How to Find and Prevent Mold: Basements, Crawl Spaces, & Concrete Floors


Article Summary: In this article, I give lots of examples of where to find mold in basements, crawl spaces, and on poured concrete slabs. However, no list can ever be all inclusive. Consequently, I’ve decided to also teach people about why mold forms in relation to concrete foundations as a result of moisture movement and temperature differences. With this understanding, the reader is empowered to look at their own particular situation and uncover those places where mold is most likely to take root. Along the way, we get into plenty of construction details for those thinking of building new and also touch on mold remediation a bit. February 22, 2016


Footing Slab Foam Plastic Detail

  1. Misleading Information
  2. Mold Basics
  3. Cooler Air is More Humid
  4. Concrete Breaths Moisture
  5. Concrete Wicks Moisture
  6. Crawl Spaces & Mold
  7. Crawl Space Ventilation Codes
  8. Crawl Space – Vented or Unvented
  9. Good Construction Practices

Misleading Information

Foundation Wall

I thought I’d take some time and write down what I know about finding mold as it relates to foundations. Before I do, I want to take a minute to discredit articles I’ve been reading lately that are clearly written primarily for financial gain. I’m not talking about well-meaning individuals trying to share what they’ve learned. What I’m talking about are authors writing content in part to increase sales.

Relative to the topic at hand, I recently read a piece that offered the most obvious of suggestions related to crawlspaces – like checking crawl spaces for standing water. From where I stand, trivial articles written to increase sales without educating the reader are a real disservice. My concern is that people that are really sick will be misled. I know from experience that when I was really sick, it was very tempting to latch on to simple explanations rather than really digging in.

Clearly a big part of the motivation behind these types of articles is a desire to sell stuff, over and above empowering people with quality information. After all, I’m sure these savvy business people know full well that if you educate the reader, they may be less inclined to buy your services. On top of that, it takes a lot of time and research to write quality material. From a business perspective, it’s better to just mostly scare readers or feign expertise.

Well, maybe that’s being a bit too harsh. I’m sure the authors of these fluff pieces have some good intentions too. I’m sure they’ve convinced themselves that their services have value. Writing oversimplified articles brings in traffic, and to the degree that what’s being offered is appropriate, all is well and good. However, these authors need to seriously reflect on why its important to help educate those with CIRS first and to make money second. Pounding out dribble that anyone could write in 30 minutes does little to educate people and increasing knowledge of mold and CIRS.

As always, I strongly suggest that those with CIRS educate themselves. Following the advice of sales people will not keep mold at bay. Getting better from CIRS means you’re going to need to become an expert when it comes to mold. In addition to teaching you how to find and prevent mold, I’m also going to get into quality home construction and will touch a bit on the beginning steps of mold remediation.

By the way, if you’re looking for a well done checklist, read Do-It-Yourself Mold Prevention, Inspection, Testing & Remediation by Phillip Fry. (archived)

Mold Basics

Since this is the first of a series of articles on finding mold, let’s discuss a few basics regarding mold. I’m sure most of you know by now that mold will grow whenever there is moisture present. In other words, mold spores (mold seeds) are everywhere. It is how nature works. We need mold to break down organic matter so it can return to the soil and be reused. Mother Nature figured this out long ago and consequently, the world is loaded with spores just waiting for a little moisture to allow them to get to work. If we didn’t have mold, the planet would quickly fill up with debris of all sorts and die.

Likewise, food for mold is everywhere. Mold doesn’t need a big chunk of wood to grow. A tiny bit of dust will have enough organic matter to support a thriving little mold colony. Mold isn’t picky either. It’ll eat just about anything including jet fuel. You may also be surprised to see mold growing on inert materials like glass and plastic. It’s not that the mold is consuming these inedible substances. Rather, it’s the thin layer of dust on these materials that provides the food mold needs.

The real key that will help you understand how to find mold is that whenever moisture is present, mold will soon follow. You know how they say “follow the money” to tract down corruption. Well, when it comes to mold, the saying should be “follow the moisture”.

Please realize that when I say moisture, I don’t just mean drips from a leaky pipe or water that seeps in past a basement window. Certainly larger leaks are a problem. However, there are molds that thrive in all types of moisture conditions. In fact, whenever the humidity is above 50%, you can expect to find mold of one type or another growing. As noted in Mold Testing – Common Misconceptions, molds like Wallembia Sebi thrive in fairly dry conditions.

So this is a key point. When you’re looking for mold, you need to look for moisture conditions ranging from sopping wet due to large leaks, to damp conditions caused by condensation, to low level moisture conditions from elevated humidity levels. You need to consider all three. You’ll get a better sense of what this means from a practical standpoint as we progress through this article. Since it’s impossible to discuss all the scenarios that create moisture levels conducive to mold growth, my hope is that by giving you enough examples and fundamentals, you’ll be able to apply what you’ve learned to your own situation.

