Last Updated on August 25, 2026 by Umar Farooq
Almost everything written about this is written by someone who sells pest treatment, so it ends where the spraying starts. Here is a more useful way to look at what is in your basement.
These animals cannot survive without a specific amount of moisture in the air. They have been sitting in your basement measuring it for you. What you have, and where you found it, is a reading.
First, which one is it?
Silverfish and firebrats look nearly identical and are constantly confused, and they want opposite conditions. Telling them apart localises your problem immediately.
| Silverfish | Firebrat | |
|---|---|---|
| Temperature | Cooler. Develops in places from about 71 to 90 °F | Above 90 °F, optimum 98 to 102 °F |
| Humidity | Above 75% at normal temperatures | Thrives at relative humidity as low as 30% |
| Where you find it | Cool damp areas. Usually the basement itself | Near the furnace, boiler or hot pipes |
| What it is telling you | You have a humidity problem | You have a hot dry corner. Not a damp reading at all |
The 75% figure is the one number on this page worth remembering, and it comes from a state integrated pest management programme rather than from anyone selling a service. Firebrats are the more drought-tolerant of the two, but they need heat to make up for it, and females only lay eggs above 90 °F.
They can drink the air
This is the mechanism, and it explains why humidity rather than puddles is the variable that matters. These insects do not need to find water. They can absorb it directly out of humid air.
The measurements were made on the firebrat, and the thresholds are precise:
Above about 63% humidity the animal gains water from the air. Below about 43% it loses water and dries out. The uptake happens through the rectum, which is a peculiar detail but a real one.
An honesty note, because this gets misquoted. Those figures are for the firebrat. The silverfish is a close relative using the same rectal mechanism, and the family generally absorbs vapour, but we could not reach a primary paper giving the silverfish its own number. So treat 63% as the firebrat’s figure and 75% as the silverfish requirement, and do not let anyone hand you a single number for both.
The number you were already aiming for
Here is the part nobody has put side by side, and it is genuinely convenient.
The environmental regulator’s target for a house, set for mould rather than insects, is that indoor relative humidity should be kept below 60 percent, ideally between 30 and 50 percent.
Silverfish need above 75 percent. Atmospheric water uptake in this family fails below about 43 percent.
So the humidity you would want anyway, for reasons that have nothing to do with insects, sits well under the level they require. You do not need a separate insect strategy. You need the humidity target you already had.
We are not going to tell you they die at 60 percent, because nobody has published that. What is published is that they need more than 75, and that the drying you would do for mould control takes you a long way below it.
A house centipede is a longer-baseline signal
The fast one with too many legs is not eating your house. It is eating the other things in your basement.
Now follow that through. Its listed prey are themselves moisture-dependent animals. A centipede population therefore needs a sustained population of damp-dependent prey, which needs sustained damp. One silverfish might be a visitor. A resident centipede population is a longer-running humidity signal than a single sighting.
Extension guidance follows the same logic when it lists control measures, putting reduction in the centipede food source first, and stating that correcting the problems that lead to centipedes will often eliminate the centipedes.
And it is not dangerous
Worth saying plainly, because they are alarming to look at. They are described as harmless, bites are rare because the animal is shy and its jaws are too small to break through human skin, and they are not harmful to people, food, clothes or furniture. Where a bite does happen, one extension service compares it to a wasp sting.
It should really be considered a beneficial organism. It is on your side.
The rest of the panel
Several other basement arthropods are moisture indicators, and extension services are unusually direct about what each one means.
| What you find | What it means |
|---|---|
| Sowbugs, pillbugs or millipedes, alive indoors | “If sowbugs or millipedes are frequently found alive, it means there is excess moisture indoors.” The sowbug can survive only in damp places |
| Springtails, in numbers | “When you see large numbers of springtails, it means there is a high moisture problem.” They are typically found around plumbing leaks and condensation |
| Camel or cave crickets | Attracted to cool, moist areas. They invade crawl spaces, basements and garages where moisture may be a problem |
| Earwigs | They hide during the day in dark, confined, damp areas. Excess moisture attracts them |
Where you found it is half the diagnosis
Extension guidance lists the basement harbourages precisely, and each one is a specific moisture fault rather than just a hiding place.
| Where you see them | What is actually wrong there |
|---|---|
| In or around a floor drain | A drain trap that has dried out. It is both an open route in and an evaporating source. Pour water down it |
| Coming out of a block wall | The hollow cores of concrete block are a moisture reservoir, and warm humid air is drawn through them |
| Under and in cardboard boxes on the slab | Cardboard on concrete wicks moisture out of the slab. It is a damp reservoir and their food |
| Along the wall-floor joint, behind baseboards and trim | They get in through crevices under and behind baseboards and where pipes pass through walls and floors |
| Near new concrete or fresh timber | Freshly laid concrete and green lumber supply humidity of their own for months |
Why the paper matters as much as the damp
Silverfish carry gut enzymes that digest cellulose, so they need both conditions at once. They are attracted to dry food such as cereals and pasta, paper with glue or paste, sizing in paper including wallpaper, book bindings and starch in clothing.
A basement with high humidity but nothing to eat supports far fewer of them than one with boxes of old paperwork in it. Stored cardboard is the single most common food supply in a basement, and it is also the thing wicking moisture off the floor.
You can even read the damage. Their mandibles are weak, so they scrape the surface of paper rather than biting it, and on paper and fabric the feeding begins in one area and spreads as unconnected, irregularly shaped holes.
