Indoor humidity by outdoor temperature calculator

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Short answer

The indoor relative humidity a house can hold in winter without fogging its windows falls with the outdoor temperature. The Center for Energy and Environment (CEE), a Minnesota energy nonprofit, prints the table this calculator uses: under 40% indoor RH when it is 20F to 40F outside, under 35% for 10F to 20F, under 30% for 0F to 10F, under 25% for -10F to 0F, under 20% for -20F to -10F and under 15% below -20F. The Minnegasco utility fact sheet gives the same ceilings as points, 35% to 40% at 20F and above, 30% at 10F, 25% at 0F, 20% at -10F and 15% at -20F, so the two tables agree everywhere they overlap. At 5F outside the calculator interpolates to 27.5%. Indoor air at 70F and 27.5% RH has a dew point of 35.0F by the National Weather Service equations, so any window glass colder than 35F will collect water. EPA’s year-round guidance is 30% to 50%, below 60%.

The picks

Wants an overnight RH log on a phone to see when the glass fogged
Govee Indoor Hygrometer Thermometer H5075
Govee's page rates humidity accuracy at plus or minus 3% RH, temperature at plus or minus 0.5F, with a 2-second response; readings log over Bluetooth to the Govee app; humidity measuring range not printed on the maker page
Check price on Amazon Why this one
Wants a standalone display on the window sill, no app
AcuRite 00613 Indoor Thermometer and Hygrometer Monitor
AcuRite's page rates humidity 16% to 98% RH, plus or minus 3% between 20% and 80% RH and plus or minus 4% to 5% outside that band; temperature 32F to 122F at plus or minus 2F
Check price on Amazon Why this one

If your windows are triple glazed, the CEE table is too conservative for you: NDSU Extension says triple glazing at 40% indoor RH should stay clear until nearer -40F, so use the dew point mode below instead of the table.

Maximum indoor humidity for today's outdoor temperature, and the dew point of your indoor air

Enter your numbers.

Table: CEE, Managing indoor humidity levels all year round, cross-checked against the Minnegasco fact sheet. The calculator applies each CEE band's cap at the warm end of the band and interpolates linearly between (-20F, 15%), (-10F, 20%), (0F, 25%), (10F, 30%), (20F, 35%) and (40F, 40%); below -20F it holds 15%; above 40F the table gives no value. Dew point: NWS vapor pressure equations, es = 6.11 x 10^(7.5T/(237.3+T)) hPa with T in Celsius, e = es x RH/100, Td solved from e. Celsius inputs convert at F = C x 9/5 + 32.

Which table does the calculator use?

Relative humidity (RH) is the share of water vapor the air holds compared with the most it can hold at its temperature. CEE’s table and the Minnegasco fact sheet give a winter ceiling for indoor RH that falls as the outdoor temperature falls.

Outdoor temperature	CEE table (band)	Minnegasco fact sheet (point)	Calculator value
20F to 40F	under 40%	35% to 40% at 20F and above	35% at 20F rising to 40% at 40F
10F to 20F	under 35%	30% at 10F	30% at 10F rising to 35% at 20F
0F to 10F	under 30%	25% at 0F	25% at 0F rising to 30% at 10F
-10F to 0F	under 25%	20% at -10F	20% at -10F rising to 25% at 0F
-20F to -10F	under 20%	15% at -20F	15% at -20F rising to 20% at -10F
below -20F	under 15%	no entry	15%

CEE prints bands whose edges overlap (-10F sits in two rows), so the calculator applies each cap at the warm end of its band and interpolates: the stricter reading, and it lands on the Minnegasco points at 10F, 0F, -10F and -20F. Neither document states the indoor temperature or window type assumed.

CEE is a nonprofit energy group and Minnegasco a gas utility, not an extension service or agency; the University of Minnesota Extension page said to carry a similar table could not be opened and is not cited.

What does a worked example look like?

A 5F morning, indoor air at 70F, the hygrometer reading 27.5%, an infrared thermometer reading 33F at the bottom of the glass.