Before we leave this general introduction, I should also mention that mold particularly likes hidden away spaces where there is little or no light and little or no air movement. Sunlight degrades mold and air currents bring in competing micro-organisms that make it harder for mold to thrive. Low light and little air movement aren’t required, but where ever these conditions exist, it will be much easier for mold to get a foothold.

For the remainder of this article, I’m going to try to jot down some of what I’ve learned over the years as a Residential Building Contractor combined with all my readings on mold. The focus of this article is foundations. Following articles will cover walls, bathrooms, and roofs. Hopefully, you’ll get some good insight that will help uncover hidden mold sources, as well as, helping those who are planning on building a mold-free home.

Cooler Air is More Humid

Cold Wet Glass

The first important concept to understand when it comes to foundations and mold is that when air is cooled, it becomes more humid. This is why your ice cold glass of lemonade drips condensation even on a dry day. What happens is that the air surrounding the glass is cooled by the liquid inside. As the air cools, the humidity level rises next to the glass. In fact, a thin layer of air next to the glass becomes so humid that some of the water vapor condenses out of the air and onto the glass. All that liquid on the outside of the glass is coming from the water vapor (water in its gaseous state) in the air.

So why does this matter when it comes to foundations? It matters because there is almost always a difference between outside and inside temperatures. This is in part due to the fact that the soil temperature a few feet down into the ground is around 55°F. We all know that when you dig a hole, the soil below the surface is cooler.

Of course this varies somewhat depending on the type of soil and the climate, but the main point is that the ground temperature is generally cooler than the temperature in most basements, in most crawlspaces, and in the living space above concrete floors. Again, this varies depending on whether you live in a cold climate or not, the time of year, and the type of foundation (basement, crawl space, or slab). Nonetheless, in general there are almost always temperature differences between the soil in contact with the foundation and the air inside a home. When air cools, humidity levels rise and the chances for mold growth increase.

OK so soil is generally cooler, so what? Well, as it turns out, concrete is a poor insulator. In spite of the fact that it’s so hard and dense, heat easily passes through concrete. To give you an idea of just how poor of an insulator concrete is, the R-value of a 4” thick concrete floor is 0.80. This is actually less than a single pane of glass with an R-value of 0.91. We all know how poorly single pane glass insulates against the cold. Given that the higher the R-value the better the insulator, a single pane of glass can hold in heat better than 4” of concrete!

If we add these two facts together, this is why concrete walls and floors are cool to the touch. Now I know many new homes are using insulating foam board between the soil and concrete. This is a good practice both from an energy savings standpoint and mold prevention. I’ll talk more about this later when we get into building practices. If you happen to have a home that has this improvement, it’ll definitely help keep the concrete warmer but it’s not a panacea. In terms of mold, it all comes down to humidity and temperature levels.

An easy way to start to get a handle on why this all matters in terms of mold is conveyed in the article Solve Crawlspace Moisture Problems. In the article, the author discusses the effect of exchanging the air one time in a 3-foot tall crawl space underneath a 1,500 square-foot home with outside air. The crawlspace is assumed to be at 72F and 85% relative humidity (RH). The outside air is taken to be 94F and 45RH. So the RH of the outside air is much less than the air in the crawl space – 45% compared to 85%. If you’re like a lot of people, you’re of the belief that opening the basement windows or venting the crawlspace is going to lower the humidity.

However, in the article, calculations show that the cooler crawl space air holds 13.4 gallons of moisture while the same amount of warm outside air holds 14.1 gallons of water. In other words, when you replace the crawl space air with outside air, there is a net gain of 0.7 gallons to the crawl space with one complete air exchange. Now I’m not going to get into all the details of the relationship between temperature and humidity levels but for those that are interested, the article Dewpoint & Relative Humidity along with my discussion with Tom are a good place to start.

We’ll get into crawl spaces later but this example shows that you need to consider the humidity and temperatures. The warmer the air, the more water vapor it can hold. When air is cooled it becomes more humid and if cooled enough, some moisture will fall out of the air in the form of condensation or fog.

Let’s look at another example of why when air is cooled that it gets more humid matters. Imagine you’re looking for a way to organize all the stuff in the basement. You get a great idea to build shelving and decide to place it against the outer, uninsulated concrete walls in your old basement. Using what you’ve just learned, can you guess why I would discourage this – especially in home in colder climates without insulation on the foundation walls?