The single most useful thing on this page
They gather where it is wettest, and they do it deliberately.
They also release chemical signals that pull others in, because crowding together helps regulate the temperature and humidity around the eggs and young.
So a cluster is a moisture map. Wherever you keep finding several at once, put a hygrometer on the floor there and leave it overnight. Compare that reading with the middle of the room. The insects have already done the survey and picked the wettest spot in your basement.
Why it feels like they never go away
Because you are judging a multi-year animal on a two-week timescale.
- Silverfish live about three years, with some sources putting the range at two to eight. Most household insects live weeks.
- Development is glacially slow when it is cold. Below 50 °F, firebrats can take as long as 300 days and silverfish as long as 500 days to reach adulthood.
- Eggs wait. In colder environments eggs can remain dormant for up to six weeks, hatching as soon as the temperature rises.
- They breed steadily. An average clutch is around 50 eggs, though it ranges from 1 to 200.
A spray that clears the visible population for a fortnight has told you nothing. The population turns over on a scale of years, so the only variable worth changing is the one that persists between generations. That is the humidity.
What actually works
The pest management programme says it more bluntly than we would dare to:
And, on the same page, that insecticides will not be effective unless you also remove the moisture, food and hiding places that allow these pests to thrive. Extension guidance on sowbugs and millipedes puts it in four words: pesticides are not effective if there is excessive moisture and a food supply.
- Measure first. Hygrometer where the cluster was, and in the middle of the room. You are aiming below 60 percent.
- Dehumidify. Extension guidance is direct that silverfish require a lot of moisture, so using dehumidifiers in closed spaces can help discourage them, and that ventilation from fans can reduce relative humidity to a point that is intolerable to silverfish. Note that the fan version only works if the air you are moving in is drier than the air inside, which in a humid summer it is not.
- Get the cardboard and paper off the floor. This removes the food and the damp reservoir in one move. Plastic tubs on shelving, not boxes on the slab.
- Fill the floor drain trap and seal cracks and crevices in slabs and block walls.
- Fix the water outside. Grade soil away from the foundation, and keep soil near the house able to dry between waterings.
- Put sticky traps down as sensors, not as a cure. Extension guidance introduces them under detection and monitoring, placed in corners and along edges. They tell you where, not how many.
- Judge it over a season. See the life cycle above.
Where chemistry fits, honestly: certain inorganic dust products are described as very effective against firebrats and silverfish if you apply the product directly on pests. The same guidance adds that you might need to periodically re-treat moist areas, which is the whole argument in one sentence. Leave the moisture and the treatment becomes a subscription.
Things sold for this that will not fix it
Diatomaceous earth
Not a myth exactly, but self-defeating here, and the reason is mechanical. It is a desiccant. It kills by drying the insect out.
You are deploying a drying agent into the one room where the problem is that nothing dries, against an animal that can pull water back out of the air. It is fighting the room.
You will also find confident figures for the exact humidity at which it stops working, usually 65 or 70 percent. We could not trace any of them to a primary source, and they appear only on commercial pages. A peer-reviewed test comparing 55 and 75 percent humidity found the difference was not statistically significant. So the honest position is the mechanical one, not a threshold.
Boric acid, and baits
Several of the inorganic dusts containing boric acid are described as ineffective against firebrats. And baits are not very successful against either insect, because they tend not to feed on them.
Bug bombs and foggers
University entomology guidance on these is unusually direct. Very little insecticide actually penetrates into cracks, voids and other secluded locations where household pests congregate, and foggers seldom resolve, and can exacerbate, indoor pest problems. The ingredients tend to be repellent, so insects scatter and move deeper into walls and crevices.
Look at that against the harbourage list above. Block wall cores, floor drains, under the slab. A fogger physically cannot reach where the animals are. There is also a real fire risk where pilot lights are not extinguished, which matters in a basement full of appliances.
Cedar and essential oil products
The honest objection is regulatory rather than experimental. Products of this type are sold under a minimum risk exemption, and for those the regulator states plainly that the pesticide product does not need to be registered and that it does not review exempted products.
So nobody has had to demonstrate that they work. We could not find a single university extension service recommending them for silverfish, and that absence is its own answer.
FAQ
Does seeing one silverfish mean I have an infestation?
No, but it does mean somewhere in that room is above 75 percent humidity, at least some of the time. One sighting is a reading rather than an emergency. Repeated clusters in the same spot are the thing worth acting on.
Are silverfish harmful?
Not to people. They damage paper, book bindings, wallpaper and starched fabric, which matters if you store documents or photographs down there. They are a possessions problem and a humidity signal rather than a health one.
Should I kill house centipedes?
It is up to you, but they are harmless and they eat the insects you actually mind. If they have enough prey to sustain themselves, killing them individually changes nothing about the underlying conditions.
How long before they go after I dehumidify?
Months rather than weeks, and the life cycle above is why. Development can take over a year in a cold basement and eggs sit dormant. Judge it across a season, using the hygrometer rather than the sightings.
They are in my bathroom too, not just the basement.
Same reading, different room. Bathrooms and laundry rooms are the other classic locations because they are humid, and extension guidance names laundry rooms specifically. Extractor fans that actually vent outside are the fix there.
Do I need a pest control company?
For these two, usually not, and a treatment applied to a basement that stays above 75 percent humidity is a recurring cost rather than a fix. Spend it on the dehumidifier and on getting the cardboard off the floor.
