Why is my basement damp? (With illustrations)

Are damp basements inevitable? As I went house hunting last month, I saw one basement that was very humid and I went online to try and find out why basements get damp.

There are three main culprits for damp basements. Concrete channels water inside our homes by capillarity – a fancy term for water traveling through concrete’s tiny pores. Water vapor from the warmer outside air can also condensate in the cooler basement walls and floor. Other condensation sources can be caused by your own doing as well, such as a shower without a fan, an unvented dryer, or even stored firewood.

UPDATE: I just published a new article about fixing your damp basement, follow the link to check it out!

Coping With Moisture

If homes were perfect, the basement, crawl space, or concrete foundation slab of every single one would be desert-quality, arid, throat-parching. dusty, bone dry.

There’d be no wet doors at damp walls. No fungal growth, wood rot, or smelly mildew. No peeling paint, stained wallpaper, or rusting nails. You could even lay a hardwood floor without worrying about the boards curling and warning. And your basement would make first-class living space.

But all too often, basements are too damp to be comfortable. Those solid concrete or concrete block foundations that support our homes and make up the basement walls — even concrete slabs that lie on top of the soil — not only get damp, but even worse, channel moisture into our homes rather than blocking it out. As we sketch out the reasons our foundations seem to work against us, we’ll get a good picture of why it’s so hard to get rid of basement dampness.

Stopping Leaks

Strange as it may seem, it’s often easier to stop basements from leaking than it is to put an end to other dampness that makes them so uncomfortable.

Most basements leak because rainwater or snowmelt collects near your home’s foundation and then works its way in through cracks or holes. The simple solution is to channel that water away. Extending downspouts well away from the foundation and sloping the soil away generally do the trick. It also helps to fill low areas of your yard so water doesn’t puddle and slowly seeps toward the foundation. Slope walkways away from the house, too.

These steps will solve most leak problems, and will certainly reduce dampness. They might even stop it, but dampness can be persistent because it enters your home in several completely different ways.

How Concrete Channels Water

Dampness from saturated soil after rain can seep against the basement wall. Capillarity in the concrete draws on through to the inside. Capillarity also draws water up from saturated soil at the water table. This water can enter the footings and floor from below.

Let’s take a close look at the concrete foundation first. As solid-looking and strong as concrete is, it’s actually full of tiny pores.

Even watertight concrete is porous. While the concrete won’t necessarily leak, that is, allow water to run right through it, those pores literally draw water into themselves. Like tiny suction pumps.

Since these pores interconnect with others in a vast network, they continually draw the water onward, progressively filling all open spaces. This process, known as “capillarity,” goes in all directions, even upward, soaking up water from any available source and wetting drier areas. In solid concrete or masonry structures, capillarity can easily raise water up to the second floor!

Fortunately, a concrete wall or floor can hold a lot of water without leaking because the pores retain the water. they don’t let it leak out. However, when water soaks all the way through a wall or floor to the inside surface, it’ll wet anything it touches, like wood, carpeting, and insulation. That’s the main source of moisture, which causes mold, mildew, and rot. Moisture will also evaporate off the wet walls, saturate the air, and make everything else feel damp, too.

Of course, for capillarity to work, it has to have a water supply. It has three common sources:

  1. Water steadily works its way upward through the soil from the reservoir in the ground (the water table) by capillary action, too, and commonly soaks into concrete foundations and slabs from below (see image)
  2. Rainwater, as it sinks downward through the soil, seeps against the foundation and can soak in.
  3. Rainwater that splashes up against the sides of the foundation can soak into the concrete.

You might be able to limit the splashing, but you’re obviously not going to have any luck blocking the water table or stopping the rain.

Identifying Capillarity

It might take a little detective work, but it’s usually not difficult to figure out whether capillarity is at work in your basement.

The big clue should stand out clearly on the bare concrete: a powdery white line that mysteriously shows up on your walls or floor.

If you look at the powder closely with a magnifying glass, you’ll see that it consists of many tiny crystals that look like table salt. They are indeed a variety of mineral salts but not the kind at salt you flavor your food with, so don’t taste it!

What is efflorescence?

These salts, called “efflorescence,” are the sure sign of water movement.

Efflorescence originally comes from the ground outside or sometimes from mineral residues left in the concrete. Water dissolves these minerals and carries them along through the concrete, finally depositing them when it evaporates off the inside wall or floor.

While you can find the crystals anywhere where there is moisture, they’re concentrated at the highest point of water penetration, leaving a telltale line on the wall. This “tide line” might be several inches wide, as the efflorescence rises and falls with the ebb and flow of moisture in the soil around your house.

Efflorescence, minerals deposited as water evaporates, commonly appears at the highest point that capillarity carries water. Efflorescence often occurs on the exterior near ground level as well as on the inside.