What happens to the air in the small spaces behind all those boxes and the foundation walls? It cools quite a bit. If you’re not running a dehumidifier in your basement like you should be, then the humidity level is already higher. When this air gets into the cool spaces behind storage items and the outer basement walls, it gets even more humid. Often, it’s more than enough to promote mold growth. Later, we’ll learn about how concrete wicks up water that further exacerbates this risk. However, even if we set this second factor aside, mold loves to hide and grow in the cooler spaces in basements.

Band Joist Fiberglass Insulation

In fact, I would urge you all to consider your basements and closets on any level. In the basement, another common occurrence is to find mold growing behind the insulation that is tucked in-between all the floor joists above that make up the basement ceiling (perimeter band joists). This is especially true when fiberglass is used in colder climates and it isn’t covered over with plastic. If you have this type of insulation, start by looking in the corners. Why? It’s because the warmer basement air passes through the porous fiberglass and cools against the wood framing. As the air cools, it becomes moister resulting in mold growth.

If you do happen to have loose fiberglass like this, very carefully remove the insulation between the floor joists. Wear a good mask (has some plastic parts) with P100 Carbon Filters. Use a flashlight shining at a glancing angle to the wood surface. Look very closely. You have to really look hard; mold is often the same light color as bare wood and cardboard. You may be unpleasantly surprised at what you find.

The same holds true for closets. Closets against outer walls on any level, or closets with concrete floors can be a real problem. Once again, when these spaces are packed full, mini micro-environments are created. Away from air movement and light, along with higher humidity levels due to the cooling effect of the concrete, or cold outer walls in winter, mold takes off. Any good inspector knows this. That’s why they go poking around in closets. Now if you were like me in first hearing this, you’re shaking your head incredulously at learning just how little it can take for mold to grow.

OK, so now I’ve given you some examples, can you think about other places where this might be a problem in your own place? What about underneath those basement stairs packed full of boxes? What about furniture with flat bottoms or cardboard boxes resting on concrete floors? Look very closely. Consider taking a mold swab and sending it in for analysis. Mold can be hard to detect by the untrained eye. I bet you can now see why I strongly recommend using shelving to hold boxes away from walls and off the floor in basements.

At a minimum, buy yourself a few inexpensive humidistats and place them around your basement, crawl space, and other suspect areas. Don’t just hope that everything is OK. If the humidity levels start to get above 60%, purchase a dehumidifier and connect it directly to a floor drain with a hose. That way you don’t have to worry about remembering to empty it. Keep a close eye on the dehumidifier for mold growth too. Use a little hydrogen peroxide in the bucket from time to time.

Portable Dehumidifier

Whatever you do, don’t buy a dehumidifier that uses larger diameter flexible hoses that stick out the window like the one shown. Like all dehumidifiers, they work by cooling the air making it more humid. In conventional dehumidifiers, moisture condenses on the coils and drips into a bucket, or runs down a hose that leads to a floor drain. This constant dripping is somewhat self cleaning.

In contrast, dehumidifiers that use hoses that connect to the outside, send cooler humid air through a length of hose that is stubbed out a window. Not surprisingly, debris builds up inside those large hoses. The air is moist. You know what comes next. In another article, I’ll get into why and how to clean dehumidifiers and mini-split air conditioners on a yearly basis.

I’m going to end this section with one more example. Let’s say you decided to finish the basement walls in your new home. You’ve done some homework, and diligently waited over a year to allow the water in the concrete to dry out. You’ve also read that it’s a very bad idea to use a plastic vapor barrier over concrete walls especially in the cold climate where you live.

At the same time, you’re energy conscious, so you decide to frame out wood finish walls just inside the concrete walls using wider 2×6 lumber. In addition, you’ve decided to fill the deep spaces between the wood studs (stud bays) with quality fiberglass insulation. You figure that being made of glass fibers, the insulation is impervious to mold. Also, because of your research, you’ve select a latex paint to finish the 1/2″ drywall because is breathable. Having done much of the work yourself, you smile with satisfaction at the sight of your finished basement.

A year later, the kids start coming down with “asthma”. You’re finding it harder and harder to get out of bed in the morning. One day, someone suggests that it might be mold. You hire a professional inspector that does a spore trap inside your finished basement walls and finds they’re loaded with aspergillus/penicillium mold. No!

So what went wrong? Well, think about what we just learned. During the winter, the concrete walls were especially cold particularly in the top few feet of the wall where the insulating quality of the ground is less. Although you were right in making your walls breathable, this also meant that the warm indoor air naturally moved toward the colder concrete walls.

