Dew Point in a House: Window Fog and When to Ventilate

Why equal room temperatures can leave different windows wet

For a wet window, compare its measured inside surface temperature with the room dew point. For drying by ventilation, compare outdoor dew point with indoor dew point. These are different checks: room RH alone cannot tell you whether a pane will stay dry or whether outdoor air contains less moisture.

If the window surface is colder than the room’s dew point, moisture can collect on the glass or frame. The colder the surface and the longer the condition lasts, the more likely you are to see droplets, damp curtains, or wet sills. This does not prove a hidden problem, but repeated wet materials should be corrected because moisture that remains in place can damage finishes and storage.

Measure room air and compare the window surface

Measure room temperature and relative humidity in the occupied part of the room. Use those readings in a dew point calculator. A window or frame temperature requires a separate instrument suitable for the surface. Follow its instructions; an infrared reading on reflective glass or metal can include reflected surroundings. Do not treat an ordinary spot reading as an independently verified glass temperature.

Record timing. Morning condensation in a closed bedroom can reflect overnight moisture, cooling surfaces and local air movement. Water appearing during cooking gives you a moisture-producing event to compare, but does not prove the cause. Summer condensation on basement windows may involve humid air meeting cooler surfaces. Write down whether the water is on the glass, the frame, the sill, nearby drywall, or fabric.

A cold bedroom frame below the room dew point

This hypothetical bedroom uses assumed air and frame temperatures, not measured window data. Dew point is calculated from the example air readings; the frame temperature is a separate assumed input, not a prediction from humidity alone.

A north-facing bedroom reads 69 F and 58% relative humidity at 6:30 a.m. The Dew Point Calculator for Indoor Rooms estimates a dew point around the low 50s F. The center of the room is not extreme, but an aluminum frame beside the glass reads 47 F on a cold morning. That surface is below the room dew point, so condensation on the frame makes sense.

The fix is not simply “lower the thermostat.” First, reduce the moisture pulse and expose the cold area. Leave the door open after waking, pull the curtains back, wipe the sill dry, and run bathroom exhaust longer if showers happen nearby. If the same window is wet even when the room reads closer to 40%, the surface may be unusually cold or poorly mixed compared with the rest of the room.

How much does a small temperature difference mean?

The calculator displays tenths of a degree, but your instruments may not measure that accurately. A surface 0.1°C above the calculated dew point is not proof that the whole window will stay dry. Room sensor error, surface sensor error and local variation all affect the comparison.

Record the actual surface and method: center glass, lower edge and frame are different locations. Repeat uncertain measurements according to the instrument instructions. Do not fill a missing value with outdoor temperature or assume that double glazing has a particular surface temperature.

For immediate actions and water-location checks, use why windows are wet inside. This page explains the comparison and its uncertainty; a calculated threshold cannot identify a repair.

Fix water damage promptly. Paint, wood, drywall, and fabric that stay wet can deteriorate. Do not rely on a calculator when materials are already damaged.

Compare air dew point with a cold surface

The table below is an IndoorComfortKit planning preset calculated with the same Magnus approximation used by the site’s dew point calculator. It shows why relative humidity cannot be read without temperature. The figures are rounded site calculation results, not official thresholds or measurements of a particular pane or frame.

Room air	Relative humidity	Estimated dew point	What a 12 C window surface suggests
20 C	40%	6.0 C	Surface remains above the estimated dew point.
20 C	60%	12.0 C	Surface is at the estimated condensation threshold.
24 C	40%	9.6 C	Surface remains above the estimated dew point.
24 C	60%	15.8 C	Surface is colder than the estimated dew point.

Planning preset calculated from entered temperature and RH; it is not an EPA, DOE, or AMS threshold table. Alduchov and Eskridge’s cited paper supports the Magnus-form approximation family used for the calculation, but it does not define this table’s room interpretations. The U.S. EPA guide supports controlling moisture and responding to condensation. Current U.S. Department of Energy guidance supports the explanation that window thermal performance and condensation resistance affect interior moisture conditions.

Contrast example

The following window conditions are hypothetical comparisons. Any surface temperature in the example is assumed, not a value the room-air calculator can measure or verify.

Room A is 20 C at 60% RH and has an interior glass edge near 12 C. The air and surface are near the calculated dew point, so a thin line of morning water is plausible. Reducing the room moisture pulse and uncovering the glass may change the pattern. Room B is also 20 C at 60% RH, but its glass stays near 17 C. The same room reading does not predict condensation there because the surface remains warmer.

Now compare two windows in Room A. The center of a double-pane unit stays clear, while an aluminum edge and lower frame are wet. That local pattern points toward surface-temperature and air-mixing differences rather than proof that every wall in the room is damp. Dry the water, repeat the center-room reading after the sensor settles, and note whether curtains, blinds, or furniture isolate the cold edge overnight.

What dew point cannot tell you about a window

Dew point is a useful estimate, but it does not inspect the window, wall assembly, flashing, or insulation. EPA provides general moisture and condensation guidance; the U.S. Department of Energy explains how window-frame and glazing performance affect cold edges and condensation. Both sources are U.S.-oriented. Local climate, window construction, storm windows, heating systems, and building practices affect surface temperatures. This guide is not building inspection advice, HVAC engineering advice, medical advice, or safety certification.

What dew point should you open windows at?

There is no universal outdoor dew-point cutoff. For moisture removal, outdoor dew point needs to be lower than indoors at comparable air pressure. In a calculated example, 10°C / 80% RH outdoors gives about 6.7°C dew point, while 21°C / 55% RH indoors gives about 11.6°C. The higher outdoor RH percentage still describes drier air.

Use the indoor–outdoor dew point comparison with readings from the same time. Air quality, weather, security and heat loss remain separate checks. Do not reduce required ventilation based on this moisture comparison; it cannot predict a drying time or final room RH.

What does a window condensation-resistance rating mean?

NFRC’s optional Condensation Resistance rating compares window products under standardized conditions. A higher rating indicates greater resistance; it is not a room humidity percentage or a promise that a window will stay dry in your weather. Glass, edges and frame can behave differently.

The window condensation calculator compares a measured surface or a simplified U-factor scenario with room humidity. It does not calculate an NFRC rating or identify the coldest part of a real window.

Common questions

How does dew point cause window condensation?

Dew point does not cause water by itself. Condensation appears when the window glass, frame, pipe, or wall surface is at or below the dew point of the nearby air.

Why does one room have condensation and another room does not?

Rooms can have different humidity, air movement, window temperature, and sun exposure. A closed bedroom with heavy curtains may trap colder air near the glass even when the hallway looks fine.

Should I lower humidity or warm the window area?

Often both matter. Lowering the room dew point helps, but exposing the glass, improving air movement, and keeping wet surfaces dry can reduce condensation without overcorrecting the whole room.

For the heating season, compare a settled room reading with winter humidity and cold-window scenarios.

Related calculators

Use the Dew Point Calculator for Indoor Rooms first. Then check the Room Moisture Watch Calculator if sills, curtains, or nearby walls stay damp. The Indoor Humidity Calculator helps track whether the whole room is humid or only the window area is cold.