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Exposed Mineral Wool Insulation

Bob Harshman | Posted in General Questions on

Is it advisable to paint exposed mineral wool batt insulation with a latex paint or intumescent paint in order to limit/prevent air born mineral fibers?

We are planning to use Rockwool safe and sound batts on our basement ceiling to reduce noise transmission. We would like to find a way to leave the mineral wool bats uncovered for access to the joists. 

There is no vapor barrier/retarder on the bottom side of the sub floor. Will installing the mineral wool insulation and then applying a vaper retarder (painting) the insulation be an issue for moisture? We live in zone 4. The basement walls are insulated.

Additionally – Would using an intumescent paint be beneficial for covering exposed wiring(romex) and plumbing(pex/pvc)? 

Thank you ~ Bob

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Replies

  1. Expert Member
    Zephyr7 | | #1

    I don't see any issue with painting the batts, just use a sprayer and some cheap latex paint. All the paint would be doing is acting as a sort of binder. I haven't really had any issue with mineral wool shedding fibers though -- it should be fine if it's not disturbed.

    Intumescent paints and sealants expand when heated, and are used for fire blocks. I would NOT use such a paint on the mineral wool. It's not needed for fire proofing either, the mineral wool itself is already essentially fire proof (and is used for fire stopping).

    There is no benefit to putting intumescent coatings on wires and pipes. The only place you'd normally put such a coating would be where wires or pipes penetrate a fire wall. If you want to control pipe sweating, use pipe insulation. There are spray-on pipe insulations, and also spray-on fire retardent materials (all of which, in my experience, are messy to work with both now and forever).

    Bill

  2. Bob Harshman | | #2

    Bill - Thank you for the quick response! Do you think there will be any moisture issues installing the mineral wool insulation directly to the bottom side of the sub floor (no vapor retarder between sub floor and mineral wool) and then applying a layer of latex paint to the mineral wool? My understanding is that mineral wool itself acts as a vapor retarder due to its density.

    The walk out basement is unconditioned and has 2 garage doors that are not perfectly sealed. Zone 4 with high humidity.

    Thanks in advance ~ Bob

    1. DCContrarian | | #3

      There can be moisture issues having conditioned space over unconditioned space. This article gives details:
      https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

    2. Expert Member
      Zephyr7 | | #4

      Mineral wool is NOT a vapor retarder, not even a little bit. Mineral wool is air permeable, and mositure can migrate right through too. Any fibrous insulation is going to be this way, regardless of density. The only commonly used insulating materials I'm aware of that are true vapor retarders are the rigid foams when used in thick enough thicknesses.

      I don't think the latex paint will be an issue in reasonable thicknesses. "Reasonable" here would be a quick spray applied coating to act as a binder. I don't really think you'll have any problems with shedding though.

      While I haven't seen mositure issues with this type of assembly myself, the article DC linked to shows some problems that can come up. I suspect you're most likely to see these issues if you're in an area with very long periods of very high average humidty levels.

      BTW, is this a new build or a retrofit? There are other ways to help with sound transmission if it's a new build, or if you're redoing the floor over the basement.

      Bill

  3. Irene3 | | #5

    I have exposed mineral wool batts in the upper walls of my basement with no issues as far as I can tell (admittedly I'm in Seattle, which does not get high humidity). Even when cutting the stuff I didn't notice any obvious shedding of fibers (though of course it's a good idea to wear a mask during installation regardless). The advice I was given was to insulate either the walls *or* the ceiling of the basement, not both. Also, from https://www.energy.gov/energysaver/where-insulate-home "In most cases, a basement with insulation installed on its exterior walls should be considered a conditioned space. Even in a house with an unconditioned basement, the basement is more connected to other living spaces than to the outside, which makes basement wall insulation preferable to ceiling insulation." (As I understand it, while Safe 'n Sound isn't sold as thermal insulation, and is therefore not rated to have any particular R-value, it does act as a thermal insulator of a slightly lesser R-value than Comfortbatt.)

    1. Expert Member
      Zephyr7 | | #6

      Safe'n'sound has the same R per inch value as "regular" mineral wool -- about R4.3 per inch. The difference is that Save'n'sound is only about 3" thick, to leave an air gap in a 2x4 studwall which helps attenuate sound when you're doing soundproofing. This makes Safe'n'sound a bit shy of R13 in terms of thermal performance.

      I've been told that you can request a letter from the manufacturer stating an R value for Safe'n'sound if you need it to make inspectors happy, although I've nevery tried asking them myself.

      Bill

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