The psychrometric chart, explained for architects

The axes: dry bulb and humidity ratio

The horizontal axis is dry-bulb temperature, the air temperature an ordinary thermometer reads. The vertical axis, usually on the right, is the humidity ratio: the mass of water vapour per mass of dry air, in grams per kilogram (g/kg). In buildings it typically ranges from 0 to about 20 g/kg, so water vapour is at most 1 to 2% of the air's mass.

The humidity ratio only changes when moisture is added or removed, which makes it the natural axis for tracking what happens to air as it is heated, cooled or humidified. Charts are drawn for one barometric pressure, usually sea level (101.325 kPa); versions for higher altitudes exist and are more accurate for mountain sites.

The curves: relative humidity, dew point, wet bulb and enthalpy

Given any two of the properties below, the state of the air is fixed and the others can be read from the same point.

• Relative humidity (RH): the ratio of the water vapour's partial pressure to its pressure in saturated air at the same temperature. RH curves sweep from lower left to upper right; the top curve is 100%, the saturation curve.
• Dew point: the temperature at which water starts to condense when air is cooled without changing its moisture content. From any point, move horizontally left to the saturation curve and read the temperature there.
• Wet-bulb temperature: the lowest temperature air can be cooled to only by evaporating water into it. Wet-bulb lines slope up to the left. The gap between dry-bulb and wet-bulb temperature, the wet-bulb depression, grows as the air gets drier.
• Enthalpy: the total heat content of the moist air, sensible plus latent, in kJ per kilogram of dry air. Enthalpy lines are straight, parallel and slope up to the left.
• Specific volume: many charts also show lines of cubic metres per kilogram of dry air.

Reading processes on the chart

• Heating or cooling without adding or removing moisture: a horizontal line. Heating lowers the relative humidity even though the moisture content is unchanged, which is why heated winter air can feel dry.
• Cooling below the dew point: the point reaches the saturation curve and then moves down along it as water condenses. This is how cooling coils dehumidify.
• Evaporative cooling: the point moves up and to the left, roughly along a wet-bulb line. The air becomes cooler and more humid, and the achievable cooling is limited by the wet-bulb depression, so it works in dry air and costs water.

Comfort zones on the psychrometric chart

A comfort zone is the area of the chart where most people, for a given activity and clothing, feel neither too warm nor too cold. Its position is not fixed. In ASHRAE 55-2013 as amended by Addendum g (2016), for example, the graphic comfort zone applies only to occupants at 1.0 to 1.3 met wearing 0.5 to 1.0 clo, with air speeds up to 0.2 m/s, no direct sun and a humidity ratio up to 0.012 kg/kg (a dew point of 16.8 °C). Outside those conditions the standard points to its other methods, such as the analytical method based on the predicted mean vote (PMV) or the elevated air speed method.

Bioclimatic charts go a step further. Olgyay drew the first one from outdoor conditions; Givoni's building bioclimatic chart, later expanded with Milne, draws on the psychrometric chart the extra conditions that ventilation, thermal mass and evaporative cooling can make comfortable indoors. For naturally ventilated buildings, adaptive comfort models relate the acceptable indoor temperature to recent outdoor temperatures instead.

Plotting a climate's hours

Plot every hour of a typical-year weather file and the chart becomes a climate diagnosis. A cloud of points to the lower left means a heating-dominated climate. A cloud stretching to the right at a low humidity ratio means hot, dry air, which usually comes with large day-night swings and a large wet-bulb depression, the conditions for night ventilation and evaporative cooling. A cloud pressed towards the upper right means hot, humid air, where air movement is the usual remedy and dehumidification matters.

Count the hours rather than judging the shape: the share of hours inside the comfort zone, and inside each strategy zone, is what supports a design decision. Colouring the points by month or hour shows whether the difficult hours fall when the building is occupied.

How C4B shows the psychrometric chart

C4B plots every hour of the site's typical year on a psychrometric chart in its energy analysis, available on the Free plan with weather-station data. In projects on paid plans, the Passive Design Strategies for Indoor Environments module reports the share of hours covered by seven strategies - internal gains, passive solar heating, solar protection, natural and forced ventilation, mass cooling with night ventilation, evaporative cooling and high thermal mass - ranked month by month. The chart itself, with those strategy zones drawn on it, is in the Designer module on the Technical Plan, alongside an hour-by-hour carpet of the year and a monthly breakdown.