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Community and Q&A

Rock wool insulation?

Christopherpcampbell | Posted in Energy Efficiency and Durability on

For my partially sub grade bedroom space, I am averse to spray foam insulation because of the inconclusive endorsements I get when I ask if it off-gasses long term, even if correctly applied. No one from your organization or elsewhere seems to offer definitive endorsement.

So in considering alternatives, I read on your posts that rock wool insulation should not be used against a basement wall because air gaps allow penetration that will result in condensation. So Im thinking of lining the poured cement walls with rigid insulation and then using rock wool in the framing. And Im also thinking of using rock wool in the basement ceiling – between the basement and the first floor, partly because of its sound mitigation properties.

1- what do you think of this approach for the walls?

2-Is there a concern about micro dust fragments of rock wool gettiing pulled into the forced hot air system and getting into the air over time? Especially due to constant vibration on the basement ceiling from foot traffic on the floor above.

3. If that is a concern, we considered spraying the surface of the rock wool with some adhering product to trap loose particles on its surface and adhere it to itself. What do you think of this approach?

Hard to have good air quality with an insulated space below grade!

Thank you,

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  1. GBA Editor
    Martin Holladay | | #1

    Insulating the interior side of a basement wall with rigid foam is quite common. Once you have installed a reasonable thickness of rigid foam (more in cold climates, less in warm climates), you can install a stud wall on the interior side of the rigid foam if you want -- with or without mineral wool between the studs. For more information, see How to Insulate a Basement Wall.

    In you want to install insulation between the floor joists above your basement, it makes sense to install a ceiling to enclose the batts. The most common type of ceiling would be a drywall ceiling, but you can install OSB or plywood if you prefer.

  2. Expert Member
    Dana Dorsett | | #2

    As long as the ratio of the R value of the rigid foam to total-R is high enough to prevent moisture problems at the above grade portion, a rock-wool insulated studwall trapping the rigid foam to the foundation wall works fine. Using the same prescriptives found in IRC 2015 chapter for 2x4 walls with Class-III vapor retarders is about right. See Table 702.7.1:

    Be sure to put an inch of EPS under the bottom plate of the studwall as a capillary and thermal break, to keep the framing above the dew point of summertime air too.

    Rock wool doesn't have the same friable fiber problem commonly seen with fiberglass, but you still want to cover it up with reasonably air tight wallboard. There is no need to spray any sticky (probably high VOC) stuff on it to keep bits of fiber from wandering. Even where it's installed in attics without covering it up, the amount of airborne fiber drops to extremely low levels in a matter of a day. See this old but decent site measurement study done in the UK getting onto 3 decades ago:

    Unless there is active and substantial air pressure differences accompanied by air leaks through the wall (not likely with concrete & foam on one side, wallboard on the other) there's really nothing to drive any particles out of the walls and into conditioned space. This is true for fiberglass as well, but there are many attics with fiberglass insulation, air-leaky ceilings, and an air hander & ducts driving pressure differentials. That's the scenario with nearly ALL houses found to have high levels of airborne fiberglass particulates.

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