Humidity and substrate moisture are not the same thing
Relative humidity is the amount of water vapour in the air at a particular temperature. Substrate moisture describes how much water is held in the enclosure material. A box can briefly show high air humidity after misting while the lower substrate remains dry, or it can have damp lower layers without the air feeling tropical.
For many tarantulas, the useful husbandry question is whether the spider can access appropriate moisture and water while the enclosure still breathes.
Using humidity readings sensibly
A target such as 70% or 80% can tempt a keeper to keep spraying whenever a cheap hygrometer reads low. That can create a continuously wet surface and poor air exchange even when the species would naturally use a drier surface with moisture deeper down.
Different sensor positions also give different readings. A number near damp substrate is not the same as a number near the lid. Treat percentage figures as rough context, not as a command.
Use a moisture gradient
For moisture-dependent terrestrial and fossorial species, damp lower substrate with a less-saturated surface often gives the spider more choice than uniformly wet soil. A water dish adds a reliable drinking source.
Dry-country species may need much less retained moisture once established, while tiny slings can be less tolerant of complete desiccation than large adults. Species and life stage matter.
Ventilation is part of moisture control
Moisture without airflow can produce persistent condensation, mould and stale substrate. Ventilation without any retained moisture can dry a sensitive sling too quickly. Good enclosure design balances both.
Cross-ventilation is particularly useful in small sling and arboreal enclosures because it moves air through the occupied space rather than relying only on holes in the lid.
Do you need a hygrometer?
You can keep tarantulas successfully without chasing a digital humidity display. Substrate appearance, water availability, ventilation, species ecology and the spider's behaviour are often more useful information.
If you use a hygrometer, treat it as one data point. Do not let it override obvious enclosure conditions such as standing water, condensation everywhere or completely desiccated substrate in a moisture-dependent setup.
Practical checks
Look at whether the water dish is full, whether the substrate holds the intended moisture, whether the enclosure dries at a sensible rate and whether mould or condensation persists. Those checks tell you what the spider is actually living in.
When changing moisture, do it gradually unless there is a genuine emergency such as flooding. Stable husbandry usually beats repeatedly swinging the enclosure between soaked and bone dry.
Room humidity is not enclosure husbandry
A UK home can have dry winter air or damp summer air, but room humidity alone does not tell you whether a fossorial burrow has retained moisture or whether a sling tub is drying too quickly. Enclosure volume, ventilation and substrate depth change the local conditions substantially.
This is another reason not to manage a spider by the weather-app humidity number. Look at the enclosure itself and the needs of the species.
Misting is a tool, not a requirement
Misting can be useful for adding a small amount of water to a sling enclosure or a particular surface, but it is not necessarily the correct way to maintain every species. Spraying the walls every day because a care sheet says '70%' can leave the setup wet without creating useful substrate moisture.
For many setups, carefully adding water to part of the substrate or overflowing a water dish slightly gives a more stable gradient than repeated surface misting.
How slings change the equation
Tiny slings have less body mass and can lose water more quickly than large adults, so a completely bone-dry micro-enclosure may be inappropriate even for species that are kept fairly dry later in life. At the same time, tiny containers become stagnant quickly if they are soaked and poorly ventilated.
Use small amounts of water, good ventilation and observation. Sling husbandry requires finer adjustment, not necessarily a higher target humidity number.
Species ecology still matters
A desert Aphonopelma, a moisture-dependent Theraphosa and a tropical arboreal should not all be managed with the same spray bottle routine. Broad ecological origin and known captive behaviour give context to the moisture strategy.
That does not mean recreating rainfall statistics inside a plastic box. It means recognising that substrate depth, retained moisture and airflow should be adapted to the animal rather than copied across the collection.
Signs your moisture strategy needs changing
Persistent condensation, sour-smelling substrate, mould that keeps returning and a surface that never dries are signs the enclosure may be too wet or too poorly ventilated. At the other extreme, a moisture-dependent sling in powder-dry substrate with no reliable water source needs attention.
Do not diagnose moisture from one behaviour alone. A spider can sit near a water dish for reasons other than dehydration, and wall climbing can have several causes. Read the enclosure condition and the animal together.