What a musty smell in the house is usually telling you

The water came off the roof, and the clay will not take it away

A crawlspace does not make water. Everything wet under a house arrived there, and the roof is the largest collector on the property. Aim a downspout at the base of the wall and the foundation is being watered on purpose, every time it rains.

Sand takes water and gets rid of it. The red clay under most of this county holds it: the particles are fine and flat, so a hard rain does not soak in so much as sit and spread sideways. It goes down a little, then out along the top of what it cannot get through, and it leaves by the nearest opening. A foundation vent, or the joint where block meets footing.

The dirt against the wall makes that worse, because it is backfill: dug out to build the footing and shoveled back afterward, never as tight again as the ground beside it. A hole dug in clay and filled back in behaves like a bucket, so water put there stays there.

Clay changes size with the water in it too. A dry August opens cracks in a yard you can put a finger into, so the first storm after a dry spell does not soak in, it pours straight down them and reaches the footing while the yard still looks parched.

• Downspouts discharging at the wall, or into a buried line nobody has checked in years
• Gutters running over, which sheets water along the whole wall instead of at four corners
• Ground or paving that runs flat against the house, or tips back toward it
• Mulch and flowerbeds built up above the siding line, or over a vent
• A dryer vent or a condensate line ending under the house
• A supply or drain leak, the only one here that does not care whether it rained

Three kinds of wet, and they want opposite fixes

Damp is not one condition, and the wrong answer makes it worse. Sealing a crawlspace up while the floor is still wet traps the water in with the wood.

Condensation is the one most often misread, because summer is when people open vents. Warm outside air carries a lot of water, and under the floor it meets ductwork, pipes and a subfloor that all sit cooler than it does.

The plastic test tells those apart and costs nothing. Tape a square of clear plastic flat to the dirt, sealed on all four edges, and come back the next day. Water underneath is coming out of the ground. Water on top is coming out of the air.

• Soil wet or shining after rain, drying back between storms. Bulk water from outside, fixed outside.
• Soil dark and damp all the time, nothing running. Ground moisture, and it wants the dirt covered once drainage is right.
• Dry soil, water beading on ductwork in summer. Condensation, and an air problem.
• One patch wet through a dry week, under a bathroom or a kitchen. A leak, and a plumber's job.

The musty smell, and the order you notice everything else

Smell comes first, and it reaches you through the house rather than from under it. Air leaves a house at the top, and what replaces it is pulled in at the lowest openings there are, which on a crawlspace house means the crawlspace. So the first report is usually a musty smell in a closet on an outside wall.

Metal is next, because rust shows on the small steel under a floor before the wood says anything: hanger nails, duct straps, the screws in duct seams. Then insulation, since wet batts get heavy and let go. Then the ground itself, a silt line where water stood and drained back, and a chalky bloom on block that is what the water left behind on its way through.

The floor is last. By the time one gives underfoot the wood has been wet a long while, and it is a subfloor question rather than a flooring one.

Where this stops being work we do

The outside half of the fix is ordinary repair work and it is ours. Clearing gutters so they run, re-securing and re-pitching a sagging section, extending downspouts. Repairing soffit and fascia an overflow has softened, and cutting out soft subfloor and band board where nothing load-bearing is involved.

The other half is not. Regrading a yard, cutting in a curtain drain, setting a sump, encapsulating a crawlspace and anything touching the foundation are excavation and waterproofing work. Plumbing past a fixture swap on the pipework already there is a licensed plumber, a condensate line is HVAC, and a dehumidifier or a fan under the house needs a circuit run to it, which is its own trade rather than part of this work.

Sequence decides more than budget does. Water outside first, then cover the ground, then the air, then repair wood. Replacing a soft subfloor while the downspout still empties at the wall is paying for the same repair twice.

Start outside, and do it in the rain

A downspout only tells the truth with water coming out of it, so the walk worth doing happens during a storm. Follow the perimeter and watch where each one discharges. The dirt should fall away from the wall rather than sit flat.

Extensions are a stock hardware store item and they push onto what is already there. Two things decide whether one works. Where it ends: the water has to go past the backfill, onto ground that was never dug, or it runs back down the same loose dirt it was meant to miss. And what it ends on, since one discharging into a low spot has only moved the puddle. Where a downspout feeds a buried line, check that water comes out of the far end during rain, because a silted or root-filled line backs up and empties at the wall.

Then go under with a light and old clothes. Standing water, a vapor barrier pushed aside or never laid, a wet patch under a fixture rather than under a wall. Whether the wood is sound is a push test with a screwdriver, not a judgment on how it looks.

Plenty of this is honestly a homeowner job. Extensions and splash blocks are hardware store items, and taking a flowerbed off the siding or shoveling the grade so it falls away is an afternoon. If the whole problem is two downspouts feeding one corner, buy the extensions and keep your money. Stop at a wet roof, a ladder on a two-story house, and wood that has already gone soft.

Wet wood, wet ground, or damp air

The line is not how wet the dirt is. It is whether the wood is wet, and whether water is still being delivered.

Now, if there is standing water under the house after every rain, or if the wood at the joist ends and the band board takes a screwdriver. Wood that takes a screwdriver is past the point where drainage on its own finishes the job.

Soon, if the soil never dries but the wood is still sound. Get the outside water handled before the wet part of winter. It is much the smaller of the two jobs, and doing it first keeps the other one small.

It can wait if the soil is dry, the wood is sound, and what you have is a musty smell or condensation on the ductwork in August. Watch it across a season, tape a square of plastic down and see which side the water is on, keep the gutters running.

Questions that come with this one