Notice No. 100/RH · posted for the attention of all occupants
Issued in response to one passing remark that a building was freezing cold and also at 100 % humidity, as though these were two separate grievances. They are one grievance. Arguably zero.
Section 1 — Summary of findings
Not marginally more. Roughly double for every 10 °C. This ceiling is set by temperature alone and it moves whether you want it to or not.
100 % means "this air is holding everything it can at this temperature." In a cold building, everything it can is close to nothing.
The large glass usually contains more water. Heating a room does not dry it out by removing moisture — it dries it out by enlarging the glass.
Section 2 — Demonstration
Set the temperature and the humidity reading you are so upset about. The point plots itself on the psychrometric chart below. Then heat the air and watch the point travel horizontally — same water, rising ceiling, collapsing percentage.
Section 3 — The part that ends the argument
0 °C / 32 °F at 100 % RH
4.8g/m³
"It is unbelievably humid in here."
21 °C / 70 °F at 30 % RH
5.5g/m³
"It is so dry my lips are cracking."
Same building. Same air, even. The room everybody calls swampy contains measurably less water than the room everybody calls bone dry. The instrument was honest with you. You read it wrong.
Section 4 — In fairness to the complainant
All of this — the domain, the chart, the appendices — exists because one person mentioned once that they were cold. Honesty requires conceding that their complaint had something real buried in it. Splitting the difference:
In a cold building it means close to the opposite. There is very little vapour in that air; it simply has nowhere left to put more. Warm it up and the identical air reads 40 % or less. No water left the room. The ceiling rose.
They are one problem wearing two hats. The cold is why it reads 100 %. Fix the temperature and the humidity figure fixes itself — a deeply unsatisfying thing to be told while shivering, which is why this notice is annoying.
It means the dew point has caught up with the air temperature, so every surface at or below room temperature is actively condensing. That is not a comfort complaint, it is a mould-and-rot complaint. A building sitting at saturation has moisture getting in, no ventilation, or both.
Cold damp air pulls heat out of a person far faster than cold dry air, and perspiration has nowhere to evaporate to. "Cold and clammy" is a real sensation with real physics behind it. It is simply not the physics you cited.
The remedy, for the record, is heat and ventilation: warm the air so it can carry the moisture, then exchange it with outside air so the moisture actually leaves the building. A dehumidifier in a cold room is fighting uphill — it is trying to wring out a glass that was barely full to begin with.
Section 5 — Appendices
Every figure on this page is computed live from the Magnus–Tetens approximation for saturation vapour pressure over liquid water, accurate to a fraction of a percent between −40 and 50 °C. That comfortably covers your office.
Saturation vapour pressure, hPa — this is "the ceiling" es(T) = 6.112 · exp( 17.67·T / (T + 243.5) ) Actual vapour pressure, hPa — RH is literally this ratio e = (RH / 100) · es(T) Absolute humidity, grams of water per cubic metre of air AH = 216.7 · e / (T + 273.15) Dew point, °C γ = ln(RH/100) + 17.67·T / (T + 243.5) Td = 243.5·γ / (17.67 − γ) Heating the same air from T₁ to T₂ — e is unchanged, es climbs RH₂ = RH₁ · es(T₁) / es(T₂)
Section 1 told you that warm air "holds" more water, like a sponge. That is a lie told to schoolchildren, and this notice repeated it because it is a useful lie.
Air is not a container and nitrogen is doing you no favours. What actually sets the ceiling is water's own vapour pressure at that temperature — evaporation and condensation reaching equilibrium. You would measure the same numbers in a vacuum. Keep the sponge if it helps you argue in Slack; just don't specify an HVAC system with it.
Section 6 — Distribution
Best delivered in a thread, with no preamble, roughly four minutes after somebody says the thing. Do not explain it. Do not add a winking emoji. The domain is doing the work.