Houston runs an annual average of 74.7% relative humidity with a dew point of 72.1 °F. Hold 45–55% indoors and the rest of this page is how.
- Annual relative humidity
- 74.7%
- July dew point
- 72.1 °F
- Annual rainfall
- 49.77 in
- Days at or above 90 °F
- 102.4
- Indoor target
- 45–55%
The climate, in numbers that matter to a building
Houston sits in a humid subtropical climate (Köppen Cfa) on the Gulf, and the number that defines it is not the temperature but the dew point. A July dew point of 72.1 °F means the outdoor air is carrying enough water that any surface below 72 °F is condensing — which in an air-conditioned house includes almost every interior surface, every supply register, and every duct running through the attic.
The relative humidity figures tell the same story from another angle. The annual average is 74.7%, and it barely moves across the year: the driest month averages 72.7% and the wettest 76.8%. What changes seasonally is the temperature, not the moisture. Morning readings routinely exceed 90% and fall to around 60% by mid-afternoon, which is a much smaller daily swing than inland Texas gets and leaves far less opportunity to ventilate usefully.
Nearly 50 inches of rain a year, on 104 days, means the ground around a Houston house is rarely dry for long. Combined with a high water table in much of the region and expansive clay soils that move with moisture, this is a climate where the water outside the building is as much of a design problem as the water inside it.
Through the year in Houston
| Season | What the outdoor air does | What that does to a house here |
|---|---|---|
| Spring | Dew points climb through the sixties; May and June are the most humid months of the year | Air conditioning starts running for moisture rather than heat; this is when clammy 74 °F rooms begin |
| Summer | Dew points in the low seventies day and night, over 100 days above 90 °F | Peak load. Ducts sweat in hot attics, supply registers drip, and anything stored in a garage or unconditioned space grows mildew |
| Fall | Dew points fall slowly; the first genuinely dry air often arrives in late October | The first weeks when opening windows actually helps. Good time to dry out and clean up what summer did |
| Winter | Mild, with brief cold snaps and dew points that swing widely within a week | Short cold spells put condensation on single-pane glass; the rest of the winter is the easiest time of year for a house here |
What this climate does to a house here
The problems below are the ones that turn up repeatedly in Houston specifically, rather than the general list.
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Air conditioning that cools without drying
The signature Houston complaint: the thermostat says 74 °F and the house still feels wet. An oversized system reaches the set temperature in a few minutes and shuts off, and because moisture is only removed while the coil runs, the water stays in the house. The reading to check is the humidity, not the temperature — anything in the sixties with the system satisfied confirms it.
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Ducts and registers sweating in the attic
A Houston attic in July runs far above the outdoor temperature while the air inside the ducts is around 55 °F. Any gap in duct insulation is a surface far below the attic dew point, so it condenses, drips onto the ceiling below, and produces a brown stain that gets misdiagnosed as a roof leak every summer.
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Mold in closets, on exterior walls and behind furniture
Air conditioning cools wall surfaces below the dew point of any humid air that reaches them. In a closet on an exterior wall — no airflow, no supply register, a door closed all day — the local humidity runs ten to fifteen points above the room and growth appears on shoes, leather and the wall itself.
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Garages, sheds and anything unconditioned
Unconditioned space in this climate spends the summer at or near the outdoor dew point. Tools rust, stored cardboard collapses, and anything organic in a garage or shed grows something. This is not a fault in the building; it is what 72 °F dew points do to unconditioned volumes.
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Flooding and the drying that follows
Houston floods, and the humidity climate makes recovery much harder than it would be elsewhere. Materials that would air-dry in a week in Denver may never dry in Houston without mechanical dehumidification, because the air being used to dry them is already carrying most of the water it can hold.
What is your reading telling you?
- Dew point
- 53 °F
- Mold risk, cold walls
- Moderate
Still inside the accepted range, but this is where cold surfaces begin to sweat.
Every fix that works here, cheapest first
The full ladder for a Houston house, cheapest first. Unlike a cold-climate list, ventilation is not near the top here — for most of the year the outdoor air is the source of the problem rather than the cure.
Stop ventilating with summer air
From roughly April to October the outdoor dew point is higher than the indoor one, so every open window and every hour of a whole-house fan adds water. Check the outdoor dew point rather than the percentage before opening anything. This single habit is worth more in Houston than anywhere else in the country.
Cut the moisture you make
Lids on pans, the bathroom fan running during and fifteen minutes after every shower with the door shut, and no laundry dried indoors. In a climate where the air outside cannot absorb your surplus, indoor sources matter more, not less.
Open up closets and move furniture off exterior walls
Two inches of clearance behind a dresser and a closet door left ajar raise the surface temperature and let air move. Costs nothing, and it removes the exact conditions that grow mold on cooled exterior walls.