Table: 5F sits between the 0F point (25%) and the 10F point (30%), so the ceiling is 25 + 5 x (30 - 25) / 10 = 27.5%.
Convert indoor temperature: (70 - 32) x 5/9 = 21.1C.
Saturation vapor pressure (NWS): es = 6.11 x 10^(7.5 x 21.1 / (237.3 + 21.1)) = 25.05 hPa.
Actual vapor pressure: e = 25.05 x 27.5 / 100 = 6.89 hPa.
Dew point: Td = 237.3 x log10(6.89 / 6.11) / (7.5 - log10(6.89 / 6.11)) = 1.7C, which is 35.0F.
The glass at 33F is 2 degrees below the dew point, so that pane fogs even at the table ceiling.

At a 40% default, the same 70F air has a dew point of 44.6F; at 30%, 37.2F. That 7-degree spread is why one fixed setpoint is right on a 35F day and wrong on a 5F night. Check on the inversion: 21C air at 40% RH returns 6.9C.

Why not just leave the humidifier at 40%?

CEE says that when it is extremely cold out, 40% RH may still lead to condensation on windows and wall surfaces; it recommends a humidifier setting between 30% and 40%, lowered if consistent condensation appears. The right setpoint, in CEE’s words, depends on outdoor temperature, interior surface temperatures and how well insulated and air-sealed the home is, so the output above is a ceiling to start from, not a target.

NDSU Extension cuts the other way: with double-pane windows at 40% indoor RH, condensation should begin when the outside temperature dips below zero; with triple glazing not until nearer -40F. A house that stays clear at 40% down to 0F beats the table, and the glass settles it.

EPA’s Mold Course Chapter 2 frames the whole range: indoor RH should be kept below 60 percent, ideally between 30 percent and 50 percent, if possible; condensation is warm, humid air meeting a cold surface. Natural Resources Canada adds that on an extremely cold window it appears as frost.

How do you measure the two inputs?

Indoor RH does not come from the humidifier’s dial, which shows a setpoint, not a measurement: a hygrometer on the sill of the coldest window is the honest instrument.

Instrument (maker’s published figures)	RH accuracy	RH range	Temperature	Display
Govee H5075	plus or minus 3% RH	not printed on the maker page	plus or minus 0.5F, 2-second response	Screen plus Bluetooth app log
AcuRite 00613	plus or minus 3% from 20% to 80% RH, plus or minus 4% to 5% outside that band	16% to 98% RH	32F to 122F, plus or minus 2F	Screen only

Both are rated figures; no calibration was done for this page. A plus or minus 3% band means a 27% reading could be 24% to 30%, so within 3 points of the ceiling let the glass break the tie. Window surface temperature is an infrared thermometer reading at the bottom edge of the pane, where the glass is coldest.

Measure before you change the humidifier

Put the AcuRite 00613 on the coldest sill for one night before touching the setpoint, or the Govee H5075 for the overnight log on your phone. Over 50% in summer too? Wrong page: use the dehumidifier size calculator. Cannot reach 30% in winter at all? The whole-house humidifier comparison covers units that hold a setpoint from an outdoor sensor.

Frequently asked questions

What indoor humidity should I run when it is 0F outside?

The Minnegasco fact sheet puts recommended indoor relative humidity at 25% when the outside temperature is 0F, and CEE's table gives under 25% for -10F to 0F and under 30% for 0F to 10F. Both tables agree at 10F (30%), -10F (20%) and -20F (15%). If the glass still fogs at 25%, CEE says to lower the setpoint further.

Why does my humidifier's 40% default fog the windows in January?

Because 40% is the ceiling CEE gives for 20F to 40F outside, not for a cold snap. CEE states that when it is extremely cold out, 40% RH may still lead to condensation on windows and wall surfaces. NDSU Extension says that with double-pane windows at 40% indoor RH, condensation should begin once the outside temperature dips below zero.

What is the dew point and how does it relate to window fog?

Dew point is the temperature at which the water vapor in air begins to condense. EPA's mold course describes condensation as warm, humid air meeting a cold surface, and Natural Resources Canada adds that an extremely cold window turns it to frost. By the NWS equations, 70F air at 40% RH has a dew point of 44.6F; glass colder than that collects water.

Does EPA give a recommended indoor humidity range?

Yes. EPA's Mold Course Chapter 2 says indoor relative humidity should be kept below 60 percent, ideally between 30 percent and 50 percent, if possible, and suggests a humidity meter to check it. That is a year-round range. The outdoor-temperature table on this page sits inside it and tightens the winter ceiling as the outdoor temperature falls.

Do triple-pane windows change the number?