The fact that there was so much insulation created the conditions for the moisture to condense onto the inner face of the concrete wall. This happened because the indoor heat was so thoroughly trapped by the layer of insulation that the inner face of the concrete wall remained quite cold. When the water vapor from the house approached this cold surface, the moisture condensed into droplets just like what you see on a cold glass of lemonade. Furthermore and unbeknownst to you, the resin used in your fiberglass could be eaten by mold. It wouldn’t have really matter though because the amount of debris the fiberglass picked up when you rolled it out onto the construction site floor to cut it to length would have been more than enough to support mold growth.

I’m including a chart from Estimating Dew Point Temperature for Water Cooling Applications to help make this point clear. In the chart, you can see that if you assume the indoor basement was at 70°F with a fairly low wintertime relative humidity (RH) of 30% that the water vapor in the air will condense when the temperature drops to about 45°F. I can assure you that in colder parts of the country, this is entirely possible when the only insulation on the concrete wall is on the inside.

Dew Point vs. Temperature Chart

Concrete Breaths Moisture

Concrete Surface

Another important understanding when it comes to mold and foundations is that concrete is breathable. Air passes right through concrete albeit very slowly. Given that air always carries a certain amount of moisture in it in the form of water vapor, this means water in its gaseous state moves through concrete. So why does this matter.

It matters because regardless of whether a foundation consists of a basement made up of concrete walls resting on concrete footings, or a concrete slab on grade, foundations are made up of concrete. So building foundations are made of concrete and concrete is permeable to moist air. Since moisture is the life blood of mold, it’s important to understand the impact of water vapor passing through concrete.

Now I’m guessing that hearing moist air moves through concrete may be a bit surprising to some. After all, look at the puddles that form on concrete slabs. It seems like concrete is impermeable to liquid water let alone water in its gaseous state. Many a home has suffered badly from mold caused by water vapor when builders and home owners didn’t take its movement through concrete into account. Let’s take a closer look.

As mentioned, water vapor is just humid air; it’s air with molecules of H2O bouncing around. The “water” is in a gaseous state. We all have an appreciation for what humidity means and this is what we’re talking about here. We’re not talking about liquid water. We’ll look at how liquid water moves through concrete later.

When it comes to water vapor, nearly every material is permeability to water vapor to some degree. In engineering parlance, this is referred to as “permeance”. For example, the “permeance (perm) rating” of a 4” thick slab of concrete is 1.25 perms. Permeance of a material refers to how easily moist air can move through the material. The higher the permeance, the easier moist air can move through the material.

In comparison to concrete, the perm rating of drywall is 50 while plastic is 0.03 perms. Relatively speaking, when it comes to retarding water vapor movement, a 4” slab of concrete is considered “semi-impermeable”. This means that although it does impede the flow of moisture considerably, it does not stop it completely. In comparison, an 8” thick concrete block wall has a perm rating of 2.4 and is considered “semi-permeable” when it comes to water vapor movement.

The bottom line is that moist air (water vapor) moves through concrete. I should mention that I’m talking about cured concrete. I’m not talking about the moisture that is given off by fresh concrete. Anyone who’s ever watched concrete being made knows it’s loaded with water. In fact, fresh concrete in a new building will transpire 1-2 gallons of water into the indoor air during the curing process that lasts a couple of years.

Instead, I’m talking about moist air passing through cured concrete and consequently making the other side more humid. So here’s where it gets interesting. You may be thinking to yourself that water vapor that does move through concrete foundations moves from outside into the building making the indoor space more humid. After all, we all know that if you dig down a bit into soil, the soil is moist. Since there are air pockets in soil, this moist air must be moving through the concrete and into the building. However, it’s just the opposite.

In fact, air almost always moves from warm side of a material to the colder side. Recall from the discussion above that the soil temperature a few feet down is around 55°F. Knowing these facts, what this means is that in real life, the moist air in homes diffuses through the concrete foundations toward the cooler soil outside. (In hot climates, the movement may be into the home if the sun heated earth is warmer than the air-conditioned indoor air.)

Awesome, this must mean concrete is making our homes in colder climates dryer and, as a result, less prone to mold. Well, diffusion of water vapor outward doesn’t really impact the overall humidity in a basement or crawlspace. It’s such a small amount relatively speaking. When it comes to overall humidity levels, its the movement of water by wicking (sponging) into concrete has a much greater impact of indoor humidity. We’ll discuss this in a bit. Nonetheless, while water vapor diffusion doesn’t impact indoor humidity levels much, it can result in all sorts mold issues.

So how does the fact that concrete is porous to moist air, along with the fact that moist air moves from the warm to cold, result in mold? Consider the example where a person that finishes off their basement by building wood stud walls filled with fiberglass. However, unlike the example above, instead of breathable latex paint, they used impervious enamel paint in the bathroom over the 1/2″ drywall.

