Indoor Humidity in Las Vegas Homes: What to Measure and What It Means

Las Vegas is one of the driest cities in the country, but a dry climate doesn’t guarantee a dry house. Here is how relative humidity works, what EPA recommends, how to take your own readings, and when the numbers point to a moisture problem worth a closer look.

Relative humidity in plain terms

Relative humidity (RH) is the amount of water vapor in the air compared with the most that air could hold at its current temperature, shown as a percentage. Temperature matters because warm air can hold more water vapor than cool air. If you cool air without taking water out, its RH goes up. Cool it far enough and the vapor condenses into liquid water. That is why a cold soda can “sweats” on a warm day.

The same thing happens indoors. A room can read a comfortable 40% while the air against a cold duct or supply register is much closer to saturation. Surface conditions matter more than the room average.

The target range: what EPA and others say

EPA’s guide to mold and moisture says: “If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.” ENERGY STAR also calls 30% to 50% the range generally considered optimal for a building. The CDC is a little stricter and advises keeping home humidity “no higher than 50%.” In practice, the 30–50% band is a sensible target, and readings that stay above 60% are the ones to act on.

These are guidelines for controlling moisture. They are not a health threshold. If you or someone in your household has symptoms you think could be related to mold or dampness, talk with a physician. The CDC notes that damp and moldy environments can cause a range of health effects, or none, and that people with asthma or mold allergies may react more strongly.

Measuring humidity with an inexpensive hygrometer

EPA notes that relative humidity “can be measured with a moisture or humidity meter, a small, inexpensive ($10-$50) instrument available at many hardware stores.” A digital hygrometer is a useful habit-builder. Some tips for getting readings you can actually use:

• Let it settle for a while after moving it to a new room.
• Keep it away from showers, kettles, and supply registers, or it reports that spot, not the room.
• Log over days. Note the time, room, reading, and activity (showers, cooking, cooler or AC running).
• Compare rooms. One room that stays well above the rest is a better clue than the whole-house average.

Know the limits

A hygrometer measures only the air around it. It can’t tell you whether drywall or a cabinet base is wet or see inside walls. Accuracy varies by model, so check the manufacturer’s stated tolerance and don’t read much into a few points of difference. A normal reading doesn’t rule out a hidden leak, and a spike after a shower isn’t a problem by itself.

Why desert homes still get damp spots

The National Weather Service calls Las Vegas “the driest major metropolitan area in the continental United States,” with 4.18 inches of normal annual precipitation (1991–2020). Most indoor moisture here comes from inside the house:

• Evaporative (swamp) coolers. The Department of Energy’s Building America Solution Center explains that direct evaporative coolers “increase rather than decrease the humidity in the air,” and that doors, windows, or relief vents must be open so air can exit. When a cooler runs without enough relief air, humid air has nowhere to go and builds up indoors. See our swamp cooler mold guide.
• Bathrooms and laundry. Showers, baths, and unvented dryers add a lot of moisture in a short time. EPA recommends running the bathroom fan or opening a window while showering and venting dryers outdoors. More in our bathroom ventilation guide.
• Cooking and dishwashing. EPA suggests exhaust fans or open windows for these too.
• Condensation on cold air-conditioning surfaces. Supply ducts, registers, and refrigerant lines carry air or refrigerant well below room temperature. When they’re poorly insulated or sit in humid air, water can condense on them. EPA’s condensation advice includes insulating cold surfaces and reducing humidity. Drain lines and drip pans need to flow freely too. See HVAC condensation and mold.
• Localized leaks. A slow drip under a sink, a failed supply line, or a weeping water heater can keep one area wet while the rest of the house reads dry. Our moisture sources guide covers these in more depth.

Summer vs. winter in Las Vegas

The NWS notes that in July and August a push of monsoon moisture “moves into the Mojave Desert bringing higher than average humidity” and scattered thunderstorms. Heavy storm rain makes late summer a good time to check roof penetrations and window seals. High outdoor humidity also reduces how well evaporative coolers work.

In the 1991–2020 normals, the wettest months are in winter: February averages 0.80 inch and January 0.56, compared with 0.04 in June. Coolers are usually off by then. Cold windows and exterior walls can collect condensation where indoor moisture runs high, such as a bathroom without an exhaust fan or a room where laundry dries indoors.

Dehumidifier, ventilation, or fixing the source?

These tools do different jobs.

• Fix the water source first. EPA’s core message is that “the key to mold control is moisture control.” A leak, a clogged condensate line, or a cooler with no air relief will keep producing moisture no matter what else you do. EPA also says wet materials dried within 24–48 hours usually won’t grow mold. See what to do in the first 48 hours after a leak.
• Ventilate where moisture is made. Bath and kitchen exhaust fans vented to the outdoors remove moisture right where it’s produced.
• Use a dehumidifier for persistent dampness that remains after sources are fixed. ENERGY STAR notes that certified models are more efficient than non-certified ones. In a desert home, a room that seems to need one constantly is a reason to keep looking for the source.

When readings suggest a problem worth investigating

• One room or closet stays noticeably higher than the rest of the house for days, with no obvious source.
• Indoor readings regularly stay above 60% even though you use exhaust fans and give an evaporative cooler plenty of air relief.
• You see condensation on ducts, registers, windows, walls, or pipes, or rust and water stains near them.
• A musty odor persists, or you see staining, bubbling paint, or warped baseboards.
• There was a leak or flood, and you aren’t sure everything dried within a day or two.

A mold inspection is limited to mold and moisture. It is not an evaluation of the home’s systems or structure. See mold inspection in Las Vegas and our process.

Sources

• A Brief Guide to Mold, Moisture, and Your Home — U.S. EPA
• About Mold and Your Health — CDC
• Dehumidifiers — ENERGY STAR
• Evaporative Cooling Systems — Building America Solution Center, U.S. Department of Energy
• Climate of Las Vegas — National Weather Service Las Vegas
• Precipitation Overview, 1991–2020 Normals — National Weather Service Las Vegas / NOAA NCEI