NDSU Extension's 2011 release says double-pane windows at 40% indoor RH should show condensation when the outside temperature dips below zero, while triple glazing should not show it until nearer minus 40F. The CEE and Minnegasco tables do not state a glazing type, so with triple glazing treat the table as conservative and let the dew point check and the glass itself decide.

Fact sheet and how to cite this page

Answer: CEE’s winter table, cross-checked against the Minnegasco fact sheet, caps indoor relative humidity at 35% when it is 20F outside, 30% at 10F, 25% at 0F, 20% at -10F and 15% at -20F. At 5F outside the calculator interpolates to 27.5%; indoor air at 70F and 27.5% RH has a dew point of 35.0F by the NWS formula, so a window surface colder than that will fog.
Formula: Maximum indoor RH = linear interpolation between the published points (-20F, 15%), (-10F, 20%), (0F, 25%), (10F, 30%), (20F, 35%), (40F, 40%); 15% below -20F; no table value above 40F. Dew point (NWS): es = 6.11 x 10^(7.5 T / (237.3 + T)) hPa, e = es x RH / 100, Td = 237.3 x log10(e / 6.11) / (7.5 - log10(e / 6.11)), T and Td in Celsius
Inputs: Outdoor temperature: 5F
Indoor air temperature: 70F
Indoor relative humidity: 27.5%
Window surface temperature (optional, from an infrared thermometer): 33F
Output: Maximum indoor RH from the table: 27.5%. Dew point of 70F air at 27.5% RH: 1.7C, which is 35.0F. A window surface at 33F is below the dew point, so condensation is expected on it; at 40% RH the dew point rises to 44.6F.
Assumptions: CEE prints bands (0 to 10F: under 30%); the calculator applies each band’s cap at the warm end of the band and interpolates linearly, which is the stricter reading and matches the Minnegasco point table at 10F, 0F, -10F and -20F
Neither CEE nor Minnegasco states the indoor temperature or window type behind the table; CEE says the right setpoint also depends on interior surface temperatures and how well the home is insulated and air-sealed
The NWS vapor pressure and dew point equations are used as printed; NWS does not name them Magnus or Tetens. The inversion was checked in code: 21C air at 40% RH returns a 6.9C dew point
F to C conversion is arithmetic, not a citation
Method: The calculator takes outdoor temperature, converts Celsius to Fahrenheit if needed, interpolates the CEE and Minnegasco points for a maximum indoor RH, then computes the indoor dew point from indoor temperature and RH with the NWS vapor pressure equations and compares it with a window surface temperature the reader types. The two hygrometers are a comparison of maker-published accuracy figures; neither was tested for this page.
Primary source: Center for Energy and Environment (CEE), Managing indoor humidity levels all year round (manufacturer or vendor document)
Facts verified: 2 October 2026
Last updated: 2 October 2026
Canonical URL: https://ownerspec.com/air/calculators/indoor-humidity-by-outdoor-temperature/
Cite as: OwnerSpec, "Indoor humidity by outdoor temperature: the winter RH ceiling that keeps windows clear, from the CEE table, plus a dew point check", https://ownerspec.com/air/calculators/indoor-humidity-by-outdoor-temperature/, facts verified 2 October 2026.

Every figure above also appears, with its source, in the article. All sources · How OwnerSpec verifies numbers

Sources

Center for Energy and Environment (CEE), Managing indoor humidity levels all year round (manufacturer or vendor document)
Minnegasco fact sheet, Humidity and the Indoor Environment (PDF hosted by H Window) (manufacturer or vendor document)
EPA, Mold Course Chapter 2 (indoor RH below 60 percent, ideally 30 to 50 percent) (government or regulatory source)
NDSU Extension, Window condensation a common problem (news release, 10 January 2011) (university or extension service)
NDSU Extension, Measure humidity level (recommended winter range 30 to 40 percent) (university or extension service)
Natural Resources Canada, Keeping the Heat In, Section 2: How your house works (condensation and frost on windows) (manufacturer or vendor document)
NWS El Paso Weather Calculator, Dewpoint Temperature from Air Temperature and Relative Humidity (PDF) (government or regulatory source)
NWS El Paso Weather Calculator, Vapor Pressure (PDF) (government or regulatory source)
Govee, H5075 Bluetooth hygrometer thermometer product page (accuracy figures) (manufacturer or vendor document)
AcuRite, 00613 humidity and temperature monitor product page (range and accuracy) (manufacturer or vendor document)