Bathroom humidity, condensation and mold in Central Ohio, month by month
A bathroom gets wet twice over in Central Ohio: in winter it is the one damp room in a dry house, and in summer the whole house starts out humid. This page gives the numbers behind both, the dew point and relative humidity at the Columbus airport month by month from the National Oceanic and Atmospheric Administration, explains condensation in plain words with those figures, and sets beside them what the Environmental Protection Agency says about mold and what Ohio's residential code asks of a bathroom fan and an outside wall.
Last updated September 2026 · Weather figures are NOAA normals for 1991 to 2020 and NOAA hourly observations; guidance is the EPA's and the code is the Residential Code of Ohio, each read September 24, 2026 · No prices on this page; the Estimate Builder carries those · Part of the knowledge base
Two words that explain a wet bathroom
Dew point is the temperature at which air holding a given amount of water vapor becomes saturated. Any surface colder than the air's dew point collects water: a mirror, a window, a cold-water pipe, a toilet tank, or the cold back of an outside wall. Relative humidity describes the same water in relation to the air's temperature. Warm the air and its relative humidity drops, though the amount of water in it has not changed; cool it and the relative humidity climbs until, at the dew point, it reaches 100 percent.
That is why the same bathroom fogs in January and sweats in July for different reasons. In winter the room is the wet place in a dry house. In summer the whole house starts out damp before anybody showers.
Columbus, month by month
The National Oceanic and Atmospheric Administration keeps dew point normals, its thirty-year averages for 1991 to 2020, at only one station in the seven Central Ohio counties: the Columbus airport, John Glenn. NOAA publishes those dew point normals hour by hour, not month by month, so the monthly figures below are the averages of NOAA's hourly normals for every hour of each month. The morning and afternoon relative humidity comes from NOAA's comparative climate table, which covers 1948 to 2018. The hours at or above 60 and 65 degrees are counted from NOAA's hourly observations at the same airport for 1991 to 2020. The last column is arithmetic, not a measurement: the relative humidity that month's average outdoor air would have if it were warmed to 70 degrees indoors with no water added.
| Month | Normal high / low (degrees) | Average dew point (degrees) | Relative humidity, 7 a.m. / 1 p.m. (percent) | Hours a month with dew point 60 or higher | Hours with dew point 65 or higher | Outdoor air warmed to 70 degrees (percent relative humidity) |
|---|---|---|---|---|---|---|
| January | 37.1 / 22.0 | 22.3 | 78 / 68 | 0 | 0 | 16 |
| February | 40.8 / 24.2 | 23.4 | 78 / 64 | 1 | 0 | 17 |
| March | 51.1 / 32.0 | 29.8 | 76 / 57 | 2 | 0 | 22 |
| April | 64.1 / 42.2 | 39.4 | 75 / 52 | 24 | 0 | 33 |
| May | 74.1 / 52.4 | 50.6 | 78 / 53 | 191 | 41 | 50 |
| June | 82.2 / 61.6 | 59.5 | 79 / 54 | 400 | 202 | 69 |
| July | 85.4 / 65.4 | 63.4 | 82 / 55 | 537 | 335 | 80 |
| August | 84.1 / 63.9 | 62.4 | 85 / 55 | 500 | 284 | 77 |
| September | 77.8 / 56.5 | 56.0 | 86 / 55 | 265 | 114 | 61 |
| October | 65.5 / 44.8 | 44.3 | 83 / 55 | 65 | 14 | 40 |
| November | 52.3 / 35.0 | 33.9 | 80 / 61 | 7 | 0 | 26 |
| December | 41.5 / 27.4 | 27.1 | 80 / 68 | 1 | 0 | 20 |
| The year | 42.8 | 80 / 58 | 1,994 | 991 |
Read down the dew point column and the year has two halves. From November through March the average dew point sits below 34 degrees, and the air brought in and heated is desert-dry. From June through August it sits near 60 or above, and on an average July day the dew point is at or above 60 degrees for most of the day's hours. Relative humidity is higher at 7 a.m. than at 1 p.m. in every month, and the 7 a.m. figure peaks at 86 percent in September.
