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Performance of Wet Mineral Wool / Below Grade Mineral Wool Board

joejoemiller | Posted in Energy Efficiency and Durability on


I’m building a new home with a slab-on-grade foundation.  I’d like to insulate the foundation with mineral wool board below the foundation.  My contractor is uncomfortable with the idea of using mineral wool below grade because he’s heard that it doesn’t perform well when it gets wet.  I thought because mineral wool is hydrophobic, it would never get very wet, and therefore would continue to perform well below grade even after torrential rains. 

Can someone help me understand, will I still get R-4 per inch with mineral wool board installed horizontally below grade even when the surrounding ground is very wet?

Thank you in advance for your help.

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  1. user-2310254 | | #1


    You should contact the manufacturer. Does the contractor think the slab will stay wet for some reason? You are installing a gravel layer as a capillary break, correct?

    Also... Rockwool is expensive. Reclaimed rigid foam is cheaper and greener. Why not use the foam?

  2. SierraWayfarer | | #2

    Rockwool will dry after a big rain event if there is somewhere for the water to go. First into the gravel below the rockwool and slab then into the soil. Does your soil drain well and how much rain do you get? Call Roxul, and talk to them?

    I think the expense is based upon where you are.

  3. Jon_R | | #3

    +1 on #1. Last I checked, Comfortboard was 4X more $ than new EPS. So even if there are some environmental differences, spending the savings elsewhere is probably the better bet.

  4. ab1989 | | #4

    I never used mineral wool below grade but I did use 3 inch mineral wool on exterior. Prior to putting it on fully I did screw on a sheet and kept it out to elements. It dried very quickly when there was sun and showed no changes in shape after a wet and dry phase.
    That being said it would almost be impossible to answer your question without going to manufacturer in my opinion. I would be asking the following questions
    1. How deep am I going below grade
    2. Are there ants/critters that could go deep enough to bite on alternative such as Styrofoam
    I would say mineral wool. Might be the best bet in most cases if the manufacturer can tell you about how fast it can dry below grade and if it's strength is strong enough for your application I.e. house slab vs garage floor with equipment parked on. A call to manufacturer is best bt to find these answers

  5. severaltypesofnerd | | #5

    You'll absolutely have to get that from the vendor. As the insulation works by trapping air, and water will displace the air, I doubt you'll have any insulating value when the material is wet. The binding between fibers is likely to degrade as well, leading to support issues.

    1. Expert Member
      Michael Maines | | #6

      This study suggests no long-term degradation of mineral wool used below grade. But you're right about water; it's the air spaces in any insulation that do the actual insulating, so if they are full of water they aren't insulating.

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