Humidity trips up more new invertebrate keepers than almost anything else. Too dry and your isopods desiccate, your millipedes struggle to molt, your beetle pupae shrivel. Too wet and you get mould explosions, bacterial blooms, and drowned mancae. The sweet spot depends on what you keep, but the principles for controlling it are the same across the board.
Measuring humidity
You cannot manage what you cannot measure. Buy a digital hygrometer. The analogue dial gauges sold with reptile starter kits are consistently inaccurate by 10-20%, which makes them worse than useless because they give you false confidence.
A decent digital hygrometer with a probe costs about five to eight pounds. Place the probe inside the enclosure, ideally at substrate level where your animals actually live, not stuck to the lid where the reading will be different. I have three or four scattered across my collection, and I rotate them between enclosures when I want to check on things.
Some keepers invest in combination units that read temperature and humidity together, which saves space and gives you both numbers at a glance. These are worth it if you are buying new anyway.
Factors that affect humidity
Ventilation
More ventilation means faster moisture loss. A tub with a fully mesh lid will dry out much quicker than one with a few small holes drilled in the sides. This is the primary dial you turn when adjusting humidity. I cover this in more depth in a separate post on ventilation, but the short version: reduce vent area to raise humidity, increase it to lower humidity.
Substrate
Wet substrate releases moisture into the air. Sphagnum moss holds water brilliantly and releases it slowly. Coco coir retains moisture well too. Plain topsoil dries from the surface down. A deep, moisture-retentive substrate mix acts as a humidity buffer, keeping the air inside the enclosure stable even if room conditions change.
Room conditions
UK houses in winter with central heating running can drop to 30-40% ambient humidity. That dry air pulls moisture out of enclosures faster than you might expect. In summer, ambient humidity is higher and you will need to mist less often. If you keep your collection in one room, a room humidifier can take the edge off winter dryness and reduce how often you need to mist individual enclosures.
Enclosure material
Glass retains humidity better than plastic tubs, and plastic tubs retain it better than mesh cages. For most invertebrate species, a glass or plastic enclosure with controlled ventilation holes is the right choice. Full-mesh enclosures are designed for chameleons and other reptiles that need constant airflow. They are unsuitable for humidity-dependent invertebrates.
How to raise humidity
- Misting. A hand spray bottle is fine for a small collection. Mist the substrate and enclosure walls, not the animals directly. For jumping spiders, mist the walls only. They drink from droplets, and direct misting can trap small spiders in water.
- Damp sphagnum moss. A clump of wet sphagnum in one corner acts as a localised humidity source. Isopod keepers use this to create a moisture gradient: wet sphagnum at one end, drier substrate at the other, so the animals can move to wherever they are comfortable.
- Reduce ventilation. If your enclosure has a large mesh panel, partially covering it with tape or cling film will slow moisture loss. Adjust gradually and monitor with your hygrometer.
- Water features. A shallow dish of water or damp moss near a heat source increases evaporation. This works well in larger enclosures but can be risky in small tubs where there is no room for a dry retreat.
How to lower humidity
- Increase ventilation. More airflow, less moisture. Drill additional holes or enlarge existing ones.
- Reduce misting frequency. Obvious, but worth stating. If your hygrometer reads above your target range, just mist less.
- Change substrate. If your substrate is waterlogged, the air will be saturated. Mix in dry coco coir or replace the top layer to bring levels down.
- Improve drainage. A layer of LECA or hydro balls at the bottom of the enclosure prevents water sitting in the substrate. This is standard in bioactive setups and worth doing in any enclosure where moisture management is tricky.
Species-specific ranges
These are approximate, and you should always check care guides for the specific species you keep, but as a rough guide:
- Tropical isopods (Cubaris, A. maculatum): 75-90%. These need it damp. A moisture gradient is important here so they can dry off if needed.
- Hardy isopods (A. vulgare, P. scaber): 50-70%. These tolerate drier conditions and are forgiving of fluctuations.
- Millipedes: 70-85%. Consistent moisture for healthy molting. Substrate should feel damp but not soggy.
- Beetle larvae: 60-80%. Flake soil should hold together when squeezed but not drip water. Too dry and the larvae dehydrate. Too wet and mould takes over.
- Jumping spiders: 40-60%. They need much less humidity than other invertebrates. A light mist every couple of days for drinking water is enough.
The moisture gradient trick
Rather than trying to hit one exact humidity percentage across the whole enclosure, create a wet end and a dry end. This lets the animals choose for themselves. In practice, you put damp sphagnum and leaf litter at one end, and keep the other end drier. Mist only the wet end. Your hygrometer reading will be an average, but the animals are moving to wherever they feel right.
This is especially important for isopods. Females carrying eggs in their marsupium tend to seek out the damper spots. Males and non-breeding adults are less fussy. A gradient gives everyone what they need without you having to micromanage the numbers.