Dew Point Calculator for Your Home
Most dew point calculators are written for weather forecasting. This one answers the question you actually have indoors: at your room temperature and humidity, which surfaces in the house are cold enough to collect water tonight. Enter two readings from any cheap hygrometer, and it returns the dew point and a condensation risk assessment for windows, walls and ceilings. Everything runs in your browser.
What the dew point tells you
Warm air holds more water vapour than cold air. Cool a parcel of air and its capacity shrinks until, at one specific temperature, the vapour it carries is all it can hold. That temperature is the dew point, and any surface sitting at or below it takes water out of the air as liquid. Mist on a cold drink, a fogged bathroom mirror, and the film on a bedroom window at 7am are the same event at three temperatures.
The calculator uses the Magnus formula, the standard approximation for dew point from temperature and relative humidity, accurate to a fraction of a degree across the whole indoor range. The useful number it produces is the spread: the gap between your room temperature and the dew point. A wide spread means only unusually cold surfaces can condense water. A narrow one means ordinary walls are at risk, and a home whose air sits within 3 degrees of its dew point has a moisture problem whether or not anything looks wet yet.
Indoor surfaces are colder than the room they sit in, which is the detail generic calculators skip. Window glass runs closest to the outdoor temperature, commonly 5 to 8 degrees Celsius below the room on a cold night for double glazing and further for single. Exterior wall corners and ceilings under an unheated loft typically run 3 to 5 degrees below the room, which is why condensation mold draws its familiar map: window edges first, then corners, then the ceiling perimeter.
Will the windows stream tonight?
Take the room reading in the evening, then estimate the glass temperature: on a cold night, single glazing sits within 2 or 3 degrees Celsius of outdoors, and decent double glazing roughly halfway between outdoors and the room. Put that figure in the optional surface field, and the calculator tells you whether the glass drops below the dew point overnight and the exact humidity that would keep it dry. That second number is the practical one, because glass temperature is set by the weather and the windows, while evening humidity is set by the household.
Bedroom windows and walls
Two sleeping adults release around a pint of moisture into a closed bedroom overnight while the heating is off and the glass cools. That combination is why bedrooms stream when no other room does, and why growth appears behind wardrobes on exterior walls, where furniture blocks the warmth of room air from reaching an already cold surface. Leaving the door ajar, cracking the trickle vents, and keeping headboards and wardrobes an inch off exterior walls all widen the spread where it is narrowest.
Bathroom ceilings
A shower drives the bathroom close to 100 percent humidity, which puts the dew point at the air temperature itself; every surface condenses, and the ceiling, coldest of them because of the loft above, dries last. Growth overhead follows that physics, arriving in corners and along edges where the surface runs coldest. The guide to mold on a bathroom ceiling covers telling condensation growth from a leak and the safe way to clear it.
Vents, ducts and cold pipes
Air conditioning ducts and cold water pipes are often the coldest things in a house, and in humid weather they condense water inside cavities where nobody sees it happen. Dark speckling around a supply register is frequently this mechanism at work, and mold in air vents traces it back to the coil and duct surfaces upstream of the grille you can see.
Taking readings the calculator can trust
A digital hygrometer costing a few pounds or dollars supplies both inputs. Place it at chest height on an interior wall, away from radiators, direct sun, kitchens mid-cooking and bathroom doorways, because each of those skews either the temperature or the humidity reading by enough to move the dew point a degree or two. Give it half an hour to settle after moving it between rooms.
Timing matters as much as placement. The reading that predicts streaming windows is the late-evening one, taken after cooking and showers have added their moisture and before the heating drops for the night. A morning reading in the same room typically shows higher relative humidity at a lower temperature, which is the overnight condensation cycle caught in the act. Readings worth comparing: the dampest-feeling room against the hallway, upstairs against downstairs, and the same room in the evening against first thing.
Expect seasonal movement. Summer air indoors commonly sits at 50 to 65 percent relative humidity and the dew point runs high, but surfaces are warm, so little condenses. Winter air reads drier, often 40 to 55 percent, while glass and exterior walls run cold enough that the smaller amount of moisture lands on them anyway. The spread between room temperature and dew point, which the calculator reports with every result, is the number that stays honest across both seasons; the relative humidity figure on its own flatters winter and slanders summer.