The most recent decade ran damper than the thirty-year average. Counted the same way from the same airport's hourly observations for 2015 to 2024, the dew point was at or above 60 degrees for about 2,307 hours a year against 1,994 for 1991 to 2020, and at or above 65 degrees for about 1,188 against 991. Ten years is a short window, and no source read for this page attributes the difference to anything, so the two periods are set side by side here and not called a trend.
Is the Columbus airport the whole region?
Close to it. None of the other airports in the seven counties has NOAA humidity normals, but six of them have hourly observations complete enough to compare, and their observations for 2015 to 2024, averaged the same way, put the monthly dew point within about 2.6 degrees of the Columbus airport in every month. A seventh, Bolton Field, reports hourly but had too many gaps to use. Rickenbacker and the Lancaster airport recorded more hours at 65 and 70 degrees; the Delaware airport recorded the fewest. No source explains the differences, which could be the instruments, the sites or the places themselves. The Marysville and Delaware figures rest on fewer complete months than the others.
| Station | January average dew point (degrees) | July average dew point (degrees) | Hours a year with dew point 60 or higher | Hours with dew point 65 or higher | Hours with dew point 70 or higher |
|---|---|---|---|---|---|
| Columbus, John Glenn airport | 23.3 | 64.8 | 2,307 | 1,188 | 269 |
| Columbus, Ohio State University airport | 23.4 | 64.8 | 2,262 | 1,162 | 272 |
| Rickenbacker | 23.8 | 65.3 | 2,240 | 1,325 | 371 |
| Lancaster, Fairfield County airport | 23.9 | 66.1 | 2,418 | 1,308 | 373 |
| Newark-Heath | 22.7 | 64.9 | 2,232 | 1,142 | 257 |
| Marysville, Union County airport | 23.1 | 64.1 | 2,055 | 1,148 | 246 |
| Delaware Municipal | 24.1 | 63.5 | 1,892 | 833 | 199 |
Winter: a dry house and a wet bathroom
January air at the Columbus airport averages a dew point of about 22 degrees. Heated to 70 degrees with nothing added, that air has a relative humidity of about 16 percent, which is why a Central Ohio house feels dry in winter, and why the Environmental Protection Agency's ideal indoor range of 30 to 50 percent is hard to reach without adding water.
A shower adds it all at once, to one room. The table below gives the dew point of room air at a few combinations of temperature and relative humidity, computed with the vapor pressure formula the National Weather Service publishes. The room conditions are examples, not measurements of any house. Bathroom air at 70 degrees and 90 percent relative humidity has a dew point near 67 degrees, so any surface in the room colder than that gets wet. A window pane or an outside wall, sitting nearer the outdoor temperature, gets wet first. The exhaust fan's job is to carry that water outdoors before it settles.
| Room air (temperature, relative humidity) | Dew point |
|---|---|
| 70 degrees, 30 percent | 37.2 degrees |
| 70 degrees, 40 percent | 44.6 degrees |
| 70 degrees, 50 percent | 50.5 degrees |
| 70 degrees, 60 percent | 55.5 degrees |
| 70 degrees, 90 percent | 66.9 degrees |
| 75 degrees, 90 percent | 71.9 degrees |
Where warm, wet bathroom air leaks into an outside wall, it can reach a surface near the outdoor temperature and condense inside the wall, where nobody sees it. The code's answer is a vapor retarder: Ohio's residential code places all seven counties in Climate Zone 5A, and in Zone 5 it requires a Class I or II vapor retarder on the interior side of frame walls. Kraft-faced insulation batts count as Class II. The code allows latex or enamel paint, a Class III retarder, in Zone 5 only where the wall has vented cladding outside, or continuous outside insulation of at least R-5 over a 2x4 wall or R-7.5 over a 2x6 wall.
Summer: humid before the shower starts
In July the average dew point at the Columbus airport is 63.4 degrees, and one July hour in ten runs above about 71. Outdoor July air at the average dew point sits at about 80 percent relative humidity at 70 degrees, already above the Environmental Protection Agency's 60 percent ceiling before a shower adds anything. Any surface colder than about 63 degrees sweats in air like that, and a cold-water line or a toilet tank refilled with cold water can be one. The EPA's own list of ways to prevent condensation includes insulating cold surfaces such as cold-water pipes. Opening a window in July lets in air that is already humid. Among the EPA's ways to lower indoor humidity are air conditioners and dehumidifiers, alongside running the bathroom fan or opening the window when showering.
