Winter Indoor Humidity Target Calculator
Max indoor RH before windows start sweating, from outdoor temperature and glazing type.
With double glazing at −5 °C outside you can hold ~45% RH; single glazing only ~25%. The window is your hygrometer — first condensation means back off the humidifier.
Formula
Winter Indoor Humidity Target Calculator is a free winter humidity level for HVAC designers, technicians and energy auditors — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Winter Indoor Humidity Target Calculator
Max indoor RH before windows start sweating, from outdoor temperature and glazing type. The calculation implements T_glass = T_in − (U/h_i)(T_in−T_out); RH_max = p_sat(T_glass)/p_sat(T_in) (ASHRAE Fundamentals — fenestration surface temps). With double glazing at −5 °C outside you can hold ~45% RH; single glazing only ~25%. The window is your hygrometer — first condensation means back off the humidifier.
How to use Winter Indoor Humidity Target Calculator
- 1Enter Outdoor temperature in °C.
- 2Enter Indoor temperature in °C.
- 3Enter Window U-value in W/m²K (Single 5.8 · double 2.8 · low-E triple 1.0).
- 4Read Inner glass temperature, Max safe indoor RH instantly — no submit button needed.
Why use Winter Indoor Humidity Target Calculator?
- ✓Implements the standard formula — T_glass = T_in − (U/h_i)(T_in−T_out); RH_max = p_sat(T_glass)/p_sat(T_in)
- ✓Reference cited on-page: ASHRAE Fundamentals — fenestration surface temps
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Winter Indoor Humidity Target Calculator use?+
It computes T_glass = T_in − (U/h_i)(T_in−T_out); RH_max = p_sat(T_glass)/p_sat(T_in), per ASHRAE Fundamentals — fenestration surface temps. The formula is displayed under the result.
What should I keep in mind when using this calculator?+
With double glazing at −5 °C outside you can hold ~45% RH; single glazing only ~25%. The window is your hygrometer — first condensation means back off the humidifier.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Winter Indoor Humidity Target Calculator free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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