Seal the gaps that let water in, not the ones that let it out

Quick answer: Seal cracks in the foundation, gaps around pipes and vents, and open joints at trim. Leave the bottom edge of the siding alone unless your siding maker says otherwise. Do the job on a dry day above 40 degrees, and stop if the wood you're probing is soft.

The mistake that feels responsible

You walk the house in October with a caulk gun and a plan. Every dark line along the bottom gets a bead. Forty minutes, one tube, done.

Then you've sealed a drain.

Siding sheds water, but some always gets behind it. Wind-driven rain does it. So does a failed seam higher up. The wall is built to let that water run down and out at the bottom. Vinyl siding has slots along the lower edge for exactly this reason. Lap siding in wood or fiber cement relies on a small gap and the air behind it to dry out. Seal that lower edge tight and the water gets in and has nowhere to go. It sits against the sheathing and the sill plate, which is the wood lying on top of your foundation. Rot doesn't need much time.

So the job isn't "caulk everything low." It's sorting the gaps into two piles.

Gaps that let water in

These get sealed:

• Cracks in the foundation itself. Hairline cracks in poured concrete or in the cement coating over block are common and mostly cosmetic. They're also a straight path for water, and a masonry crack sealant closes them.
• Penetrations. Hose bibs, dryer vents, gas and electric lines, cable and phone entries, the spot where an AC line set goes through. Look at the ring around each one. The gap is usually bigger than you'd guess.
• Joints between siding and trim. Corner boards, window casings and door trim that sit low on the wall. These are meant to be caulked, and the caulk fails first.
• Where the foundation meets a step, slab, or porch. Concrete moves differently than the house. The joint opens up, and water runs into it.

Gaps that let water out

These stay open:

• The bottom edge of vinyl siding. Those slots drain the wall. Don't touch them.
• The bottom edge of lap siding. The siding makers' installation instructions are the final word here. Look yours up by brand. Most want a gap and clearance above the ground, not a sealed line.
• Weep holes in brick or stone veneer. The little gaps in the vertical joints near the bottom course are intentional. Caulking them is a classic homeowner error.

Wondering about clearance? The common rule is about six inches between the siding and the soil. Check your local code and your siding instructions. If mulch or dirt has crept up against the wall, pull it back. No caulk will beat soil holding water against wood all spring.

What to buy

A dripless caulk gun, about the cheapest decent one you can find, will do. You'll also want:

• Exterior-grade polyurethane or hybrid polymer sealant, rated for the surface you're sealing. Read the cartridge. Some are paintable, some aren't, and some don't bond to masonry well.
• A masonry crack sealant for foundation cracks.
• Foam backer rod if any gap is wider than about a quarter inch. Without it, the sealant sags and thins out in the middle, and thin sealant fails first.
• A stiff wire brush, a utility knife, painter's tape, a rag, and a shop vac.
• Safety glasses and gloves.

Skip the all-purpose indoor acrylic caulk. It's cheap and it cracks within a season or two outdoors.

How to do it so it lasts

1. Pick the day. Dry surfaces, temperature in the range printed on the cartridge, usually 40 degrees and rising. Rain in the next few hours ruins a bead.
2. Cut out the old stuff. Run the knife down both edges and pull the dead caulk out. New caulk on top of old peels off.
3. Clean it. Wire brush, shop vac, then a damp rag. Let it dry completely.
4. Fill big gaps first. Push backer rod in with a screwdriver so it sits a little below the surface.
5. Tape both edges if the bead will be visible. Peel the tape before the sealant skins over.
6. Cut the nozzle small. You can always enlarge it. Hold the gun at an angle and push the bead ahead of the nozzle instead of dragging it.
7. Tool it once. A wet finger or a plastic spoon, one pass, pressing the bead into the joint. Going back over it makes a mess.

Finish by wiping the tip and capping the cartridge with a nail. Leave the walkway cleaner than you found it. The neighbors will notice that before they notice the caulk.

Test it before you trust it

Give the sealant the cure time on the label. Then pick a dry day and run a garden hose at the wall, working from the bottom upward, a few minutes per section. Have your wife or one of the kids stand in the basement or crawlspace with a flashlight and watch the inside face of the foundation wall. This is a fine Saturday job for two people, and it beats finding out in a thunderstorm.

Damp spot inside? Mark it, dry it, and go back to that section. A leak that shows up through a hose test would have shown up through a storm.

When you stop and call someone

Some things you find while you're down there aren't caulk problems.

Soft wood. Push a screwdriver into the sill plate or the bottom of the framing where you can reach it. If it sinks in easily, you have rot, and you need a carpenter or a contractor to see how far it goes. Sealing over it just hides it.

Mud tubes. Thin tunnels of dirt running up the foundation mean termites. That's a pest professional's job, and quickly.

Wide, stair-step, or horizontal cracks. A crack wider than about a quarter inch, one that steps along mortar joints in block, one that runs sideways, or one that has pushed one side out from the other is a structural question. Caulk is the wrong tool. A structural engineer or a foundation specialist should look at it.

Water coming in during a hose test that you can't trace. The leak may be from grading, gutters, or a failed drain. Fix the source, not the symptom.

One safety flag on older houses

If your house was built before 1978, the paint on the trim may contain lead. Scraping or sanding it makes dust. Buy a lead test kit at the hardware store before you disturb painted surfaces, and read the EPA's guidance on lead-safe work practices. Keep kids and pregnant women away from the work area, wet-scrape instead of dry-scraping, and clean up with a HEPA vacuum, not a broom. If the test comes back positive and the area is big, hire a contractor certified in lead-safe work.

Make it a habit

The seal that holds in 2025 is the one that gets checked in 2026. Put a walk around the foundation on your fall list, next to the gutters and the furnace filter. The home maintenance calendar has a place for it. If you're buying a house, the inspector's list is the other side of this walk. And if what you find is bigger than a tube of sealant, here's how to know when to stop and how to get a quote you can trust.

A tube costs less than a pizza. Rot behind a wall costs more than a truck payment. Spend the afternoon.