In this example, the home is located in a warmer climate. As a result, the basement space is actually cooler than the upper level of the soil heated by the sun outside. As we’ve learned, water vapor moves toward the cold. In this case, it moves from the moist soil outside into the wall cavities filled with fiberglass. Unfortunately, its movement is stopped by the relatively impervious enamel paint. Humidity levels rise inside the wall. It may be a small amount of moisture movement but the space inside the wall is small and confined. You know what comes next – mold.

So the whole point of these examples isn’t to freak out those with finished basements. It’s meant to empower you by showing you the way water moves in its various forms and to get you to think about your own situations. We now know that all air has water vapor in it and that all air moves right through concrete toward whatever side is colder. As the air cools it becomes more humid and may even condense out liquid water if cooled enough. If we don’t plan to accommodate this movement and the resulting increases in humidity, mold and bacteria will show up as unwanted guests.

Concrete Wicks Moisture

Just as important as the movement of water vapor through concrete is the movement of liquid water through concrete. In effect, concrete acts like a big sponge (hygroscopic) that can become saturated with water over a large area from a small point source of water. If protective measures aren’t taken, this movement of water can cause the interior facing surfaces of concrete walls and slabs to become damp. This excess moisture then evaporates into the building driving up humidity levels.

Remember how I mentioned that basement corners and underneath cardboard boxes could be places for mold to grow due to the lower temperatures? Now add to that the moisture that is wicking up through the concrete because the contractor did everything on the cheap, or maybe because your home was built before builders knew better. For example, maybe a cheap tar-based damp-proofing spray was used to coat the outside of the basement walls. Alternatively, maybe plastic wasn’t used directly below the concrete floor to prevent it from wicking up moisture. Masons hate to pour concrete over plastic because it takes longer for the concrete to set up before it can be finish troweled.

Remember, as you dig down into soil, the ground becomes moist. This is especially true in soils with more fine (clay) particles. This moisture in the soil is going to wick up into the concrete if precautions aren’t taken. This happens on walls and floors.

In the days when my grandfather was a builder, they knew better than to backfill around foundation with regular dirt like they do nowadays. They didn’t have some of the awesome products available today to keep the walls dry. Instead, they used gravel roughly 3/4″ in diameter all of the same size (classified as “poorly grade” or “washed”) because it would prevent wet soil from pressing up against the concrete.

The gravel also acted as a “drainage plane”. Rainwater would easily trickle to the bottom of the gravel and be collected in a clay pipe at the base of the foundation walls. The clay pipe (tile) carried the rainwater over to a sump crock so it could be pumped out onto a lower portion of the lawn away from the foundation. It was a pretty good system.

Nowadays, things are done differently. Most builders backfill with soil from the site. One of the better solutions available today for concrete walls is to install thick sheets of a waterproofing peel-and-stick membrane to the outside of concrete walls. This is followed by a heavy sheet of dimpled plastic to prevent rainwater from being able to press up against the concrete. The dimple sheeting plays the same role as the gravel. Take a look at these two HomeTime TV videos with Dean Johnson. I really enjoyed watching Dean in my younger days.

So consider this example. Let’s say that the basement contractor did a great job on the outside using the details noted above. However, the insulation contractor failed to isolate the concrete floor inside from the footings with closed-cell foam board and plastic. Take a look at Slab-on-Grade Construction Details to see various properly done foundation systems. Notice how they all separate the concrete floor slab from the footing and foundation wall with foam board and plastic. This is a detail that is often non-existent especially in older homes.

The foam board keeps the cold from transferring up into the floor. If you are planning on installing finish basement walls up next to the concrete walls, it’s important that the floor stays relatively warm. This will prevent the water vapor that we know is going to be passing through the finished wall, as it moves toward the cooler concrete, from cooling too much and becoming too moist.

Footing Slab Foam Plastic Detail

The plastic is even more important. It separates the floor from the walls and footings. This prevents water from wicking up into the floor via the walls and footings. University of Minnesota and Oak Ridge National Laboratory has some good foundation sectionals.

Without the plastic, the bottom board (plate) of wood walls in a finished basement will become damp via moisture it absorbs from the damp concrete floor. Even if this plate is treated against decay as specified in building codes, the bottom of walls will begin to rot from the excess moisture. Preventing water from wicking into concrete walls and floors is very important.

To test your own situation, tape a one foot square piece of clear plastic to the concrete in question. Wait and watch a few days. In many cases, you’ll see beads of trapped moisture develop under the plastic. If you were thinking about covering over those concrete surfaces, ask yourself how you’re going to deal with that amount of moisture to prevent moldy conditions.

