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Community and Q&A

Rockwool (Roxul Insulation) in 2×6 walls

lookloan | Posted in General Questions on


I am in Zone 5, 2×6 walls on a single floor home.  I have spoken with three insulation companies and WOW is close cell spray foam expensive.  The plan is to do R49 close cell under the roof deck to the seal plates, then Roxul Rockwool R23 for the 2×6 walls.  My first concern is Rockwool safe?  (I think also called mineral wool but I could be mistaken) 

I ask because when I searched the web, I hear complains of abestos like fibers, dust and off gas.  This week I installed some thick R-23 Rockwool batts on the side of two walk in shower units on the side that faces the exterior wall.  I found the Rockwood fit in nice and was easy to work with.  So I have been thinking of doing all Rockwool on the walls with the R23 batts.  Have any of you builders or DIYs installed Rockwool and I was wondering if you could provide any helpful feedback. Basically is it safe – I am also told I go with Rockwool, I will have to face the inside insulation with Poly.  I don’t care for Poly and I was wondering if there is a Tyvek type paper I could staple over the Rockwool.

The installers are saying every inch of close cell is R7 and I was wondering if there is a certain type of closed cell spray foam that is R7 as most of what I read it’s R 6.5.   If it’s R7, they would be putting 7 inches (7×7=49R) but I am skeptical I am being cheated if it’s really 6.5  ( 6.5×7=R45.5)  Thank you in advance.

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  1. andy_ | | #1

    Rockwool is good stuff. Much prefer it over fiberglass and haven't had any issues with it so far after using it in a few houses.
    Instead of Poly you should look at Certainteed Membrain. It will allow the cavity to dry.

  2. lookloan | | #2

    Thank you - it's ironic that I had ordered the Certainteed Membrain about an hour before your post. I think I am going with the Roxul - I hope there are face masks that can be bought in the next month.

  3. Expert Member
    Akos | | #3

    You definitely don't want to breath in the fibers from mineral wool. Best is to get the vapor barrier over it as soon as possible, this keeps the fibers down in the place.

    If you are doing the vapor barrier install, it is worth while to detail it as a secondary air barrier. Caulk around the perimeter, tape the seams and use air tight electrical boxes on the outside wall.

    As for the SPF in your roof, depending on your roof construction you might be better off to meet code on U value. This takes into the whole assembly's performance including thermal briding from the structure. For example if you have 2x4 top chords on your roof trusses, 3.5" of SPF between the trusses covered by 2.5" of continuous SPF meets code on U factor basis and saves you 1" over an R49 roof.

    Also consider to go with a hybrid insulation approach where you only use enough closed cell SPF for condensation control and the rest is open cell. Take a look at the tables here:

    Open cell is much cheaper and if you have dimensional lumber rafters, the thermal bridging from the rafters negates most of the benefit of the high R value you would get with closed cell SPF.

  4. Expert Member
    BILL WICHERS | | #4

    Mineral wool is good stuff. It doesn’t really have any offgassing issues, it’s not a chemical blend in the way sprayfoam or even rigid foam is, so there are no issues with the “mix being off” or anything like that.

    Dust wise, treat mineral wool like fiberglass. You don’t want to breath the fibers, but they’re not carcinogenic like asbestos. Where a mask, safety glasses, and gloves. A long sleeve shirt is good too. Mineral wool can be itchy, but it’s not quite as bad as fiberglass. It’s a good product, and I use it myself whenever I need batts.

    I agree with Akos too, with both using MemBrain and that it’s worth detailing your vapor barrier as an air barrier. Use a good urethane caulk and good tape. A pneumatic staple gun will make the installation MUCH easier too.


  5. user-723121 | | #5

    Avoid using urethane caulk indoors for air sealing, some nasty VOC content. There are much more people friendly sealants available.

  6. Benneaf | | #6

    I don't understand the need for the CertainTeed Membrane to help with drying. The membrane goes on before the sheetrock right? So how is it going to help anything dry? Any moisture still has to evaporate through gaps or the sheetrock right?

    1. Expert Member
      Peter Engle | | #9

      Membrane is a variable-permeability vapor retarder. Poly is a vapor barrier. This difference is important. When Membrane is dry, it has a very low vapor permeability, so it stops the moisture in humid air from penetrating into the wall. This limits condensation within the wall cavity, which is what we want. However, if the wall cavity does become wet, or if you have reverse vapor drive issues causing condensation within the wall during air conditioning, the Membrane pores open up and it transitions to a vapor-open material, allowing that moisture to dry to the inside. Think of it as a one-way moisture valve. It lets moisture out of the wall but not in. Even when you tape the seams to make it an air barrier, moisture can still get out by diffusing through the Membrane itself.

      Poly, OTOH, traps moisture in the wall and can cause it to rot. Poly should not be used in walls south of Zone 8.

  7. lookloan | | #7

    If I understand it right, by code you need to have a vapor barrier facing the warm side so since Rockwool does not have a paper face like faced paper fiberglass, you have to add one. I was curious if something like Tyvek could be used since it paper like instead of the plastic stuff. I'm going with Certainteed Membrain but curious if there was a paper product like fiberglass has.

