Humidity · The advisory and building-science practice of Saniservice

The room is comfortable. The wall is not.

Humidity is not one number for a building. It is a relationship between the moisture in the air and the temperature of whatever the air touches. A room at 24 °C and 65% humidity feels fine to the people in it; the same air, on a duct at 15 °C, is water. On this coast, for one hour in every four, the air outside is wetter than a cooled room can hold, and wherever it gets in, the coldest surface is where the surplus goes. This page explains why, lets you try it, and tells you what a measurement would establish.

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Sibling page: Mold, what grows and why it comes back

Three ideasHUM · v1.0

Dew pointthe temperature at which the moisture in air turns back into water
Cold surfaceanything below that temperature gets wet
Timewet long enough, and mold follows

Comfort is measured in the middle of the room. Mold grows at its edges.Air · Surface · Time

What the number means

Relative humidity tells you about the air. It does not tell you about the wall.

Relative humidity, the percentage on a hygrometer or a thermostat, is how much moisture the air holds compared with the most it could hold at that temperature. Warm air can hold more than cold air. So the same moisture that reads 65% at 24 °C reads 100% at about 17 °C: at that temperature the air is full, and the surplus becomes water on whatever it is touching. That temperature is the dew point.

Every room has a dew point, set by its temperature and its humidity together. Every surface in the room has a temperature. Wherever a surface is below the room's dew point, it is wet, whether or not anyone can see it. That is why a humidity reading on its own cannot say whether a room is at risk; it has to be read together with the temperature of the coldest surface the air can reach.

24 °C · 55% → dew point 14.4 °C
24 °C · 65% → 17.0 °C
24 °C · 72.1% → 18.7 °C

The benchmark. Dubai Municipality TG 142 asks for indoor humidity below 60%, ideally 30 to 50%; the practice's report benchmark is 60%. Those numbers are for the air in the room. They keep a comfortable room comfortable; they do not make a cold duct dry.

The model

One room, three dials. Watch where the water goes.

The model is one interior as a box, with the outdoor air on one side, a cooled room inside, and a duct passing through it. Set the room's humidity, choose how well the duct is insulated, and set the room's pressure against outdoors. The readouts give the room's dew point, the humidity at the duct's surface and the margin between the two. When the margin drops to zero, the duct is at the dew point; below zero, it drips.

Surface humidity is how full the air is at the duct's own temperature. Above roughly 75 to 85%, sustained, most building materials will grow mold (Johansson, Svensson and Ekstrand-Tobin, 2013). At 100%, water.

Margin is how many degrees the duct is above the room's dew point. The readout writes it in K, for kelvin; a difference of one kelvin is a difference of one degree. At 0 K the duct is at the dew point; below it, condensation is predicted.

Pressure shows which way air moves through the walls. Negative, and outdoor air is pulled in through every crack and gap in the outer walls and roof; the reading says the direction, not how much.

What the model does not do. It assumes the duct's temperature rather than calculating it; it treats the moisture at the surface as the same as in the room; it does not compute how much air the pressure moves. It is for seeing the relationship, not for sizing a system. The full statement of the model, its assumptions and its limits is in Research Note 01 · The Control Volume.

Why it happens here

The air outside is wetter than a cooled room can hold, one hour in four.

The outdoor dew point in the coastal cities of the UAE, Dubai and Abu Dhabi among them, sits above 22 °C for roughly a quarter of all hours in the year; in the summer months it is often in the mid-twenties, and the cooled rooms inside are held at or below that. Whatever outdoor air gets inside, through the ventilation, through leaks, through a door opened a hundred times a day, carries that moisture with it, and it meets the coldest surface first.

Inland it is different: at Al Ain the outdoor dew point exceeds 22 °C in fewer than one hour in ten. The coast is the problem, and the coast is where most of us live.

Source and method: Research Note 01

Moisture that gets into a cooled building can go three ways: the air-conditioning removes it, as water draining from the cooling coil; it stays in the air, and the humidity reading climbs; or it lands on something cold. The storage room on our home page shows the second and the third at once: 72.1% in the air, and water on the duct.

Whether the system keeps up depends on its capacity, its settings and how much air is coming in. Three failures cover most of what we see: a room over-cooled until its own walls are below the dew point; a duct that has lost its insulation; and a building that pulls in more outdoor air than its system can dry. Each has its signature, set out under Mold.

What a measurement establishes

Three readings, taken together, say whether a room is at risk.

The room's dew point

Air temperature and humidity in each room, from which the dew point is computed. This is the number the surfaces are measured against.

The coldest surfaces

Surface temperatures where a cold surface is suspected, and a thermal camera pass over walls, ceilings, ducts and pipes to find the ones nobody suspected. A surface below the room's dew point is wet, whether or not it shows.

The pressure and the air

The pressure of each room against its neighbors and against outdoors, which says which way air, and the moisture it carries, is moving through the walls; and where the question is whether the system can hold the room, its capacity and performance, ventilation and duct condition, under layer iv of Diagnostics.

What you receive Each room's temperature, humidity and dew point; the cold and wet locations with their readings; the pressure map; and, for every surface whose temperature was taken, whether it sat below the dew point, by how much, and what that indicates. Where a surface temperature or a pressure could not be measured on the visit, the report says so.

One question, one report: Diagnostics The cause, managed: Advisory

Frequently asked

What people ask when the reading is high.

My thermostat says 60%. Is that a problem?

For the air in the room, 60% is the upper edge of Dubai Municipality's guideline and of our own benchmark, so it is worth bringing down. Whether it is a problem for the building depends on the coldest surface that air can reach: at 24 °C and 60%, the dew point is about 16 °C, and any duct, pipe or wall below that is wet.

Should I buy a dehumidifier?

A dehumidifier lowers the humidity of the air in the room it stands in, which lowers that room's dew point and can keep a surface dry that was marginal. It does not change a cold surface's temperature, and it does not stop moisture arriving from outside; if the building is pulling in humid air, the dehumidifier runs continuously and the cause remains. In this climate a portable unit fills its tank in hours; if it cannot drain, it stops. It is a reasonable holding measure and a poor substitute for finding the cause.

Should I set the air-conditioning colder or warmer?

Neither, without knowing why the room is humid. Colder makes every surface in the room colder, including the walls and floor that face warmer, more humid air next door or outside, and those are the surfaces that go wet: this is the over-cooled room described on the mold page. Warmer does little to the moisture already in the air, so the dew point barely moves and a cold duct stays wet; and if the system is too powerful for the room, so that it cools the air in a few minutes and switches off, a warmer setting makes it run even less, and it removes even less moisture. What removes moisture is the system running long enough for its coil to condense it. That is a question of capacity and settings, not of the number on the thermostat.

Why is there water on my AC duct or diffuser?

Because the duct or diffuser is below the dew point of the air around it. Supply air leaves the cooling coil at about 12 to 15 °C; a duct with failed insulation runs close to that, in a room whose dew point may be 18 °C or more. The fix is insulation and, if the room is that humid, the reason it is.

Is this the same page as the mold page?

No. Humidity is the cause; mold is the evidence. This page is the physics and the measurement. The mold page is what grows, why it comes back and how it is investigated.