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Urgent help needed: Give up on mineral wool for free XPS?

matt2021 | Posted in Energy Efficiency and Durability on

Sorry for the “urgent help needed” post.  For my addition, four-season room in New Jersey, I have been planning to use, as insulation, (some) mineral wool for the floor (two inches of XPS, which I happen to have, and 5.5″ of Rockwool), and 5.5″ of Rockwool for the knee wall and the portions of wall above the windows (in addition to Zip System R6 sheathing).  The Rockwool’s cost will be around $1,000.

I might have an opportunity to acquire, for free, new XPS boards of various thickness.  Shall I get as many of them as possible, and save on all that Rockwool purchase?  In case the XPS foam boards are not enough for the whole project, are there reasons to use them more in the wall cavities or the floor cavities, or vice versa?

Thanks to anyone who can share their thoughts on this!

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  1. kbentley57 | | #1

    What you'll find with the free materials is a lot of frustration, and you may spend a lot of money on accessories making it kind of work, like cans of spray foam, etc.. It can be made to work, but if you're not so hard up that you shouldn't be doing a project in the first place, I'd just spend the cash on the rockwool.

    By all means acquire the free insulation, and do something with it, I just wouldn't try to cobble it into the walls / floor when a 5.5" batt already exists for that purpose.

    1. matt2021 | | #2

      Thanks Kyle! From what you say, it seems that rigid foam in cavities is appropriately used only if sealed against the studs and the like? I assumed that, with the Zip R6 sheathing properly sealed on the outside, and the inside carefully sealed (as I am planning to do) with some good-quality tape, the rigid foam could just be cut to size, for a good fit, and taped. One thing that concerns me is the stability of XPS v. mineral wool; you seem to think that the assembly with rigid foam in the cavities might work that well though.

      (I'll do the project regardless. Yet, the Rockwool I will need is $1,000, hence the appeal of using free rigid foam insulation, which would even have a higher R value, at least nominally, if indeed it is offered to me.)

  2. kbentley57 | | #3


    You're correct. I would only use rigid foam in a cavity if I were certain it could be well sealed. With a rigid product, it's hard to retrofit and do it well. Perhaps in a new build, if you use a table saw to make strips of the foam 14.5" wide (or 22.5"), you might be able to do it well enough, but that's a lot of time tied up in getting the insulation done, even if that time is free, it effects the schedule for other parts.

    While they can both be done, and both can work, my vote would still be to use the rock wool in the cavities unless the lead time on it was longer than you can stand in your region, and then I'd use the foam.

    I don't want to be that guy, but if you're using fibrous insulation, R21 fiberglass insulates about as well as R23 rockwool for 1/3 the price, especially since you've already got a head start with the Zip R.

    1. matt2021 | | #4

      Thanks again, Kyle. There is a lot of wisdom in what you write, including this:

      "I don't want to be that guy, but if you're using fibrous insulation, R21 fiberglass insulates about as well as R23 rockwool for 1/3 the price, especially since you've already got a head start with the Zip R."

      I'll research fiberglass again. I know it decays more than mineral wool; yet maybe that should not be a deal breaker. I imagine you're thinking of faced batts, right?

      1. kbentley57 | | #5

        I think that lately fiberglass has gotten something of a bad rap. It's probably a mix of new products, good marketing, and all that. When installed well, and in a good envelope, the sound characteristics and insulating properties are very near mineral wool, for much less price, and loading / unloading / packing / lifting. If you're building new, it shouldn't get wet, eh?

        It doesn't perform as well in a fire. Beyond that the characteristics are largely very similar, especially for the high density fiberglass batts.

        In NJ I'd get the faced batts.

        1. matt2021 | | #7

          I've run into discussions about the danger of adding a vapor barrier when using the Zip system R6 (or R9). The concern is that the vapor barrier on the room side, combined with the foam under the sheathing would trap humidity inside the wall cavity. Is that something I should worry about? See this discussion:

          1. kbentley57 | | #9

            That's good reading, and I'm glad you're taking such a thorough approach!

            You're right to be concerned about the humidity/vapor barrier. There seems to be something of an ambiguous recommendation for the vapor barrier to be on the outside in hot climates where it's always more humid outside (keep the moist air outside of the structure), and keeping the vapor barrier on the inside where it's extremely cold and dry in the winter (keep the moist air inside the structure).

            NJ has some muggy days in the summer, so it's not just a cold climate year round. I believe the kraft facing on the batts would provide enough of a vapor barrier to keep the moist air mostly in, in the winter, yet still have enough permeability to let the wall dry out in cases of high humidity.

            Keep researching it, you're doing well!

  3. user-5946022 | | #6

    One other way to look at it is this: What happens to the xps if you don't take it?
    - Does it end up in a landfill? In that case I think I might be inclined to take it and use it.
    - Does it go to someone else who has a more appropriate use for it? In that case I'd let them have it.
    If your addition is such that it would work, you could also use the xps either on the outside of the zip sheathing as exterior insulation (I'd strongly consider this) or at the roof which adds other complications, but it is a possibility.

    1. matt2021 | | #8

      Thanks! Good questions. I think the insulation would go to someone else.

      I would LOVE to find a carpenter/contractor who would apply insulation to the outside. Unfortunately, every time I have mentioned this, I have not gotten the feeling they would be completely comfortable with it. So, I decided to go for the more expensive Zip R6 solution, as the installation will be more straightforward.

  4. matt2021 | | #10


    Thanks! I do need to keep researching. If money were not an issue, I suspect that mineral wool, with no added vapor barrier or retarder, would have been the way to go. With fiberglass, it might be the same; yet, unfaced fiberglass will likely to be a bit more of a pain to install. We are talking about just one room, with three external walls, largely interrupted by glass, and one floor.

    Since I already have some mineral wool batts (not enough for anything, but maybe I will be able to find more sold by people who have leftovers), maybe I should reserve those for the walls, and go for unfaced fiberglass in the floor (above the XPS panels I am planning to have against the existing floor, with a vapor barrier under those panels). Depending on the actual thickness of the floor cavity, if the unfaced fiberglass ends up being compressed by a 1/2", that might not be too bad in terms of loss of R value.

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