What the Environmental Protection Agency says
The EPA's guide to mold and moisture says to keep indoor relative humidity below 60 percent, and ideally between 30 and 50 percent, and to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth. For a bathroom it names running the exhaust fan or opening the window when showering, and it says condensation can be a sign of high humidity. Its cleanup page says that more ventilation and more frequent cleaning will usually keep bathroom mold from coming back, and that a moldy area smaller than about 10 square feet, roughly 3 feet by 3 feet, is in most cases one a homeowner can handle; above that it points to its remediation guide.
What Ohio's residential code asks of a bathroom
These are the sections of the Residential Code of Ohio, in effect since July 1, 2019, that bear on bathroom moisture. The fan rate matches the International Residential Code's table, except that Ohio's title adds three-family dwellings. The window-or-fan rule and the fan rate are set out with the rest of the bathroom's electrical rules on the page on bathroom electrical in Ohio, and what a fan does in plain words is on the page on the bathroom exhaust fan.
| What it covers | What the code requires | Where |
|---|---|---|
| A window or a fan | A window of at least 3 square feet of glass, half of it openable, or artificial light and a local exhaust fan discharging directly to the outdoors | Section 303.3 |
| How much the fan moves | An exhaust capacity of 50 cubic feet a minute run as needed, or 20 run all the time | Table 1505.4.4 |
| Where the air goes | Directly to the outdoors, not recirculated, and never into an attic, soffit, ridge vent or crawl space | Sections 1505.2 and 1501.1 |
| The rating | The fan listed and labeled for its airflow under the standard test | Section 1505.3 |
| How long the duct may run | At a 50 cubic-feet-a-minute rating, 56 feet of 4-inch flexible duct or 114 feet of 4-inch smooth duct, less 15 feet for each elbow, unless the maker or a measured flow allows more | Table 1504.2 |
| Where the cap goes | At least 3 feet from a property line and from an operable window or door, and 10 feet from a mechanical air intake unless 3 feet above it | Section 1504.3 |
| The damper | Automatic or gravity dampers that close when the fan is off | Section 1103.6 |
| Inside the outside wall | A Class I or II vapor retarder on the interior side of frame walls in Climate Zone 5, which covers all seven counties | Section 702.7 and Table 1101.7 |
What the ENERGY STAR label on a bath fan means
An ENERGY STAR certified bathroom fan must move at least 2.8 cubic feet a minute per watt if it is rated 10 to 89 cubic feet a minute, and at least 3.5 if it is rated 90 to 200, and a fan up to 200 must be no louder than 2.0 sones. It must also still deliver at least 70 percent of its airflow at the higher of two test pressures, which ENERGY STAR describes as a fan operating under a heavier load. The Department of Energy's Building America Solution Center summarizes the ventilation standard ASHRAE 62.2 and the model residential code the same way the Ohio table reads: at least 50 cubic feet a minute run as needed, or 20 run continuously.
What these numbers do not say
Every figure here describes the air at an airport weather station, averaged over years. None of it measures a house, and no source read for this page gives the surface temperature of a window or a wall in a Central Ohio home, so this page gives none. A hygrometer in the room says what one bathroom actually has.
The short version. At the Columbus airport the average dew point runs from about 22 degrees in January to about 63 in July, averaged from NOAA's 1991 to 2020 hourly normals, and the dew point is 60 degrees or higher for about 1,994 hours in an average year. Winter air heated indoors is dry, near 16 percent relative humidity, so a shower makes the bathroom the wet room in a dry house; summer air is already near 80 percent at 70 degrees. The EPA says keep indoor humidity below 60 percent and dry wet materials within 24 to 48 hours. Ohio's code asks for a bathroom fan of 50 cubic feet a minute as needed or 20 continuous, vented directly outdoors, and a vapor retarder on the inside of frame walls in all seven counties.
Sources
Every figure, date and quotation on this page comes from one of these documents, read on the date given. Where two of them disagree, the page says so.
- National Oceanic and Atmospheric Administration, National Centers for Environmental Information, U.S. Climate Normals, 1991 to 2020, hourly normals (dew point) and monthly normals (temperature) for Columbus John Glenn, station USW00014821, through the NCEI data service; read September 24, 2026.