Now before I leave this subject of water wicking through concrete, let’s take a look at warmer climates where slab-on-grade foundations are common. These foundation systems don’t have a basement. Instead, a slab of concrete that is thicker around the perimeter is poured in a single piece. Not having to worry about frost heave, this is a nice way to get both a foundation to support the heavy load-bearing perimeter walls and a floor in one go.

Just as in any foundation system, it’s important to keep moisture from wicking up into the concrete. However, consider the consequences of a builder who fails to waterproof the outside of the roughly 18” tall face of concrete around the outside of the concrete slab as discussed in BSI-082: Walking the Plank. This exposed concrete is just waiting to soak up moisture.

Slab Water Wicking

Along comes an owner that loves their shrubbery next to the building. Due to the warm climate, the plants are watered often. Just as the article describes, water wicks up into the floor and begins to rot out the walls, floor coverings, or whatever else is in close proximity to this serious water influx. It’s really important to control capillary movement of water when it comes to foundation work.

I realize that most folks reading this article aren’t going to be building new homes. What’s important to realize for everyone else is that almost every building is going to suffer from one or more of these design flaws. Now that you understand about how water moves through concrete and the effect of cooler temperatures on mold growth, you can begin to look at where you’re living with a more critical eye. You’ll know better where to look for mold and just as importantly, you’ll know how to avoid creating conditions that will promote its growth.

Crawl Spaces & Mold

Crawlspace New Construction

So now that we’ve got the basics down, we’re ready to take a good look at crawl spaces. Not surprisingly, crawlspaces and basements with dirt floors transpire a lot of moisture into the air. After all, there’s nothing to slow the movement of moisture from the soil into the crawl space. In fact, unless the soil and concrete walls were properly covered with a continuous, heavy plastic during construction before the wood floor above was put down, you can pretty much plan on finding mold in the crawl space regardless of whether it has bare dirt or a concrete floor. This includes both old and new homes!

OK, there was a lot in that last paragraph so let me “unpack” it a bit. A lot of older crawl spaces don’t have any covering over the bare dirt floor. This virtually guarantees mold growth for reasons that’ll I’ll discuss as we go. Older homes simply don’t have any of the water management construction details that are important to keep foundations and the crawl space dry.

Sealed Crawl Space

However, even on new homes where the builder does realize the importance of covering the exposed dirt and follows other important moisture details related to the concrete foundations, quite often the dirt isn’t covered with a heavy 20-40mil sheet of plastic until the house is framed, sided, and the insulation crew is called in. This is way too late. In the months it took to get the house to that point, the humidity in the crawl space most likely got too high. The image from AquaGuard shows what a properly sealed crawl space.

This happens because crawl spaces are fairly small and typically don’t have drain tile at the base of the foundation walls on the inside to carry water away. As a result, the soil gets quite damp due to the fact that the floor above protects it from the sun. At the same time, the soil is drenched with rainwater. Combine this with a lot of sawdust and you get the picture. Most builders simply don’t appreciate how critical it is to get the crawl space hemmed in before the floor framing above it is finished – let alone waiting until the walls and roof is up.

Setting new construction aside, recall what we now do about how concrete wicks up water and increases in humidity with lower temperatures. What happens to moisture in the form of rain, sprinkler water, warm humid outside air, and the occasional high water table? Without proper moisture control measures, this moisture finds its way into the cooler craw space and humidity levels rise. These factors combined with the closer proximity of the wood framing to the soil and low air flow are a prefect recipe for mold growth regardless of where in the country the home is. If it’s an older home, the crawl space is guaranteed to be moldy. If it’s a new home, chances are still quite good that it’s moldy.

In fact, I’ll go so far as to say that if you’ve got CIRS and a bare dirt basement or crawlspace, move. Don’t even bother looking. Don’t waste another minute of time pondering whether you can stay. You can’t. Unless you’ve got a ton of money, don’t even think about trying to remediate. Just move. Places that have never had a proper plastic vapor barrier have been moldy for a long time.

The reason I take this strong position relates to the extent of mold you’re likely to find along with the fact that mold spores and toxins in crawl spaces move up into the building. This happens as the cooler air in the crawl space is warmed by the wood floor above. This temperature differential creates a natural convective flow moving air from the cool basement or crawl space up into the warmer house. Homes with mold growth in basements and crawl spaces will be inundated with mold. Even worse, because it is coming from below, large quantities of mold toxins will be sandwiched between the sub-floor and the finished flooring.

As a former builder, I’m always amazed to see how much dirt gets trapped between finished floorings like tongue-and-groove wood planks and the sub-flooring below. The same holds true for carpeting. Every time someone walks over a floor like this that’s loaded with mold toxins, this reservoir of toxins puffs into the air. I mean if you’ve got money to burn, by all means, remove the baseboard, tear out all the finish floorings, clean, and then reinstall.