    1. Expert Member
      BILL WICHERS | | #8

      Tyvek is plastic, just vapor permeable. Tyvek is also NOT a vapor “retarder”, it’s a water barrier (think “keep rain out”). MemBrain is a vapor retarder with variable permeability dependent on humidity level, which is why it’s “smart” — more moisture means more vapor open which allows for better drying.

      I suppose you could use kraft paper as a vapor retarder, but I wouldn’t do that unless you can find a version rated for the purpose.


      1. Expert Member
        Peter Engle | | #10

        Remember that the Kraft paper facing on FG batts is not just paper. It is asphalt-saturated kraft paper. It is the asphalt that does most of the work to make it a vapor retarder. It is also a "smart' vapor retarder, with permeability of 1-5 when dry, and as much as 30 or so when wet. Depends a lot on manufacturing. But without an airtight seal, the leakage around the edges and penetrations is much greater than diffusion-carried moisture anyhow, and that's how many Kraft-faced walls fail in cold country. Membrain permeability is 10 when wet. And with the ability to make it airtight, you control moisture from both air leakage and vapor diffusion.

  8. KauaiBound | | #11

    Rockwool (no longer called Roxul) and almost all mineral wool producers use a formaldehyde binder which will continually off-gas.

    Fiberglass manufacturers got rid of formaldehyde binders a long time ago because of off-gassing concerns, so I'd think hard about using Rockwool in a living space.

    However, Rockwool does make one no-formaldehyde-added product, but availability is very limited (check with Rockwool to see if there's a local supplier).

    1. andy_ | | #12

      Yes, but... The formaldehyde is used in the manufacturing process so almost all of it is burned off when it's made. It's not like the old fiberglass that had it in the final product.
      Rockwool has gotten very high marks for indoor air quality and lack of VOC.
      I was told that the AFB Evo variant was made for applications where customers have severe restrictions like in chemical labs, but was kept around for people who didn't want to have a product that listed formaldehyde anywhere. kinda like an "organic" label for marketing.

      1. Expert Member
        Akos | | #13

        I've had cheap wood flooring show up on a handheld formaldehyde sensor, never mineral wool.

  9. _Stephen_ | | #14

    I have Rockwool used as sound insulation between the mechanical room and the rest of the basement.

    Its mostly there because the ERV and solar inverter make a fair bit of noise at full chat.

    No off gassing to report in the slightest.

  10. Stripes7 | | #15

    With any mineral wool I use, I have always had a horrible rash around my wrists and it itches like crazy anywhere my skin touches a *cut* edge of mineral wool. Experiencing this, I started to wear a tyvek suit and had no problems that way. Either I am allergic to something or people are not being quite real about saying you can just work in short sleeves and cut it and fit it with no problems. If you don't have to cut the product at all, it's very easy and clean. But the moment you take a knife to it and start peeling it, be wearing some clothes to protect your skin and ABSOLUTELY wear a mask rated for particulates. I found that the P100 respirator mask style was the most effective. I had N95 flapper valve masks that didn't do as good of job. I did a 1300 sqft home in about 18 hours. This was from opening the first bundle to installing the smart membrane and sweeping up all the subfloor so I could throw away the tyvek suit and mask filters when I was done.

    As others have said, try to get it installed and just be done with it. Don't do a little of it and go do another job then come back, it's something you want to install, clean up, and be done. I know that doesn't sound like a good advertisement for it, but I believe it is a superior product in every way possible compared to fiberglass or cellulose.

    1. Expert Member
      MALCOLM TAYLOR | | #16


      I agree it's a better batt product that fiberglass, but there are some wall assemblies where the moisture-buffering characteristics of cellulose make it a better choice, and cellulose wins hands-down in trussed attics.

      1. lookloan | | #17

        Malcom - can you or some help me understand the insulation of cellulose in trusses ? I picture a cloth or plastic material stapled to rafter's underside then the cellulose is blown in between the rafters and held in the rafers by the material that was stapled. So in the end, the rafters look like half round pertruding pillows from in between. Trying my best to describe but I am sure there is a more professional description

        1. Expert Member
          MALCOLM TAYLOR | | #18


          If the roof is trussed there are no rafters. The bottom chord of the trusses (the ceiling) gets drywall, and in some places also gets a vb, and in others strapping. The, loose-fill cellulose is blown through the access hatch. As long as you use 5/8" or 1/2" ceiling rated drywall, the cellulose doesn't bulge.

          It does make for two trips for the insulator, but here anyway, It's a separate crew from the guys who install the batts and vb.

          The main advantage over batts is continuity at the truss webs.

          1. lookloan | | #19

            Thanks Malcolm - I'm torn on the way to go after the $21K to $22K foam estimate. The attic trusses I have are 10 inches for the chord or the trusses.
            I meant to say roof rafters of the trusses, which are right under the roof decking. They are 10 inches also and 24 inches apart. Do you know if it was an option to fill the roof rafters if there was a strong material that is stapled to the bottom of the roof rafters and the cellulose is blown in rather than having spray foam under the deck? Not sure if it is a dumb question but I thought I'd seen this done.

            Editing post - just found this but not sure I could get R49

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