- National Centers for Environmental Information, Comparative Climatic Data, table Average Relative Humidity, Morning and Afternoon, Columbus, period of record 1948 to 2018; read September 24, 2026.
- National Centers for Environmental Information, Integrated Surface Database, ISD-Lite hourly observations, seven Central Ohio airport stations, 1991 to 2024, hours counted for this page; read September 24, 2026.
- National Weather Service, Vapor Pressure calculation notes, the formula used for the computed columns; read September 24, 2026.
- U.S. Environmental Protection Agency, A Brief Guide to Mold, Moisture and Your Home, last updated February 18, 2026; read September 24, 2026.
- U.S. Environmental Protection Agency, Mold Cleanup in Your Home, last updated June 4, 2026; read September 24, 2026.
- Ohio Administrative Code (Ohio Board of Building Standards), Residential Code of Ohio: rule 4101:8-3-01 (chapter 3), rule 4101:8-7-01 (chapter 7), rule 4101:8-11-01 (chapter 11) and rule 4101:8-15-01 (chapter 15), each effective July 1, 2019; read September 24, 2026.
- International Code Council, International Residential Code, Chapter 15, Exhaust Systems (2018 edition); the Code Council site refused the request on September 24, 2026, so the table was not re-read there.
- ENERGY STAR, Ventilation Fans Key Product Criteria, undated; read September 24, 2026.
- U.S. Department of Energy, Building America Solution Center, Bathroom Exhaust Fans, last updated December 4, 2022; read September 24, 2026.
Common questions
What is the average dew point in Columbus in summer?
Averaged from NOAA's 1991 to 2020 hourly normals at the Columbus airport, the dew point averages 59.5 degrees in June, 63.4 in July and 62.4 in August. NOAA's hourly observations at the same airport put the dew point at 65 degrees or higher for about 335 hours in an average July, and at 60 or higher for about 537 of July's 744 hours.
How humid is Columbus in the morning and in the afternoon?
NOAA's comparative climate table gives Columbus an average relative humidity of 80 percent at 7 a.m. and 58 percent at 1 p.m. across the year, for 1948 to 2018. The mornings run highest in August and September, at 85 and 86 percent, and the afternoons lowest in April, at 52 percent.
Why does a Central Ohio house feel so dry in winter?
Outdoor air in a Columbus January averages a dew point of about 22 degrees. Heated to 70 degrees with no water added, that air has a relative humidity of about 16 percent, below the 30 to 50 percent the Environmental Protection Agency calls ideal indoors. Showers, cooking and breathing add water back, and a shower adds a great deal of it to one room at once.
Why do bathroom mirrors and windows fog after a shower?
Any surface colder than the dew point of the air around it collects water. Bathroom air at 70 degrees and 90 percent relative humidity has a dew point near 67 degrees, so a mirror, a window or an outside wall colder than that gets wet. A window in winter, sitting nearer the outdoor temperature than the rest of the room, gets wet first. An exhaust fan ducted outdoors carries that moisture out of the room.
What indoor humidity does the EPA recommend to prevent mold?
The Environmental Protection Agency says to keep indoor relative humidity below 60 percent, and ideally between 30 and 50 percent. It also says to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth, and that more ventilation and more frequent cleaning will usually keep bathroom mold from coming back.
What does Ohio's residential code require of a bathroom exhaust fan?
Section 303.3 of the Residential Code of Ohio asks for a bathroom window of at least 3 square feet, half of it openable, or else artificial light and an exhaust fan. Where there is a fan, Table 1505.4.4 sets its capacity at 50 cubic feet a minute run as needed, or 20 run continuously, and sections 1505.2 and 1501.1 require the air to go directly to the outdoors and never into an attic, soffit, ridge vent or crawl space.
Does Ohio require a vapor retarder in bathroom walls?
In frame walls, yes. The Residential Code of Ohio places Franklin, Delaware, Licking, Fairfield, Union, Madison and Pickaway counties in Climate Zone 5A, and section 702.7 requires a Class I or II vapor retarder on the interior side of frame walls in Zone 5, with exceptions for basement walls, below-grade walls and construction that moisture or freezing will not damage. Kraft-faced insulation batts count as Class II.
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