Of course, that’s after you spend tens of thousands to have all the wood in the crawl space cleaned and encapsulated along with installing a proper vapor barrier and foam insulation. I could go on but suffice it to say that this is in no way a complete accounting of all that’s required to do proper remediation. Remediating a home with a moldy basement or crawl space is a real challenge. If it’s an older home, chances are the foundation leaks too. Just to give you an idea of what it takes to properly remediate an old foundation, take a look at BSI-041: Rubble Foundations and Waterproofing Best Practices.

Crawl Space Ventilation Codes

The latest 2015 International Residential Building (IRC) R408.1 code specifies that crawl spaces with an exposed dirt floor must have 1 square-foot of venting for every 150 square-feet of floor area. So basically, code says that if you have a 30-foot by 40-foot ranch over a bare dirt crawlspace, you need (30×40)/50= 24 square-feet of vents. If you’ve been paying attention, you already know why I think this is crazy.

First, the typical 8”x16” foundation vent installed into the crawl space foundation walls has an area of 0.9 square-feet. That means for the example given, you’d need to install 27 of these vents to meet code. That’s one vent every 5 feet! Second, are they nuts?! We just got done talking about how unless the air outside is colder than the air in the crawl space, you’re just driving more and more moisture into the crawl space by venting warmer outside air into it.

When it comes down to it, codes are first and foremost pushed by insurance companies and the building industries as a way of limiting liability. Happily, they also serve as a starting point to Builders that really care about doing quality work. Luckily, these Builders go beyond the codes in their work and sometimes are also are willing to wrangle with the code bureaucracies to make the standards better through the amendment process.

I’ve digressed. Getting back to code, another IRC code R408.3 provides an alternative to the massive venting required by IRC R408.1. Instead, this code says if you install a “vapor retarder” (plastic) over the floor that extends up onto the concrete walls, then you can install a small fan that blows 1 cubic-foot for every 1,500 square-feet of floor to the outside. This is a fraction of the ventilation required in IRC R408.1.

The code goes on to say that you have a couple choices regarding where the 1 cubic-foot ventilation air comes from. One option is to cut an opening (covered by a floor grill) that connects the crawl space to the first floor above. Another option is to install supply and return ductwork into the crawl space that connects to the main ductwork. Given that the house’s ductwork almost always runs through the crawl space, it’s a simple matter to cut in an opening into a supply and return plenums – see Ductwork Basics. Given what we know about pulling warm air into cooler spaces, IRC R408.3 is a much better option. Let’s examine some of the details of this code.

It’s great that IRC 408.3 allows for drastically reducing ventilation into crawl spaces when they’re seal up. It’s a step in the right direction. However, the code specifies that the vapor barrier plastic only needs to extend 1-foot up the walls with the remainder of the wall covered in foam insulation. Given what we know about water movement through concrete, my preference would be to run it to the top of the concrete walls.

In addition, IRC 408.3 still specifies venting warm are into a cool space – albeit much less than IRC 408.1 It doesn’t matter if the warm air comes from the heating and cooling ductwork or is passively drawn in by running an exhaust fan in the crawl space. Luckily, it’s not much air; it probably makes sense to get a little fresh air into the crawl space.

However, I would strongly recommend that anyone that does have a crawl space runs a dehumidifier in it and places a dehumidistat in each corner in order to monitor humidity levels. Of course, I’m assuming the floor has been properly covered with 20-40mil plastic that’s sealed at the seams and runs up the foundation walls. In other words, that it’s not moldy. If it is moldy, it’s time to think long and hard about what it’s going to take to clean it up.

EPA Crawl Space Exhaust Fan

When it comes to what ventilation method to use, my preference is to vent to the outside with a small fan and install a single opening into the first above – opposed to using air from the ductwork. In so doing, you’re creating slightly lower air pressure in the crawl space (suction). As such, air in the crawl space will be prevented from migrating up into the house. This is a good thing as there are other issues like radon to consider. Creating a slight negative pressure in the crawl space ensures air moves from the living area into the crawl space and not the other way around.

Crawl Space – Vented or Unvented

So let’s examine some data related to vented versus unvented crawl spaces. Maybe you’ve Google’d crawl space ventilation and found all sorts of opposing views. Don’t be led astray. Take a look at data published by the Advanced Energy funded by the U.S. Department of Energy at Crawl Spaces. In their study, they carefully monitored humidity levels in homes with vented crawlspaces and plastic covering the floor along side homes with zero ventilation and plastic that covered both the floor and walls.

