Filament Dryer Humidity Not Dropping? What the RH Reading Actually Means

If your filament dryer humidity is not dropping—or it reaches 10–20% and you are wondering whether the spool is actually dry—the key is to understand what the RH reading can and cannot tell you.

What does the humidity reading on a filament dryer actually measure?

The display is normally reporting relative humidity (RH) inside the dryer chamber. NIST calibrates relative-humidity sensors by exposing them to air with a known moisture content, and Prusa likewise describes its drybox hygrometer as measuring the humidity of the air inside the enclosure.

That distinction matters because the sensor is not embedded inside the filament. A spool can still contain absorbed moisture even when the surrounding air is relatively dry.

Why can filament dryer humidity stay high or stop dropping?

There is no single cause. Work through the dryer as a system instead of assuming the spool is still wet just because the display has stopped moving.

What you see	What it may mean	What to check
RH falls slowly after startup	The chamber, spool and sensor are still reaching a new temperature and moisture balance	Give the dryer time to stabilize and avoid repeatedly opening the lid
RH rises after heating begins	Moisture leaving the spool may temporarily enter the chamber air	Check that the dryer’s ventilation or dehumidification path is operating as designed
RH stays high for a long time	Air exchange, seals, vents, sensor placement or a very wet load may be limiting the drop	Check the lid, filament outlets, fans, vents and the manufacturer’s operating instructions
RH is low very quickly	Heating can lower RH because RH depends on temperature as well as moisture content	Do not stop a recommended drying cycle solely because the display looks low
RH rebounds after the heater stops	Cooling changes relative humidity and moisture can redistribute inside the chamber	Judge the spool by the drying cycle and print result, not one post-cycle RH number

Why temperature changes the RH number

Relative humidity is temperature-dependent. The U.S. National Weather Service defines RH as the amount of water vapor present relative to how much the air could hold at its current temperature. That means a dryer can show a lower RH as the chamber heats even if the filament has not yet completed a proper drying cycle.

This is one reason a number such as 10%, 15% or 20% RH should not be treated as a universal “done” signal.

What humidity should a filament dryer show?

There is no single RH target that proves every filament is dry. Different materials, dryer designs, sensor locations and chamber temperatures make a universal cutoff unreliable.

For example, Prusa publishes drybox storage targets such as below 20% RH for PETG and TPU. Those values are useful for storage conditions in that drybox system; they are not a direct measurement of moisture inside the filament and should not automatically be reused as the endpoint for every active dryer.

How do you know when filament is actually dry?

No consumer hygrometer can give you a direct polymer moisture percentage from across the dryer chamber. For practical troubleshooting, use multiple signals together.

1. Use the correct drying procedure. Check the filament manufacturer’s recommendation or a source-backed drying reference.
2. Keep the test controlled. Do not change temperature, retraction, flow and speed at the same time.
3. Repeat the same print. Compare popping, bubbles, surface quality, extrusion stability and stringing before and after drying.
4. Stop blaming moisture if nothing changes. If correct drying produces no meaningful improvement, investigate retraction, temperature, clogs, feed resistance or other causes.

If you are not sure whether the original symptoms point to moisture at all, use the FilamentFix moisture diagnostic or compare the signs in Wet Filament Symptoms.

What if the dryer RH never gets very low?

Do not compare your dryer blindly with screenshots from another model. Dryer designs handle moisture differently. Some use active exhaust or dehumidification, some circulate heated chamber air, and some include humidity-control logic. SUNLU, for example, documents automatic chamber-humidity control on the FilaDryer S4, while Creality documents active dehumidification on the SpacePi X4.

Check your specific model’s manual for how its vents, fans and humidity controls are supposed to behave. A dryer that is designed to vent moisture should not be diagnosed the same way as a more sealed heated box.

Common questions

Is 15% humidity enough for filament?

It may be a good chamber reading, but it does not prove that moisture inside the filament has reached a safe level. Complete the appropriate drying cycle and compare print behavior.

Why does my filament dryer humidity go up while drying?

One plausible reason is that moisture is leaving the heated filament and entering the chamber air before being vented or removed. Temperature changes also affect RH. Check the dryer’s intended airflow and dehumidification behavior before assuming the dryer is failing.

Should I follow drying time or the humidity reading?

Use the material-specific drying temperature and time as the primary procedure. Treat the chamber RH as supporting information, then confirm the result with the same print test.

Does a low dryer RH mean my PETG or TPU is dry?

Not by itself. PETG and TPU can both absorb moisture, and the chamber sensor is measuring the surrounding air. Use the appropriate drying guidance for the exact material and product.

Useful next checks

• Filament Drying Temperature & Time Chart
• Does New Filament Need to Be Dried?
• PETG Stringing: Moisture vs Retraction vs Temperature
• How to Dry TPU Filament Safely
• Filament Storage Guide