Last Updated on October 24, 2026 by Umar Farooq
The ideal indoor humidity in summer is 30-50% relative humidity, with 60% as the ceiling you do not cross. In a warm house, 45-50% is the realistic target; there is little to gain from chasing 30%.
That is the same band the EPA gives all year round. What changes in July is not the target but how hard it is to hit, because the air outside is now doing the opposite of what it does in January.
What is the ideal indoor humidity in summer?
Three limits cover it, and all three come from sources I opened rather than from memory:
- 30-50% relative humidity is the range the EPA calls ideal, and ENERGY STAR puts the optimum for a building at the same 30-50%.
- Below 60% is the EPA’s outer limit: “if possible, keep indoor humidity below 60 percent”. Above it you are in the territory where damp smells, condensation and mould risk start to turn up.
- A dew point under about 13°C (55°F) is where the US National Weather Service says summer air feels “dry and comfortable”. Between 13 and 18°C it turns sticky, and above that, in the Service’s words, oppressive.
For the general bands across the whole year, including the too-dry end, see what indoor humidity should be. Winter pulls the ceiling down to protect windows, which is covered in the ideal home humidity level in winter; summer does the opposite and lets you sit near the top of the band.
My own view: in summer, aim for 40-50% and stop. Below 40% you are paying for extra drying that nobody can feel, and the cooling coil of an air conditioner will happily do some of it anyway if you let it run.
Why a thermostat tells you nothing about how muggy it is
A thermostat reads temperature and nothing else. A house at 22.5°C can be pleasant or swampy, and the display looks identical either way.
You need a hygrometer, a small, inexpensive gadget (the EPA mentions the same thing). Put it in the room you care about, away from the bathroom door and out of direct sun, and read it at the same time of day for a week. One reading at 3pm after a shower tells you about the shower.
Dew point explains a muggy house better than a percentage
Relative humidity is a ratio, and it swings with temperature even when the amount of water in the air has not changed. Dew point is the actual moisture content expressed as a temperature, which is why the Weather Service uses it for comfort. Its own example makes the point: an 80°F day with a 60°F dew point is only 50% RH, yet feels more humid than a 30°F day at 100% RH.
I worked the figures in the diagram out with the Magnus formula, so treat them as my arithmetic, rounded, not as a published table. The useful result is that at 24°C the EPA’s 50% top-of-ideal lands almost exactly on the Weather Service’s 13°C comfort edge. Two independent organisations, one boundary. Rather neat.
It also explains the summer problem. If the air outside has a dew point of 18°C, air let in through a window cannot be drier than that, so a house kept at 24°C by opening windows is sitting around 70% RH. No amount of fresh air fixes it; the outdoor air is the damp one.
Why the AC leaves some summer houses clammy
An air conditioner dries the air as a side effect of cooling it. Air passes over a cold coil, water condenses out and runs to the drain. I have covered the mechanism in whether air conditioning removes humidity.
The catch is cycling. A unit that is too big cools the room quickly, hits the thermostat setting and switches off before the coil has run long enough to pull much water out. You get a cold, damp room, and by the dew point arithmetic above, 22°C at 65% RH is a damper house than 25°C at 45%, however cool it feels. Two quick checks:
- Does the unit switch on and off every few minutes? That is short cycling, and it is a sizing problem, not a settings one.
- Is the drain line flowing and the drip pan clean? The EPA specifically advises keeping air conditioning drip pans clean and drain lines unobstructed, because a blocked one becomes a moisture source of its own.
Dry or “dehumidify” mode, where the unit has one, runs the coil without chasing the temperature down, and is the first thing to try before buying anything.
Basements and cellars: summer is when they get worse
Ground-level rooms stay cool through July. Warm, humid air from the rest of the house or from an open window meets those cool walls and floor, and the air next to the surface cools towards its dew point. The air in the room can read a respectable-looking 55% while the wall is effectively damp.
That is why a basement hygrometer tends to climb from June and why throwing the windows open on a humid day can make it worse, not better. The full explanation, with fixes, is in why a basement is humid in summer.
What to do when the hygrometer reads over 55%
Work down this list and stop when the reading drops. It is ordered by cost.
- Check for a source before treating the air. Drying laundry indoors, a leaking pipe, a wet cellar floor, a humidifier someone forgot to switch off. No machine out-runs a running tap.
- Run the extractor fan in the bathroom and kitchen during use and for 15 minutes after, with the door shut so the steam goes outside rather than down the landing.
- Close windows in the muggy part of the day, not just when it is hot. Open them in the cool early hours if the outdoor dew point is lower than indoors.
- Let the AC run properly. Longer cycles, a clean filter and the dehumidify setting do more than a lower temperature.
- Add a dehumidifier if you are still above 55-60% after a week of that. A small one in a hot living room can lift comfort noticeably, and the summer-specific advice on running one covers placement and settings.
One safety note, because it is summer and people reach for whatever is to hand: do not run a dehumidifier on a long, thin extension lead. Plug it straight into the wall or use a properly rated one.
A 45-50% reading is the goal, and 60% is the line
If your hygrometer sits at 45-50% on summer mornings, leave it alone. Between 50 and 60% is a grey area to act on only if you can feel it or see condensation. Over 60% on repeated days, find the cause and deal with it, because that is where the EPA’s guidance stops being a preference and becomes a ceiling.
Short version: the ideal indoor humidity in summer is a number most houses can hold at 50% or below using the AC, the extractor fan and a closed window at the right time, with a dehumidifier as the backstop and not the plan.
FAQ
Is 60% humidity too high for a house in summer?
It is at the top of what the EPA considers acceptable (“below 60 percent”), and above its 30-50% ideal. Occasional readings around 60% on a very muggy day are not alarming; a reading that sits there for days is a sign something is adding moisture or the cooling is not drying the air.
Is 70% humidity bad inside the house?
At 24°C it works out, on my own calculation, at a dew point of about 18°C, which the Weather Service describes as oppressive. It is well above the EPA’s 60% ceiling, so it is worth finding the source and bringing it down.
Should I run the AC or a dehumidifier to lower summer humidity?
Start with the AC, since cooling air dries it, and check that it runs long cycles rather than switching on and off. Add a dehumidifier if the reading stays above 55-60% afterwards, or if the cooling is off at night when the damp builds up.
What humidity should I set the AC to?
Most air conditioners have no humidity setting, only temperature and perhaps a dry mode. Use a separate hygrometer and aim for 40-50% while the unit runs.
Is it bad to have 30% humidity in summer?
It is the bottom of the EPA’s range, so it is not harmful on paper, but it gains you nothing in comfort, and running an air conditioner for long enough to get there costs money. If it sits below 30% for weeks, a little added moisture is reasonable.
