Dew point and condensation
Why windows, walls and pipes sweat
Water appears on any surface colder than the air’s dew point. Chill the window, raise the room’s humidity, and watch the air walk left into the saturation curve.
Condensation forms whenever a surface is colder than the dew point of the air touching it [1]. That one rule explains the fogged bottom edge of a window in January, the beads on a cold-water pipe in August, the drip from an air-conditioning duct in a humid attic and the damp patch on a wall over an uninsulated stud. On the psychrometric chart it is a single move: air that touches a cold surface is cooled without losing water, so it walks to the left, and if it reaches the saturation curve before it reaches the surface’s temperature, the water it can no longer carry comes out on the surface.
Dew point, not relative humidity, decides
Most people meet humidity as relative humidity, the number on a thermostat or a weather app. It is a ratio: the water vapour in the air compared with the most the air could hold at its present temperature [2]. Change the temperature and the RH changes even though the water has not: the 0 °C, 80 % RH air of a winter morning becomes 20 % RH air once the furnace has warmed it to 21 °C. Knowing only the RH, as the EPA’s moisture guide puts it, is not much help in predicting condensation [3].
The dew point is different. It is the temperature at which the air’s own water vapour would saturate it, and it depends only on how much water the air carries (and, slightly, on the barometric pressure), not on how warm the air happens to be. On the chart it is the end of the horizontal walk to the left from the air’s point. That is why it predicts condensation so directly: if a surface is colder than the dew point, water vapour condenses onto it; if it is warmer, it does not [3].
How much colder than the room does a surface have to be?
Turn the question around and ask how many degrees of chill it takes. Figure 1 plots the gap between a room’s temperature and its dew point against relative humidity, the same relationship the EPA’s moisture guide draws as its Figure 1-19 [3]. The dashed horizontal lines show how much colder than the room the inside surface of single, double and triple glazing runs at the outdoor temperature you choose.
The curve falls steeply. At 50 % RH a surface must be about 11 °C (20 °F) colder than the room before water appears; in the EPA’s words, few surfaces in a building are that much colder than room air under ordinary circumstances [3]. A room at 21 °C has a dew point of 2.8 °C at 30 % RH, 6.9 °C at 40 %, 10.2 °C at 50 % and 12.9 °C at 60 %. At 90 % RH the gap is under 2 °C, and almost every building has surfaces that much colder than the air somewhere. At the default outdoor temperature of −10 °C, the line for single glass sits above the curve over most of the range: it is the first surface in the house to get wet.
Winter windows
In cold weather the window is usually the coldest thing on the inside of a wall, and it sets the practical limit on indoor humidity. That was the point of one of the first Canadian Building Digests, A. G. Wilson’s 1960 “Condensation on Inside Window Surfaces”, which tabulated the inside glass temperature and the highest indoor relative humidity that each kind of glazing tolerates without condensation [1]. With the room at 70 °F, still air indoors and no sun, single glass at 0 °F outdoors runs at 35 °F and fogs above 27 % RH; double glazing runs at 50 °F and tolerates 49 %; triple glazing runs at 56 °F and tolerates 61 % [1].
The figure at the top of this page uses those measurements. In Wilson’s table the inside surface of each glazing sits a nearly fixed fraction of the way from the room temperature to the outdoor temperature: about half for single glass, 0.29 for double and 0.2 for triple. With the room at 21 °C and −10 °C outside, that puts single glass at 5.5 °C, where condensation starts at 36 % RH; double glazing at 12 °C, good to 56 %; triple at 14.8 °C, good to 68 %. Turn the outdoor temperature down and every limit falls.
This is why the same humidity that is comfortable in October fogs the windows in January, and why advice to “keep the humidity down in winter” depends on the windows. The EPA advises keeping indoor relative humidity below 60 percent, and ideally between 30 and 50 [5]; in a very cold spell, with older windows, the upper half of that range already reaches the glass. Condensation on windows is also a useful signal: it can be a sign of high humidity indoors [5].
Summer: cold ducts, cold pipes and humid air
In summer the walk to the left runs the other way round. The cold surfaces are the ones an air conditioner or the water supply makes, and the humid air comes from outdoors.
Figure 2 shows the numbers. The air at A carries 15.8 g of water per kilogram of dry air. Its walk to the left reaches the saturation curve at 21 °C, the dew point; below that it can only follow the curve down, and at the duct’s 13 °C it can carry 9.2 g/kg. The other 6.6 g/kg is water on the duct. This is the EPA’s own example: in Miami, outdoor air has a dew point above 70 °F for most of the year, while a supply duct carrying 55 °F air has a surface near 55 °F, so infiltrating outdoor air that reaches the duct condenses on it and possibly on the supply diffuser [3]. Drag B above 21 °C and the sentence under the figure changes: the air cools, its RH rises, and nothing condenses. The same move makes cold-water pipes sweat in a humid basement (room air at 24 °C and 60 % RH has a dew point of 15.8 °C, so any pipe colder than that drips) and makes the outside of a cold glass wet on a summer day.
Inside an air conditioner the move is deliberate. The cooling coil is kept below the dew point of the air crossing it, so it condenses water, which runs to a drain pan. That is how an air conditioner dehumidifies, and it is step 9 of the tour of the chart. A coil that runs too warm, or air that bypasses it, leaves more water in the house.
Inside walls and roofs
The same rule holds for surfaces you cannot see. In cold weather the sheathing and cladding outside the insulation run close to the outdoor temperature, and condensation is most likely on the inside of exterior walls and roofs [3]. Indoors, a wall surface over a gap in the insulation or over an uninsulated framing member can sit below the room’s dew point long enough for water to accumulate and mould to grow; and where a hole in the air barrier lets cold outdoor air in, it can bypass the insulation and chill indoor surfaces below the dew point [3].
How a wall or roof should be built to stay dry depends on the climate, the materials and the local code, and this page does not prescribe vapour retarders or assemblies. What the chart adds is the test that every design is trying to pass: at each surface where air can reach, is the surface warmer than the dew point of that air?
Two levers: lower the dew point or warm the surface
Because condensation needs both a cold surface and air with a dew point above it, there are only two ways to stop it [3]. The first is to lower the indoor dew point: remove water from the air faster than people, cooking, washing, showers and damp basements add it. A dehumidifier or an air conditioner does it with a cold coil. Ventilation does it only when the outdoor air is drier: as the EPA notes, ventilating dehumidifies the indoors only when the outdoor dew point is lower than the indoor one [3]. In a humid summer, opening windows can add water.
The second is to warm the surface: better glazing, insulation without gaps, air barriers that keep cold air from bypassing the insulation, and room air that can reach the glass instead of being trapped behind heavy curtains. The two levers are why a house with new triple-glazed windows can be run at a humidity that would stream down the old single panes.
When to call someone. Occasional fog on a window is a sign of humidity, not a defect. Condensation that keeps returning inside walls, ceilings or attics, wet insulation, a musty smell, or mould growing on building materials points to a moisture problem that needs a building-science professional or a qualified home inspector to find; the EPA’s guide starts with finding and fixing the source of the water [5]. Air conditioners, dehumidifiers and their refrigerant circuits are serviced by licensed HVAC technicians. Nothing here is a statement of what any building code requires.
Back to the chart
Every case on this page is one move on the psychrometric chart: a horizontal walk to the left from the air’s point, ending either at the surface’s temperature, with the surface dry, or at the saturation curve, with water to spare. The calculator gives the dew point from any thermometer and hygrometer reading, in °C or °F, at your altitude.