Lowering the dew point indoors
- Run the extractor fan during showers and for 20 to 30 minutes after, with the bathroom door shut
- Air the house for ten minutes of wide-open window in winter; cold air carries the moisture out
- Keep background heat on in cold weather, since warm surfaces sit further from the dew point
- Dry laundry outdoors, in a vented dryer, or in one closed room with a dehumidifier
- Hold 30 to 50 percent humidity, checked with a hygrometer rather than guessed
- Leave a finger-width gap behind furniture on exterior walls so room air keeps the surface warm
- Drying washing on radiators, which loads around 5 pints per load into the warmest, wettest air
- Blocking trickle vents and airbricks to stop draughts, which traps the moisture with you
- Turning heating fully off in empty rooms, whose cold surfaces then collect the whole house’s moisture
- Wiping the windows every morning and treating that as the fix; the water is a reading, not the problem
- Running unvented gas or paraffin heaters, which release roughly a litre of water per litre of fuel
- Relying on scented products against a damp smell instead of finding what the moisture is feeding
From condensation to mold, and when it matters for health
Condensation is the delivery mechanism for most indoor mold that is not caused by a leak. A surface that drops below the dew point for a few hours each night collects water faster than it dries through the day, and a colony can establish within days on paint, plaster or timber that never fully dries. Liquid water is not even required at the margins: sustained relative humidity above 60 percent supports growth on its own, and some species manage it at 70 percent with no wet surface anywhere. Keeping the whole house between 30 and 50 percent removes that route entirely, and where the air will not stay in range on ventilation alone, our dehumidifier size calculator gives the capacity that gets it there.
A recurring musty smell is the same physics announcing itself before anything is visible, and the guide to tracking a musty smell to its source gives the search order room by room. Damp that has run for months carries a health dimension as well as a decorating one: for people who react to biotoxins, long exposure shows up as fatigue, brain fog and congestion that ease away from home. The free visual contrast sensitivity test screens for one measurable effect of that exposure in about 15 minutes, in your browser.
Dew point and condensation questions
Why do my windows stream with condensation at night?
Overnight the heating drops, the glass cools toward the outdoor temperature, and the moisture your household added all evening through cooking, showers and breathing stays in the air. Once the glass falls below the dew point of the room air, water condenses onto it continuously until morning. Lowering evening humidity and keeping bedroom doors slightly open both reduce it, and the calculator shows the humidity your glass temperature can tolerate.
What should indoor humidity be in winter?
Between 30 and 50 percent relative humidity, and toward the lower half of that range in cold weather. Surfaces run colder in winter, so air that was safe at 55 percent in summer condenses on windows and wall corners when the same humidity meets colder glass and plaster. Below 30 percent the air starts to dry skin, eyes and wood.
What dew point feels uncomfortable indoors?
People start noticing air as humid once the dew point passes about 60 degrees Fahrenheit, or 15 to 16 degrees Celsius, whatever the thermometer says. Above a dew point of 65 F the air feels heavy, and above 70 F oppressive. Indoors the more useful reading is relative humidity, since the condensation and mold thresholds are set in those terms.
Does opening a window help with condensation?
In winter, usually yes. Cold outdoor air holds little absolute moisture, so exchanging inside air for outside air lowers the dew point indoors even though the incoming air feels raw. Ten minutes of wide-open window moves more moisture than hours of a small gap. The exception is humid summer weather, when outdoor air brings moisture in, which is when mechanical dehumidification earns its keep.
What temperature stops condensation on walls?
A surface stays dry while it is warmer than the dew point of the air touching it. At 20 degrees Celsius and 60 percent humidity the dew point is about 12 degrees, so any wall patch below 12 degrees collects water. Raising the room temperature, insulating the cold spot, or lowering humidity all widen the gap; which is cheapest depends on whether the cold spot is one corner or the whole building envelope.
Damp House, Unexplained Symptoms?
Condensation you can see usually means humidity the whole house is breathing. If damp has been around a while and you feel better away from home, the free Visual Contrast Sensitivity test screens for a measurable effect of biotoxin exposure.