Why Does My House Smell Musty After It Rains?

Quick answer: A musty smell after rain usually means indoor humidity has climbed higher than your home can handle, or water is getting somewhere it shouldn't. In Central Texas homes, check the AC drain line and coil, the slab edge, the space under a pier-and-beam house and small leaks around windows and doors.

What does a musty smell after rain usually mean?

Usually too much indoor humidity, water getting somewhere it shouldn't, or both.

You probably know the pattern. It rains for a day, the air turns thick and warm, and by evening the house smells musty. A few dry days later, the smell fades. Then the next storm rolls through and it's back.

A smell that follows the weather is a clue. The job is to learn which moisture is behind it: the air, the ground under and around the house, or a leak.

Why does rain make a home smell different?

Rain changes two things at once in Central Texas:

• Humidity jumps. Outdoor humidity can go from comfortable to near 100 percent in an hour. Humid air finds its way inside through doors, gaps and your HVAC system, and damp air carries odors better than dry air.
• Water collects around the house. Many Austin-area clay soils swell and hold water. Rain that doesn't drain away sits against the foundation, under the house or around the AC equipment.

Most musty smells after rain come from one of those two things, or both.

Where do musty smells come from in Central Texas homes?

Is indoor humidity climbing past 55 percent?

It often is on mild rainy days, because your AC may barely run. When it doesn't run, it doesn't pull moisture out of the air, and indoor humidity climbs. Fabrics, carpet, closets and cabinets pick up that moisture and start to smell stale. An oversized AC makes this worse, because it cools the house fast and shuts off before it can dry the air.

An inexpensive hygrometer (a small humidity gauge) from any hardware store will tell you where you stand. If it reads above 55 percent on rainy days, humidity is likely part of the story. Our Central Texas humidity guide explains how to bring it down.

Could it be the AC drain line or drain pan?

Yes, and it's a frequent one. Your air conditioner pulls a lot of water out of the air, and that water runs to a drain pan and out a drain line. If the line is partly clogged or the pan holds standing water, it can smell musty, and the system blows that smell through the house. Rainy weeks, when the system works hard to dry the air, can make it more noticeable.

Could it be a dirty coil or ductwork?

It can. The indoor coil stays wet whenever the AC runs. Dust and debris on a wet coil, or in ducts running through a hot, humid attic, can add a musty smell that rides along with the airflow. If the smell is strongest right when the system kicks on, start with the HVAC system.

Mold in your AC system or ductwork? Call Gold Eagle Services, a licensed Texas mold remediation company that handles HVAC mold, at 512-489-4653. For mold anywhere else in the home, contact an independent state-licensed mold assessor or remediation company.

What about water against the slab?

It's a common cause when the smell sits along exterior walls. Most homes here sit on concrete slabs, and when gutters overflow, downspouts dump right next to the house, or the yard slopes toward the foundation, water pools against the slab edge. Concrete wicks moisture, and flooring, baseboards and carpet pad along those walls can pick up a musty smell after every heavy rain.

What if the house is on pier and beam?

Then check the crawl space under the house. Older homes in central Austin and some Hill Country houses sit on pier-and-beam foundations. That space can stay damp for days after rain, and the damp air rises into the rooms above through gaps in the floor. Our fall crawl space check shows what to look for.

Could a small leak cause it?

Yes, even one you can't see. A slow leak around a window, a door threshold, a chimney or roof flashing may never leave a visible stain. It can still dampen the wall or the attic enough to smell after a storm.

Could it be a dry drain trap?

Sometimes. A drain trap is the U-shaped bend under a drain that holds water to block sewer gas. In a guest bath or floor drain that rarely gets used, the water evaporates and sewer gas can drift in, and people often notice it more on stormy days. Many describe it as musty or "off." Our guide to smelly drains and dry traps explains the quick fix.

How do you narrow it down?

Try this the next time it rains:

1. Note where the smell is strongest. One room? Near exterior walls? Near the HVAC closet? The whole house?
2. Check indoor humidity with a hygrometer in a few rooms.
3. Stand near a supply vent when the AC starts. If the smell comes out with the first blast of air, look at the HVAC system first.
4. Walk outside during or right after the rain. Watch where gutters and downspouts send water and whether it pools against the house.
5. Look for signs of water: stains, bubbling paint, a damp baseboard, a warped floorboard, or a wet spot on the carpet near a wall.

What can you do about it?

Match the fix to what you found:

• Humidity: set the AC fan to "auto" rather than "on" during humid weather, so moisture on the coil drains away instead of blowing back into the house. Consider a dehumidifier. Our guide to choosing a dehumidifier can help.
• HVAC: have the coil, drain pan and drain line cleaned and checked. Our sister company, Gold Eagle Services, cleans coils, pans and drain lines, and can also check whether your system is sized right for your home.
• Drainage: clean gutters, extend downspouts several feet away from the house, and regrade soil that slopes toward the foundation.
• Leaks: call a roofer or a window and door pro. Stop the water before anything else.
• Anything that looks like more than a smell: if you see visible growth, soft drywall or water damage, stop and call the right licensed professional before you clean or treat anything. Our guide on who to call for indoor air problems lays out who handles what.

Where does a whole-home treatment fit?

A whole-home treatment comes after the water is handled and humidity is under control. A home that smelled musty for months or years often still carries it on hard surfaces, in closets and inside the ductwork. Once the source is fixed, Pure Maintenance Austin can help freshen what's left.

Our whole-home dry fog treatment sends a fine fog of hydrogen peroxide and peracetic acid solution, made by the patented Pure Maintenance VaPURE system, into the rooms, closets and ductwork where a rainy season's mustiness settled, without getting things wet. A surface protectant follows. On treated hard, non-porous surfaces it helps control the growth of odor-causing microorganisms, and the job takes one day. It is not a repair, though. If the next storm still puts water against the slab, the smell will come back.

Common questions

Is a musty smell after rain normal?

A faint earthy smell outdoors after rain is normal. Inside the house, a musty smell that shows up every time it rains usually means moisture or humidity needs attention somewhere. It's worth tracking down.

Why does it only smell in one room?

A smell limited to one room usually points to something local: a window or door leak, water against that side of the slab, a supply vent near a damp spot in the attic, or a closet with no airflow. Start your search there.

Who should I call first?

If there's active water, call a plumber or roofer. If the smell comes from the vents or the HVAC closet, call your HVAC company. If it's a lingering smell after everything is fixed and dry, call us.

Get a free phone consultation

Book a free phone consultation. A Comfort Advisor will call you, learn what's going on in your home and send a written quote based on square footage. Pure Maintenance Austin, brought to you by Gold Eagle Services, serves all of Central Texas, including College Station, Round Rock, Georgetown, San Marcos, Bastrop and Marble Falls.

Call 512-904-4646 or email IAQ@goldeagleservices.com. We're open Monday through Saturday, 7am to 7pm.