't see a reason to prioritize mineral wool.

    1. wiscoguy | | #6


  4. Expert Member
    BILL WICHERS | | #5

    I much prefer working with mineral wool since it holds its shape, which makes it easier to get a good installation. If you have a good installer though, there isn’t as much difference since R is R when everything is installed equally well. Mineral wool is more fire resistant, which is a plus, but it’s not going to make your home fireproof. Using mineral wool in a shared garage wall is probably the only place where the extra fire resistant is potentially a real benefit.

    Note that you can get “high density” fiberglass batts that are R15 in a 2x4 wall too, so if you go that route there is even less difference between fiberglass and mineral wool. Sometimes the high density fiberglass batts are priced between “regular” fiberglass and mineral wool, so this might be a way to get some extra performance without all of the extra cost — a sort of middle option for you.


    1. wiscoguy | | #7

      Thanks for the ideas. It’s a 2x6 wall though. I was looking at like denim insuLtion as well but the price is the issue. I suppose I could save quite a bit of money doing it myself as well.

      1. charlie_sullivan | | #9

        You might get a higher quality install doing it yourself.

        The higher density will be easier to install well, as well as having a slightly higher R-value for the same installation quality.

      2. Expert Member
        BILL WICHERS | | #10

        It’s not difficult to do your own insulating, you just need to take your time and detail things well. A bread knife is good for cutting batts (I’ve linked to one before that is cheap and works well). Where crummy clothes with long sleeves, and use a dust mask since you don’t want to breath the fibers. After that it’s just hours and hours of installation, which isn’t difficult work, it’s just tedious. You can easily save money here doing some DIY work, and you might even end up with a better install when you’re done since you’re likely to be more careful than a pro that will be going fast.

        If you go the DIY route, I highly recommend going with high density fiberglass or mineral wool since it’s much easier to cut and fit the higher density batts well. Low density batts are more squishy and harder to get a good fit. Ideally you want a flat finished batt when you’re done, with no voids and no depressions. Mineral wool is especially good about this, high density fiberglass is close, but not quite as good in this regard since it’s softer/squishier than mineral wool.


        1. wiscoguy | | #12

          How would I find this link ? I am definitely considering doing it myself. It’s not rocket science just need to find the time we are so busy building decks trimming out houses etc. my own house seems like I keep trying to sub out stuff I’d like to do.

          1. Expert Member
            BILL WICHERS | | #14

            Searching the Q+A archives is one way, but asking is another :-) Here it is:

            It's more expensive now than it was several years ago, but it's still pretty cheap and it's a surprisingly good knife -- I kinda felt bad using it to hack up insulation batts actually... It works VERY well for insulation though. Highly recommended.

            The old joke in the trades is that "the plumber's house always has a leak, the electrician's house always has a cover off" etc. I have some insulating work to do on my own home myself.


          2. CrisPA | | #22

            The bread knife Bill links to is *fantastic* for cutting bread. I've never used it for insulation but have really liked this one for that purpose:


  5. walta100 | | #11

    If this is new construction consider wet sprayed cellulous you get a perfect fit in every void. And it is the only time I can recall the greenest option also the low cost option.


    1. wiscoguy | | #13

      Does wet sprayed cellulose have a high r value? Also does the fact is wet sprayed add to the overall moisture content of the assembly? I have never done wet sprayed cellulose and to be honest don’t much about it at all.

      1. JC72 | | #23

        It takes about 2-3 days to dry stiff before the walls go up. Damp (aka wet) sprayed cellulose is typically used on walls of new construction because the added moisture and adhesive allow the product to stick to the studs/sheathing. It isn't automatically denser.

  6. walta100 | | #15

    I should say damp sprayed cellulose.

    Wet spray and dense packed cellulose will have an R3.6 -3.8 per inch.

    The amount to water used did not seem high when it was applied. It seemed dry to the touch after it seconds after it was applied.

    I am sure they used less water on the insulation than the floor and framing soaked up from rain before the roof went on.


  7. wiscoguy | | #16

    It seems like the actual materials based on what I can find locally is about 7000-8000 for the entire house so it’s quite a bit of savings if I do it to myself. Will take a little longer but seems worth it at this point for me. I’m not even taking out for windows and doors really so that number could be less plus studs and waste as well.

  8. walta100 | | #17

    Your last post does not make sense to me.

    What “actual materials” are you talking about? Cellulose, Rockwool or something else.

    “If I do it myself” Do what? Install Rockwool or cellulose?


    1. wiscoguy | | #19


  9. Patrick_OSullivan | | #18

    In my opinion, except for very small jobs, if your DIY time is limited to nights and weekends (i.e. you're not retired), insulation is not worth DIYing.

