humidity-window-fog-doctor

Use when windows fog/weep/frost on the inside in winter: computes the inner-pane surface temperature for your glazing (U-value + film coefficients), finds the condensation RH threshold for each window class, diagnoses cause (too humid / too cold glass / thermal bridging / lifestyle moisture sources), and gives the target indoor RH + ventilation fix with mold-risk timing.

Overview

Winter window condensation is a dew-point collision on a cold surface: warm moist indoor air meets an inner pane chilled below that air's dew point. Whether a window fogs is fully computable from indoor temp, indoor RH, outdoor temp, and the glazing's thermal resistance — which means the fix is computable too (either raise glass temperature or lower RH; which one is cheaper depends on the numbers).

This skill:

• Computes inner-pane surface temperature for single / double / triple glazing (U-value + indoor/outdoor film coefficients, with night-radiation and curtain-cavity effects)
• Finds the condensation threshold RH — the indoor humidity above which THAT window starts collecting water at tonight's outdoor temp
• Converts complaint → cause: "weeping at 60% RH but only on the north windows" = glazing class difference, "fog every morning only in the bedroom" = overnight moisture load (2 sleeping people ≈ 40–80 g/h water)
• Sizing the moisture sources: shower, cooking, plants, drying laundry, aquariums, unflued gas heaters (≈ 1.6 kg water per kg of gas!)
• Mold-risk framing: liquid water on organic frames/drywall paper above 0 °C for hours = mold in 24–48 h at the corner; below 0 °C = frost, less biological risk but worse on thaw
• The fix ladder: target RH for your coldest window → ventilation strategy (intermittent vs trickle vs HRV) → warm-edge spacers / low-e storm panes as the glass-side fix

When to Use

• "My windows are wet every morning / have black spots on the frames"
• Setting a humidifier in winter: what RH can this house actually carry without weeping windows? (Higher than people fear with good glass; lower with single pane.)
• Bedroom/bathroom/kitchen condensation pattern diagnosis
• Renters documenting a landlord conversation: glazing class vs indoor RH, who owns the fix
• Don't use for: summer condensation on AC supply grilles (different mechanism), whole-building HVAC design, or already-established large mold (>1 m² — remediation, see references).

Steps

1. Measure/know: indoor temp, indoor RH (any hygrometer), outdoor min temp, glazing class (single/double/double-low-e/triple), window location notes (curtains closed at night? north wall? bay window?).

   bash

   python3 scripts/windowfog.py --tin 21 --rh 55 --tout -8 --glazing double
   python3 scripts/windowfog.py --tin 21 --rh 55 --tout -8 --glazing double --curtains
   

2. Read the verdict: glass surface temp vs dew point, margin in °C/RH points, whether curtains made it worse (closed curtains chill the pane — the classic self-inflicted fog).
3. Fix ladder: target RH for tonight's temps (e.g. "hold ≤45% and the double glazing stays dry; ventilate bedroom 10 min at 7:00 and 22:00").
4. If target RH < 35% (comfort floor), the glass is the problem — the ladder says so and switches to glazing-side fixes.

The Model (summary — full in references/condensation-physics.md)

• Pane surface temp: T_s ≈ T_in − (T_in − T_out) · R_film_in / R_total with R_total from U-value, R_film_in ≈ 0.13 m²K/W (still air; less with curtains trapping a cold pocket → colder pane).
• Dew point from Magnus formula; condensation iff T_s ≤ T_dp.
• Threshold RH: invert Magnus at T_s → the RH where dew point reaches the pane. Each glazing class has a characteristic threshold curve vs outdoor temp (single ≈ 30–35% at −10 °C; double ≈ 45–52%; triple ≈ 60%+).
• Moisture balance: sources (g/h) vs ventilation removal (air changes × humidity capacity difference).

Output

Verdict + threshold table across outdoor temps (0/−5/−10/−15/−20) for your glazing, moisture-source audit checklist, fix ladder ordered by cost. --json for machine use.

Common Pitfalls

1. Treating all windows the same. Single-pane kitchen vs triple bedroom differ by 25+ RH points of threshold; fix each or accept gradient.
2. Closing curtains "to keep warm" — the cavity between curtain and glass goes colder than the room, the pane drops several degrees, and fog appears exactly behind the curtains where you can't see it. Leave a 2–3 cm gap at top/bottom for convective washing, or accept the fog.
3. Wiping instead of ventilating. Removing liquid does nothing to the vapor supply; the water returns nightly. Ventilation first (dry incoming winter air holds a huge absorption margin).
4. Humidifier chasing "healthy 50%" folklore. With single glazing at −15 °C outside, 50% RH turns windows into nightly rivers and frames into mold farms. The correct target is the threshold of your coldest window minus a 5-point safety margin.
5. Ignoring hidden sources: unflued gas heaters and indoor drying racks each add kilograms per day — the audit table converts to RH points.
6. Frost ≠ safe. Sub-zero panes deposit frost (no liquid for mold) but thaw into running water every morning; treat as condensation.

Verification Checklist

• Glass surface temp computed from the actual glazing class (not assumed double)
• Dew point and threshold RH both shown with margin (RH points and °C)
• Curtain/cavity effect flagged if user has night curtains
• Fix ladder starts with the free options (ventilation timing, source removal)
• If threshold < 35% RH → glazing-side fix recommended, not "dry the house more"