Can bats ruin attic insulation with droppings?

The honest answer to this question depends entirely on scale and time. A single bat that gets into an attic overnight and leaves does not saturate insulation — there simply isn't enough guano or urine produced in that window to reach the material in any meaningful way. What actually ruins insulation is an established colony roosting repeatedly, typically across a full maternity season or returning year after year, producing enough accumulated waste that it soaks into and collapses the loft of fiberglass batts or loose-fill insulation. If you've seen one bat once, that is a different situation from finding an active roost with visible guano accumulation.

Bat guano and urine are rich in nitrogen compounds and uric acid. Under an established roost, urine soaks into insulation fibers over repeated seasons, and that saturation is what degrades R-value — wet or compacted insulation simply doesn't trap air the way dry, lofted material does. Bacteria and fungi break the waste down, which is also the source of the persistent ammonia odor associated with bat roosts. None of this touches the roof framing or rafters beneath the insulation; the damage is confined to the insulation layer and, in long-term cases, the wood or drywall directly under the roost.

Histoplasmosis is a real, named risk, and it deserves to be taken seriously — but it is also frequently overstated relative to what CDC actually documents. The fungus Histoplasma capsulatum can grow in accumulated guano given sustained warmth and humidity, and inhaling its spores can cause illness. CDC is direct that most people who breathe in the spores do not get sick, with more severe disease concentrated in people with weakened immune systems. The actual exposure event is physically disturbing dried guano — sweeping it, vacuuming it, handling contaminated insulation — not simply having bats roost overhead. That distinction changes what you should do: leave the material alone and get the colony excluded and the space professionally cleaned, rather than treating every attic with bat sign as an active toxic emergency.

Because bats are a protected species in New Hampshire, there is no lawful extermination option for anyone, including licensed pest professionals — and that includes Anchor. The only legal fix is exclusion: one-way devices installed by a wildlife specialist that let bats leave to feed and prevent them from re-entering, timed to avoid the maternity period when flightless pups can't yet follow adults out. Once the colony is excluded, the guano accumulation stops, and remediation can address whatever insulation has already been saturated. Trying to seal an active roost yourself, especially in summer, risks trapping pups inside, which makes the problem — and the smell — considerably worse.