Humidifier Settings by Outdoor Temperature
A humidifier set to 45% feels great in November and streams water down the glass in January. The windows set the ceiling, and that ceiling drops as it gets colder outside.
The manufacturer chart
Aprilaire’s humidification guide publishes recommended indoor RH by outdoor temperature. HVAC installers print the same curve on humidistats:
| Outdoor temperature | Indoor RH, at most |
|---|---|
| 40 °F (about 4 °C) | 45% |
| 30 °F (about -1 °C) | 40% |
| 20 °F (about -7 °C) | 35% |
| 10 °F (about -12 °C) | 30% |
| 0 °F (about -18 °C) | 25% |
| -10 °F (about -23 °C) | 20% |
| -20 °F (about -29 °C) | 15% |
Aprilaire also warns that condensation on the windows usually means indoor RH is too high, and the same condensation can be happening inside walls.
How to use it
- Set for the overnight low, not the afternoon high. The coldest hour is what wets the glass.
- Check the windows each morning. Water across the middle of the pane means the setting is too high, so drop it about 5 points and check again the next day. A thin line along the very bottom edge is common because the spacer and frame run colder there.
- Use window protection if you have it. Honeywell Home thermostats with an outdoor sensor or internet weather offer a “Window Protection” level from 1 (well-insulated windows) to 10 (poor windows). Raise it one step each morning until the condensation stops.
- Room humidifiers count too. A tabletop unit in a closed bedroom can push that room well above the chart on its own. Turn it down or off when the glass fogs.
- Measure, don’t guess. A hygrometer near the bed or the coldest window shows what the room is actually doing.
Single-pane or older windows need lower settings than the chart; triple-pane tolerates more. Above freezing, the US EPA’s general guidance is to keep indoor RH below 60%, ideally 30–50%.
Sources: Aprilaire humidification guide; Honeywell Home T10 Indoor Air Quality docs; Obsidian Heating & Cooling humidistat chart; US EPA mould guide.
Originally published at https://dry-glass-notes.surge.sh/articles/humidifier-settings-by-outdoor-temperature.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.
Write a comment