    Also, I recently went through the same fiberglass vs. mineral wool exercise for my own home and ended up with open cell spray foam. It was marginally more expensive than fiberglass and much less than mineral wool. Its nature addressed the same reason I was considering mineral wool (wanting to minimize convective looping in cavities due to subpar installation).

    If dense pack cellulose was a readily available option, I would have considered it, but it simply isn't popular in my part of NJ.

    1. wiscoguy | | #20

      Isn’t open cell spray foam a vapor barrier after a certain thickness

      1. JC72 | | #24

        Anywhere from 3 - 5.5 inches depending on the foam used.

      2. Jon_R | | #25

        > Isn’t open cell spray foam a vapor barrier after a certain thickness

        Effectively no. But maybe you mean "Class III vapor retarder" or "air barrier", which is in the 3-5.5" range.

        1. wiscoguy | | #28

          Yes but it doesn’t seem like a good idea to have 2 1/2” foam on the exterior walls and then have spray foam on the interior walls as well for my case anyway.

    2. aaronbeckworth | | #21


      Just curious, in your opinion, what is worth DIYing if limited to evenings and weekends? I’ve been hashing out these same questions for my future build.

      1. Expert Member
        BILL WICHERS | | #26

        Electrical is pretty easy to DIY if you know what you're doing. Plumbing isn't too bad either, but is a little more intense since alignment is more critical with pipes than with wires. I personally think insulation is easy to DIY too, but I've done a lot of it so I can go quickly and still do a good job. Any of these can usually be spread out without too much impact on the timeline for the project since the walls are generally open long enough that you have a while to do the electrical (at least) without holding up other trades. Insulation is a little more critical, but you can get a lot done in a long weekend. Insulation is also lightweight and unlikely to hurt you if it falls on you, so it's pretty safe as a "DIY alone" project too. Electrical is MUCH easier with a helper, as is plumbing -- although less so with plumbing.

        I personally like framing, but that's difficult to do alone, and stuff can hurt you if it falls. This probably isn't a good DIY project if you don't have experience, and ideally also a helper.

        Trim can be done DIY if you have some experience. If you haven't done it before, you'll probably go very slowly and waste a lot of material. You need some not so cheap tools to do a good job too (like a good mitre saw). Hardwood flooring is similar to trim in this regard.

        Paint is pretty easy to do as a DIY. Note that finish work like paint and trim is usually done after most of the other trades are done, so you won't hold anyone up if you take a long time. The downside is obviosuly that you'll be putting in a lot of your own time. Doing flooring takes a toll on your body too -- especially knees. Keep that in mind.


  10. Patrick_OSullivan | | #27

    > Just curious, in your opinion, what is worth DIYing if limited to evenings and weekends? I’ve been hashing out these same questions for my future build.

    I'll explain my calculus, which will not be the same for everyone else.

    1. I'll do pretty much anything myself if the job is small enough. There is value in just "getting it done."
    2. I will do something myself if it's esoteric enough that the typical subcontractor in a given field is not going to have the attention to detail the job requires to make me happy.
    3. I'll do electrical work because (1) it's an expensive trade to hire (2) there is no aspect of residential electrical I'll encounter that is beyond my existing capability (3) I'm a tricky customer
    4. I'll do plumbing work because (1) it's an expensive trade to hire (2) I find a little bit of it to be enjoyable

    Insulation and drywall are at the top of my "nope" list. They are dirty, physically uncomfortable jobs and unlike some things I do, don't challenge me enough mentally.I have the utmost respect for the folks who do this work well; it's just not for me.

    1. Expert Member
      BILL WICHERS | | #29

      +1 for not DIYing drywall unless you have no choice!

      I did all the drywall finishing myself of a decent size renovation on my house. My power drywall sander was giving me problems, so I did it all by hand. Felt like my arm was going to fall off, and I dreaded each day that needed another coat. If you do have to do any drywall work, I find that hot mud (the powder you mix with water) is easier to work with because it doesn't shrink when it dries, and it ALWAYS dries in a predictable amount of time regardless of the weather. The tradeoff is it is a once and done mix, since it chemically sets like concrete. There is no "close the bucket for tomorrow" when you're working with hot mud.


  11. wiscoguy | | #30

    I don’t do roofing any more since I broke my heel bone the foot just doesn’t act the same. And I won’t do my own drywall either. However I will do most anything else. I actually do pretty much everything else in my normal day to day job. Being a small general contractor. I mostly come to the forum for questions that fall outside of what I would call construction norms at this point to try to build a more efficient and better house.

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