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Community and Q&A

Indoor air quality and rodent feces

karlb_zone6a | Posted in General Questions on

What are best practices for maintaining indoor air quality, in the face of eventual infiltration by mice, etc?

We obviously do what we can to keep them out, and to eliminate them when they get in (because they WILL get in).  We clean up their messes whenever possible.  But what do we do when those messes are inaccessible? (e.g., floor/ceiling/wall/service cavities)  Let’s not fool ourselves: there are plenty of spaces that mice will inhabit that are not accessible for inspection or maintenance or cleaning.  In a tight house, if those spaces are within the conditioned envelope, the inevitable mouse mess will have an unwelcome effect on IAQ.

Do we just count on an airtight drywall approach to exclude these inaccessible spaces from substantial air exchange? (and not worry about it?)  Do we build in “clean-out” ports the way plumbers do?

(with apologies to John Steinbeck, for the post title)

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Karl,

    I slightly edit the titles of the Q&A posts sometimes because we've learned that the more direct they are, the more the get opened and found in search. So, sorry to change your clever title, but I'm curious about how people will respond to this too.

    My initial thoughts are the the mantra "build tight and ventilate right" is appropriate here. The most important thing to do for healthy indoor air quality is source control, or limiting harmful airborne substances from the house. If mice feces is a harmful airborne pollutant, then the best thing to do is to keep mice out and building a tight house will be helpful with this. Building tight also allows us to control ventilation, which is number two for healthy air quality, and to filter that air (number three).

    Perhaps there are extra sealing steps that can be taken to keep rodents out of the house that don't have to do with air sealing, like finding a way to keep them out of a vented attic, for example. But if the attic is vented, then the ceiling needs to be the air barrier so airborne pollutants shouldn't be getting in.

    Sometimes it is recommended to slightly pressurize a building to keep airborne pollutants out as well. So that's a possibility too, if you are concerned about feces around the periphery of the envelope as a potential hazard.

  2. Expert Member
    BILL WICHERS | | #2

    Usually rodent poo is an issue when you’re actually in it working, not so much when it’s somewhere people don’t go. I have had “black rice” rain on me before when working with overhead insulation, but I’ve never seen people have problems UNLESS they disturb it.

    Any air sealing you do is going to help contain any rodent residue too. Good construction practices (tight fits, good seals) will also help to keep both rodents and insects out. If you’re really worried about mice, you can build some 1/4” mesh hardware cloth into areas you expect them to try to chew through to help keep them out. This material is inexpensive and it’s easy to sandwich it behind sheathing and other materials vulnerable to penetration by chewing.

    If you want to keep a slight positive pressure in your home (we do this in critical facilities to keep out dust and other airborne contaminants), be sure to use a good filter on the air intake. HEPA filters can filter out dander, pollen, and many other things that can be health issues for sensitive people. If you pressurize your home without using a good filter on the intake, you’re not accomplishing as much as you could.


  3. tommay | | #3

    The key is finding out how they are getting in and out. Block off these entrances and exits and your problem will disappear. Inspect around your foundation for even the smallest of holes and plug them up.
    After you seal everything up, put a bucket of water in your basement. Every living organism needs water. If there are no water leaks in your house they can find they will go for the water in the bucket, fall in and drown...if they can't get outside.

  4. user-2310254 | | #4

    Hi Karl,

    Some might minimize or pooh-pooh the risk, but the CDC says rodents can transmit a range of diseases, including Hantavirus Pulmonary Syndrome, Hemorrhagic Fever with Renal Syndrome, Lassa Fever, Leptospirosis, Lymphocytic Chorio-meningitis (LCM), Omsk Hemorrhagic Fever, Plague, Rat-Bite Fever, Salmonellosis, South American Arenaviruses (Argentine hemorrhagic fever, Bolivian hemorrhagic fever, Sabiá-associated hemorrhagic fever, Venezuelan hemorrhagic fever), and Tularemia. (See for more detail.)

    While geography is a big factor here, I think it is prudent to keep rodents outside your home.

  5. Andrew_C | | #5

    Echoing others here, my experience in several houses is that plugging holes and improving the air sealing has eliminated any rodents and also decreased the number of bugs and spiders that get inside the house. This is another benefit of air sealing that is undersold, IMO.

  6. karlb_zone6a | | #6

    Thank you all for the thoughtful replies!

    Steve: we're up in northern Vermont, so communicable diseases are a bit less of a concern than in other parts, but we're still wearing P100 respirators when dealing with the mess.

    Tom/Steve/Andrew: there's no question that eliminating mice is an important first step. As it is, I have a trap line in operation. We are also fortunate to have a cat, who will be asked to earn her keep when we move in. Eliminating all of their access points, will unfortunately prove difficult however (see below).

    Brian/Bill/Andrew: building tight makes good sense, and is relatively easy to do in a new house. We're dealing with a 200 year old house though, atop a stone foundation. In the course of those 200 years, mice have been EVERYWHERE. Removing the (modern) drop-tile ceiling alone yielded enough mouse nests to fill a trash bag. The bottom of every wall/floor cavity bears an extensive collection of pelletized calling cards, with a corresponding effect on IAQ. I'll be busy a while longer cleaning it all out.

    The existing lath-and-plaster walls are in pretty good shape, so I'd like to keep them if possible, but they're not air-tight (and I don't know that they can be made air-tight in an economical manner). The roof and siding are due to be replaced, so my plan is to wrap the house with self-adhered membrane and get my air/vapor barrier there.

    So the challenge is this:
    - assume that I'm able to clean the existing mess from cavities spaces during renovation
    - assume that mice will eventually work back in, if only temporarily
    - assume that the interior side air barrier (plaster) is leaky, and that there remain inaccessible cavities
    - how do we maintain IAQ?

    Maybe it's enough to take active countermeasures, and keep the population under control, while maintaining a tight-as-possible interior finish and the positive air pressure idea that Brian and Bill advocate. I still wonder/worry about all the places that neither I (nor our cat) can access on a regular basis, though..

    1. Expert Member
      Akos | | #7

      Dealing with critter takes the same amount of focused details as a well detailed air barrier. Think of it as critter barrier. Same design detail, you should be able to draw a line along the perimter of any cross section of the house without lifting your pen.

      Since you are re-doing the siding, the best is to deal with this at the sheathing level. Cover the existing board sheathing with OSB/CDX, tape the seams as your air barrier, than cover all transitions with wire mesh. Around rough openings, you can protect the spray foam with a bit of steel wool embedded into the foam. If you have vented roof, make sure to protect all vent openings with wire mesh.

      Stone foundation is a bit more challenging, but if you need to waterproof, you can parge the exterior before the waterproofing goes on to seal it from rodents.

      This has worked well for me in northern Ontario, gotten the occasional visitor through careless holes through the mesh for pulling wires but that is about it.

    2. Expert Member
      BILL WICHERS | | #8

      +1 for hardware cloth in the exterior layers of your building assembly. In tricky spots, I’ve used 1/4” mesh hardware cloth embedded in spray foam as a combination air seal / rodent barrier. When cutting the hardware cloth for this, do it opposite of what you’d normally do: cut through the middle of the grid and not along one of the wires. The pointy edge is another level of critter deterrent, similar to mini barbed wire.

      Note that if you have some rodent mess somewhere you can’t reach, spray foam can encapsulate the mess so that it can’t mess up your air anymore. For smaller messes, spraying in a good sealing-type primer can help too. Clean as best you can, then seal/encapsulate as necessary in problem spots.


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