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Community and Q&A

Do rodents nest in/with mineral wool batts, as they do with fiberglass?

lutro | Posted in Green Products and Materials on

In my area (NM), mice and other rodents often nest in fiberglass batts, or harvest the fiberglass and move it to nearby locations to build their nests. Do they do this just as much with mineral wool batt insulation? While major rework is needed on many aspects of this property, in making short term repairs to the insulation in a few locations, I wonder if using mineral wool would be effective in deterring rodents.

On a similar question, I’ve got one room with a roughly 3/4″ x 3/4″ gap between the edge of the hardwood flooring and the wall. I would like to fill this gap, and then install baseboards. I can imagine the rodents chewing through backer rod or fiberglass insulation. Is there a good way to fill this gap, that rodents wouldn’t destroy?

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  1. Expert Member
    Dana Dorsett | | #1

    Rodents aren't too fussy. There is some indication that the fire retardents in cellulose are enough of an eye irritant to keep them from nesting in the stuff, but it wasn't sufficient to keep a squirrel from eating a hole into a bag of new bag of cellulose I had in the garage and spreading it about. I have little doubt that a dedicated mouse or rat would be happy nesting in rock wool, though it takes a bit more work to tunnel through higher-density rock wool than low- or mid-density fiberglass.

    Foamed concrete insulation or packed pumice insulation might do it, or EPS beads in a cement/polymer matrix like Styrocrete (all about R2/inch) would likely impede them a bit.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Do rodents nest in mineral wool batts?"

    A. I don't know, but birds do. See Wrapping an Older House with Rock Wool Insulation.

  3. jackofalltrades777 | | #3

    Mineral Wool is supposedly rodent and pest resistant. Fiberglass is probably the best nesting material out there for rodents, they love the stuff. Cellulose is pretty resistant but rodents have been known to use it also. Best practice is to seal off the insulation to prevent them from accessing it but rodents love to gnaw.

    Being that you are in New Mexico, the bigger concern is that you are in HantaVirus country. While not an epidemic by any means, you should be careful around Deer Mice droppings and nests since that is where the life threatening HantaVirus resides.

  4. lutro | | #4

    Thanks to each of you for your answers. Regarding hanta virus, it is a very serious concern, but thankfully not in my part of New Mexico. I hadn't thought of pumice in this application. It's an attractive option, since it is a locally available, renewable resource. (Another eruption should be happening any millennium now).

  5. Expert Member

    I had two batts of Roxul safe and Sound that didn't quite friction fit in a joist cavity and ended up on the crawlspace floor. When I went to put them back in place I found they contained quite a large mouse nest.

  6. GBA Editor
    Martin Holladay | | #6

    A portion of the 3/4 inch by 3/4 inch gap at the perimeter of your hardwood flooring can be filled with caulk, which will reduce air leakage. Of course, the caulk is not 100% effective against rodent teeth.

  7. lutro | | #7

    Martin, please tell me more about how you would approach the caulk option. The caulks that I am familiar with discourage trying to fill a gap as large as 3/4" in any dimension. Backer rod can potentially reduce the depth to be filled, but I am unsure how to partially fill both the depth and breadth of the gap, and then caulk effectively. Note that the gap averages 3/4" x 3/4", but is irregular.

  8. mackstann | | #8

    I use door & window expanding foam for the bigger gaps, then trim it down flat with a knife. The remaining smaller gaps get caulk.

  9. GBA Editor
    Martin Holladay | | #9

    This is a classic air leakage location, and I like to caulk it where possible. Obviously, if the gap is particularly wide, it can be hard to caulk.

    Depending on the height of the drywall, possible caulk locations include the seam where the subfloor meets the bottom plate, or the seam where the drywall meets the subfloor. In some cases, it also makes sense to install a bead of caulk under the baseboard, to seal the seam between the baseboard and the hardwood flooring. (This last location is the one usually chosen by weatherization workers.)

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