Silverfish need relative humidity between 75% and 95% to reproduce. Below 50%, their eggs won’t even hatch. New York City’s summers regularly push outdoor humidity into the 65–70% range — and attic spaces in unventilated pre-war buildings go higher than that.
Unlike the rest of your home, your attic isn’t regulated by air conditioning or heating in the same way. It sits at whatever temperature and humidity the season delivers. That’s the core problem in Brooklyn brownstones, Queens row houses, and the attached two-family homes throughout Marine Park, Flatbush, Bay Ridge, and Bed-Stuy.
These buildings were built before modern insulation standards, with plaster walls, aging plumbing, and limited ventilation. They create the exact warm, dark, humid conditions silverfish have thrived in for — and this is not an exaggeration — roughly 400 million years. They are very good at surviving. They are especially good at surviving in buildings like yours.
A silverfish can live two to eight years and go without food for up to a year. An established attic population does not self-correct. It grows, and it spreads downward.