Where is mold hiding in my house?

Pick a room on the drawing. Show all eight as a list

1. 1Attic and roof

   Most attic mold starts with water from above. A monsoon storm or a tired roof lets rain in, and an attic can stay damp long after the sky clears.

   Common causes in Las Vegas homes

   • Monsoon storms push extra moisture into the valley in July and August¹, and they often roll in from the mountains around town².
   • Roof leaks are one of the places CDC names where mold grows³.
   • An air handler or ducts in the attic can sweat or drip if the drain line clogs. EPA says to keep drip pans clean and drain lines flowing⁴.

   First steps

   1. Find and stop the water first. EPA says mold will most likely come back if the water problem is not fixed⁵.
   2. Dry wet insulation and wood within 24 to 48 hours if you can⁶.
   3. Wet insulation is porous. EPA says porous materials may have to be thrown away if they get moldy⁷.

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2. 2Bathroom and shower

   Steam, splash and a slow drip under the vanity. A bathroom gets wet every day, so it only needs a small leak or a weak fan to tip into mold.

   Common causes in Las Vegas homes

   • Condensation on walls, windows or pipes is a sign of high humidity⁸.
   • Toilets leak more than people think. The water district says more than 20 percent of gravity flush toilets leak⁹.
   • A shower pan or supply line leak inside the wall stays hidden. EPA says a musty smell with no mold in sight can point to hidden mold¹⁰.

   First steps

   1. Scrub mold off hard surfaces like tile with detergent and water, then dry it fully¹¹.
   2. Do not paint or caulk over mold. EPA says paint over mold is likely to peel¹².
   3. Keep indoor humidity below 60 percent, ideally 30 to 50 percent¹³.

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3. 3Kitchen sink and dishwasher

   The cabinet under the sink is dark, closed and full of pipe joints. A slow drip can soak the cabinet floor for weeks before anyone looks.

   Common causes in Las Vegas homes

   • CDC lists leaks around pipes as a place mold grows³.
   • Las Vegas tap water is very hard, at 291 ppm or 17 grains per gallon in the water district's current figure¹⁴.
   • A meter that spins with every tap off can mean a hidden leak. The water district says to check the flow triangle on the meter¹⁵.

   First steps

   1. Shut the supply valve, empty the cabinet and let air reach the wet wood.
   2. Dry what got wet within 24 to 48 hours, and most of the time mold will not grow¹⁶.
   3. A particleboard cabinet floor is porous. If mold gets into it, EPA says it may need to go⁷.

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4. 4Laundry and water heater closet

   A washer hose, a drain box and a water heater often share one small closet. When one of them leaks, the wall behind takes the water.

   Common causes in Las Vegas homes

   • A water heater sits on the floor of that closet, and CDC lists leaks around pipes as a place mold grows³. In the City of Las Vegas, replacing a water heater needs a permit¹⁷.
   • Damage from contaminated water such as a backed up drain needs a professional with that experience, says EPA¹⁸.
   • CDC says Stachybotrys, a greenish black mold, grows on materials with a lot of cellulose like gypsum board and paper, and needs constant moisture¹⁹²⁰.

   First steps

   1. Turn off the water to the heater or washer and stop the source.
   2. Pull the appliance out and dry the floor and the bottom of the wall.
   3. If the moldy patch is larger than about 10 square feet, EPA points to its larger job guidance²¹.

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5. 5AC air handler and ducts

   An air conditioner pulls water out of the air all summer. That water drains through a pan and a line, and when either one stops up, mold has a wet, dark place to start.

   Common causes in Las Vegas homes

   • Las Vegas has a normal of 79 days a year at or above 100 degrees²², so the AC runs long and makes a lot of condensate.
   • EPA says to keep air conditioning drip pans clean and drain lines open and flowing⁴.
   • EPA says only an expert can tell for sure whether growth in a duct is mold²³.

   First steps

   1. If you know or suspect the system has mold, EPA says do not run it, because it could spread mold through the house²⁴.
   2. Fix the moisture cause. EPA says mold in ducts comes back if the cause is not fixed²⁵.
   3. EPA does not recommend routine duct cleaning, only cleaning as needed²⁶.

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6. 6Evaporative cooler

   A swamp cooler runs on water by design. A cracked pan, a stuck float or a leaking feed line puts that water onto the roof deck or into the wall below.

   Common causes in Las Vegas homes

   • SNWA says evaporative cooling is the second largest consumptive use of water in Southern Nevada²⁷.
   • Any steady drip is a moisture source, and EPA says the key to mold control is moisture control²⁸.
   • Monsoon months raise outdoor humidity¹, which is when a cooler works hardest for the least relief.

   First steps

   1. Shut the water feed to the cooler and look for stains on the ceiling under it.
   2. Check the meter flow triangle with the cooler off to see if water still moves¹⁵.
   3. Dry wet ceiling drywall within 24 to 48 hours, or plan to cut it out⁶.

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7. 7Exterior walls and windows

   Wind driven rain finds the gaps around windows and doors. The stain shows up inside on the drywall under the sill, often weeks after the storm.

   Common causes in Las Vegas homes

   • CDC names leaks around windows as a common place for mold³.
   • Nevada law counts effective waterproofing of the roof, exterior walls, windows and doors as part of a habitable rental²⁹.
   • The flood control district says July through September bring sudden downpours, even on sunny days³⁰.

   First steps

   1. Note the date of the last storm and the stain size, and take photos.
   2. Renters should report leaks and moisture to the owner or manager right away, says EPA³¹.
   3. In the City of Las Vegas, drywall repair needs no permit only if no framing is touched and the wall is not fire rated³².

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8. 8Slab and floors

   A pipe that runs under or through a concrete slab can leak for a long time before it shows. The first sign is often a warm spot, a damp baseboard or a musty carpet edge.

   Common causes in Las Vegas homes

   • Expansive soils have been documented in the Las Vegas Valley for many years, says a UNLV study³³.
   • UNLV research says a foundation sitting partly on caliche and partly on soft soil can settle unevenly³⁴.
   • New water district meters can detect very low flows, and the district may alert you to check for a leak³⁵.

   First steps

   1. Turn off every tap and watch the meter flow triangle¹⁵.
   2. SNWA offers single family homeowners a voucher worth up to $250 toward leak diagnosis by a Water Smart Plumber³⁶.
   3. In the City of Las Vegas, repairing leaking concealed piping needs a permit³⁷.

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