The presence of efflorescence doesn’t mean your concrete wall is deteriorating. It’s harmless, and you can brush it away. However, expect it to return because where minerals appear on the wall’s surface. you can bet there are more just under the surface. The only way to stop efflorescence is to eliminate the water.

Unfortunately, that’s not easy.

The time to anticipate and stop capillarity was when workers laid the foundation — from the outside.

A proper foundation wall has a damp-proofing coat (asphalt, plastic, or another impervious layer) on the exterior to keep moisture out or the walls, and good drainage alongside the walls leading to a drain pipe at the bottom (the footing).

A damp-proof layer between the footing and wall, or a coarse gravel bed beneath, stops capillarity from below. And a polyethylene (plastic) membrane or bed of coarse gravel underneath protects the concrete floor. Coarse gravel stops capillarity because the gaps between stones are too large to draw water.

Of course, you usually can’t tell how thoroughly workers damp-proofed your foundation unless you dig it up. That’s an expensive project, although perhaps worthwhile if you have serious moisture problems. Consult your local building inspector and several contractors who specialize in basement moisture problems before taking this step.

Efflorescence will definitely limit your success if you try to block capillarity from the inside.

Capillarity usually keeps the wall that’s below the tide line damp, although above that line the wall will remain dry unless moisture from rainwater soaks through higher up. Manufacturers simply don’t formulate paints or other wall coverings to stick to wet surfaces, so no matter what coating you try, it’ll fail and begin to crack and peel within a few years.

Even worse, the mineral salts temporarily blocked from emerging on the surface of the wall, will crystallize below the surface, inside the concrete pores, and crack off the surface of the concrete. So after a few years down the line when you clean off the peeling paint, you’ll find that your concrete wall has begun to crumble along the tide line or wherever there is efflorescence.

There’s not much good news when it comes to stopping capillarity once it’s in your walls and door. The best solution is to keep as much water away from your foundation as you can.

More Dampness: Water Vapor

Capillarity isn’t the only source or dampness. The second culprit is water vapor.

Moisture from high humidity condenses on and within cool walls. This moisture can also carry minerals to the wall surface.

We recognize water vapor best as the humidity in the air. Though we don’t often think of it, concrete contains air, too, in exactly the same pores that cause capillarity. In fact, water vaporizes inside concrete from the pool of water that capillarity draws up.

This water vapor spreads through the concrete, heading to wherever the air is drier. Eventually, it filters into your basement, raising the humidity and increasing the feeling of dampness.

Fortunately, you can successfully paint concrete above the tide line and block water vapor movement into your basement. “Block” is actually too strong a word here. “Retard” is better, because paint slows water vapor movement, but doesn’t completely stop it.

Of the three types of paints readily available, alkyds (enamels) retard vapor better than latex (acrylics) and concrete-based coatings, though all block most vapor.

No matter what type of paint you use, buy good quality paint specially formulated for concrete so it bonds well and lasts a long time. Scraping and cleaning a concrete surface for repainting isn’t fun — you won’t want to do it often.

Dampness From The Air

The third source of basement dampness doesn’t come from the ground or the concrete. Moisture circulates throughout your home in the damp air, during periods of high humidity, usually in summer. Sometimes, you’ll discover high-humidity sources within your basement, like a shower without a fan, an unvented dryer or stored firewood.

Basement walls and doors are cool since they’re in contact with the cooler surrounding soil. When warm humid air strikes these cool surfaces, water condenses on them. And if a wall surface is warm, water vapor can move into the wall and actually condense inside the concrete where it’s cooler. In either case, you’ll end up with wet walls or floors and possible mold and mildew problems.

What type of dampness source do I have in my basement?

It’s not always easy to determine whether the source of moisture in a basement is humidity or capillarity. The efflorescence tide line clearly shows capillarity. But condensation can be at work, too.

One way you can check for condensation is to tape a 2-ft. sheet of aluminum foil tightly to the wall or floor and examine it over several days to see it condensation develops on its surface. Another sign of condensation is water trickling down a painted wall.

But without one of these signs, you can’t easily tell where your moisture is coming from condensation or capillarity.

You’ll have to wait for an extended period of low humidity, of say, several weeks to a month. That’ll stop condensation and give your basement time to dry out should condensation be the problem. If the dampness continues, you’ll know capillarity is at work.

If condensation proves to be the culprit, you’re in luck, because you can control the high humidity that causes condensation with a dehumidifier (or air conditioner, which does virtually the same thing).

Or, if your basement is otherwise dry, you can get rid of those cold walls by insulating them on the inside and installing a vapor barrier over the insulation. (You could also insulate walls from the outside, but that’s more costly.) It’s harder to insulate cool concrete floors to stop condensation. In that case, dehumidification is probably the best defense.