The November Cold Snap That Took My Frog Room to 29 Percent

The first cold snap in November dropped my frog room from 68 percent to 29 percent in four days, and nothing inside the tanks had changed. Same timers, same depth of substrate, same animals. What changed was the house. The heating came on, the windows stayed shut, and the air in the room became a sponge that pulled water out of every enclosure faster than the misters could put it back.

This is the part that catches people. Indoor humidity in a heated house is not a fixed property of the room; it is what is left after the air has been warmed. A cubic metre holding 6 g of water vapour reads about 70 percent at 10 C and about 32 percent at 22 C. Same water. Different capacity. Your hygrometer is not broken and your substrate has not stopped working.

Heat Does Not Dry Air, It Changes What Air Can Hold

Warm air can hold more moisture, so the same absolute amount of water becomes a smaller fraction of the maximum. That is why a room that felt fine in October reads 30 percent in December with no leak and no dehumidifier. The tank is still exporting water vapour at the same rate; the room is simply pulling harder, and the gradient between a 90 percent tank and a 30 percent room is three times steeper than it was in autumn.

Glass makes it worse. A cold pane in a warm room condenses water on the inside, which looks like plenty of humidity and is actually water leaving the air. Wipe the glass and you have removed moisture from the system. Leave it and you get streaks, then mildew in the silicone, then a keeper who mists more to compensate.

Why Misting Harder Made It Worse

A keeper I know doubled his misting duration in the first week of December. His room went to 34 percent anyway, his drainage layers filled, and two of his frogs developed skin lesions within a month. The extra water went into the substrate and the drainage, not into the air, because the room was exchanging its air several times a day through a draughty window and an extractor fan.

The flooded floors then did real damage. Saturated substrate loses its air spaces, roots rot, and the bacterial load climbs. He had added water to a system that could not hold it and had not addressed the reason it was leaving.

A Winter Plan That Uses the Substrate Properly

Start with measurement at three heights: floor, mid-room, and 10 cm above the tank vents. A single hygrometer on a shelf tells you almost nothing. Log the 7 am reading for a week before the heating season starts so you know your baseline.

Then make the substrate do more of the work. A deeper humidity layer holds more reserve, and fewer, heavier soaks beat constant light misting, because a heavy soak refills the reservoir while a light mist only wets the surface. Covering 30 to 50 percent of a screen top cuts evaporation without sealing the tank. Group tanks together so they share a microclimate instead of each one fighting the room alone. Keep the room at 20 or 21 C rather than 24, since every extra degree lowers relative humidity further.

A room humidifier sized in litres per day is the honest fix if the room itself is at 30 percent, but it will run constantly and put condensation on cold walls. Insulating the room, sealing the draught, and running the extractor on a timer rather than continuously is cheaper over a winter than three litres of distilled water a day.

The Number to Watch in February

By mid-winter the failure mode shifts. Tanks that were too dry in November are often too wet in February, because the keeper overcorrected and the substrate never dried between soakings. Check weight, not just percentage: lift a corner of the cap and feel 3 cm down. If it is soggy and smells sour, you have gone past the point where more misting helps.

Tonight, put one hygrometer on the floor and one at tank level, and write both numbers down. The gap between them is the number that tells you whether you have a tank problem or a room problem, and they need different fixes.