Make the air conditioner run longer, not colder
Set the fan to auto rather than on — a fan running after the compressor stops re-evaporates the water sitting on the coil and puts it straight back into the house. If your system has a dehumidify or comfort mode, use it. Raising the thermostat a degree and letting the system run longer cycles usually lowers the humidity rather than raising it.
Exhaust fans that actually reach outside
A bathroom fan ducted into the attic instead of through the roof puts every shower into the roof space, where in a Houston summer it has nowhere to go. Check where each duct terminates, insulate the run, and add a run-on timer.
A dehumidifier, sized under the DOE 2019 test
For a damp room under 1,000 sq ft, 20 to 30 pints a day set to 50% is enough. Whole-house ducted units in the 70 to 100 pint range are common in Houston for good reason: they let the air conditioner handle temperature and the dehumidifier handle water, instead of asking one machine to do both badly.
An ERV rather than an HRV
Houses need fresh air, and in this climate the recovery ventilator you want is an energy recovery ventilator, which transfers moisture as well as heat between the incoming and outgoing streams. A heat recovery ventilator, which transfers heat only, brings the Gulf dew point straight indoors and is the wrong machine for zone 2A.
Seal the ducts, the attic and the crawl space
Sealing and insulating duct runs stops the condensation that stains ceilings. An unvented, conditioned attic keeps the ducts out of the 130 °F humid air entirely. Where a house is on pier and beam, encapsulating the crawl space with a sealed ground cover and a small dehumidifier is the durable fix — venting it in this climate pulls Gulf air onto cool joists.
Remediate what has already grown, then keep it dry
Growth under about 10 square feet on a hard surface is a homeowner job with detergent, an N95 and containment. Anything in drywall paper, insulation or ducting needs cutting out. None of it holds unless the moisture is fixed first, which in Houston means the dehumidification strategy above rather than a coat of mold-resistant paint.
Local construction, and why it changes the answer
Two construction patterns dominate the Houston area and each has its own failure mode. Post-war and modern housing is overwhelmingly slab-on-grade, which removes the crawl space problem and replaces it with a slab that stays cooler than the summer air — so anything stored directly on a garage or utility-room floor condenses on the underside. Older housing inside the loop is frequently pier and beam over a vented crawl space, which in a 72 °F dew point climate is a reliable way to condense Gulf air onto floor joists all summer.
The other local factor is the soil. Much of the region sits on expansive clay that swells when wet and shrinks when dry, which moves foundations and opens the cracks that let humid air into wall cavities and crawl spaces. Keeping roof water away from the building — working gutters, downspouts discharging well clear, and grading that falls away from the slab — is a humidity measure here as much as a foundation one.
Finally, air conditioning in Houston is not a comfort appliance but the primary moisture control for the whole house. That makes correct sizing more important than efficiency ratings: a modestly sized variable-speed system that runs for long periods will hold a house at 50% far better than an oversized single-stage unit with a better label on it.
Common questions
What should I set my thermostat to in Houston to control humidity?
Set the temperature for comfort — most people land between 74 and 78 °F — and control the humidity separately rather than by chasing a colder setting. Put the fan on auto, use a dehumidify mode if the system has one, and if the reading still sits above 55% in summer, add a dehumidifier rather than dropping the thermostat. A colder setting on an oversized system produces shorter cycles and often a wetter house.
Should I run a dehumidifier and the air conditioning at the same time in Houston?
Yes, and in this climate that combination is normal rather than wasteful. The air conditioner is sized for heat and removes water as a side effect; the dehumidifier removes water directly and adds a little heat doing it. Running both lets you hold 50% at a comfortable temperature instead of overcooling the house to squeeze water out of it.
Is a whole-house fan a good idea in Houston?
For about six weeks a year, in late fall and on cool spring nights, and not otherwise. A whole-house fan pulls the entire outdoor air mass through the building, which is exactly what you want when the outdoor dew point is in the fifties and exactly what you must avoid when it is in the seventies. Check the dew point before switching it on, every time.
Why does my ceiling have brown stains after a hot week with no rain?
That is almost always duct condensation rather than a roof leak. In a Houston attic the air around the ducts is hot and very humid while the metal and the insulation jacket are near 55 °F, so any gap in the insulation condenses and drips. The tell is that it appears during hot humid weather rather than during rain, and it appears in a line following a duct run.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions
- NOAA / NWS 1991–2020 Climate Normals, via the city climate tables on Wikipedia
- US Department of Energy — Building America climate zone definitions
- EPA — Indoor Air Quality: moisture control and mold
- DOE 10 CFR 430 — dehumidifier test procedure (2019 method)