Crawl Space Humidity Graph

The data clearly shows that when warm air is vented into cooler crawl spaces, humidity levels remain very high. The relative humidity (RH) levels in vented crawl spaces are easily enough to support mold growth. They wrote, “Comparative moisture measurements for these crawlspaces and sub-metered heat pump kWh use demonstrated that the closed crawlspace protocol produced substantially drier crawlspaces, reducing conditions for mold, wood decay and insects. It also reduced space conditioning energy use by 15 percent to 18 percent annually.”

I’ll finish this discussion on crawl spaces with an unfortunate story. A friend had a tri-level home. It was only a few years old. The area of the home over the 3-foot tall crawl space had a concrete floor poured over a plastic vapor barrier. In other words, the floor was well sealed. Although the walls weren’t sealed or insulated, the concrete floor over plastic was undoubtedly poured (dirt sealed) before the wood floor went in above it – as should be the case in any crawl space. Guess what? It was loaded with mold.

The humidity levels were just too high. There was a very fine, light colored mold behind much of the band joint fiberglass insulation in the wood floor framing above. The cardboard boxes and wood furniture stored in the space were likewise coated in this fine mold. You really had to look closely. Being inexperienced, I missed it. It took an experience Inspector who looked really closely to point this out.

By the way, one trick to detect mold is to swipe your finger over the surface in question and then rub your fingers together. If they feel slippery, it’s mold. Yep, crawl spaces are really problematic regardless of whether you have CIRS or not.

So here’s an example of a crawl space where the dirt was sealed before the wood flooring above was installed, and yet it was moldy. There were a few reasons for this. First, the walls were neither sealed nor insulated. This allowed water vapor to enter in the upper portion of the wall exposed to warm summer soil. Second, it’s really important to install proper moisture barriers to prevent water in the soil from wicking up into the walls. Third, these deficiencies could quite likely have been overcome had a dehumidifier been installed in the space. Crawl spaces take a lot of care to keep mold away.

Good Construction Practices

We’ve already started talking about what constitutes good water management practices when it comes to foundations. For the remainder of this article, I’m going to throw out some of what I’ve learned over the years. This list is in no way all inclusive. The practices used not only depend on what the climate is like but also the type of foundation. To cover this all in detail would take many books. In fact, for people that are thinking of building new or just want good information, I would recommend the Builder’s Guides and Building America Climate-Specific Guidance.

  • Hat & Boots: Every home should have a big “hat” and tall “boots”. What this means is that 6-8” of the concrete foundation should stick up above ground level. Now I know people think this looks “ugly” but to someone that understands the importance of keeping the bottom course of siding dry, it looks beautiful. Do not heap up dirt, wood chips, or anything else anywhere near the siding. If you’re building new, put some real overhangs on the house. You can size them to block the high summer sun while letting in the lower winter sun. More importantly, the large “hat” keeps much of the rainwater off the windows, doors, siding, and away from the foundation.
  • Drainage: If the ground doesn’t pitch away from the house or the gutters don’t include piping to carry the water away from the foundation, you’re asking for trouble. Personally, on new construction, I like running 4” PVC sewer pipe underground to carry rainwater well away from the building. One end stubs up out the ground to receive the downspout. When the ground slopes away from the house, the other end “goes to daylight”. Otherwise, I’ll install a popup lawn drain away from the house that allows the rainwater to flood out onto the lawn.
  • Band Joists: In general, I’ve always avoided spray foam. I just have too many concerns about off-gassing. However, when it comes to the wood floor joists that run around the perimeter of the basement ceiling, it’s really difficult to cut foam board or insulate and seal the ends of the floor joist bays with plastic. This is the only place I think a couple of inches of professionally installed spray foam makes sense. Be sure to hire a reputable company with equipment that comes on a truck. Say no to anyone that shows up with tanks the same size as what you use on your gas grill. It’s really important that the chemicals are mixed properly and applied when it’s not too cold.
  • Dimple Sheet: I love dimple sheeting on the outside of concrete walls. I remember buying the stuff when it still came from Europe and you had to wait weeks for it to arrive. On one job in particular, it became clear how well it works. The image to the right is Delta dimple sheeting.

    Delta Dimple Sheet

    I remember as the foundation hole was being dug, I started pacing back and forth because the muck coming out of the bottom of the hole was similar to what you’d see in a swamp. Even though the houses on both sides had sandy-gravel soil and were dry, there was an underground spring going right through this basement. The soil was sopping wet. I just shook my head.

    After getting confirmation from an engineer that the soil could bare the weight of the house, we got the foundation poured. At the time, they didn’t have some of the newer rubberized or peel-and-stick membranes. As a result, I decided to just install dimple sheeting and made sure to use a lot of gravel under the floor and covering over drain tile that ran on both sides of the footings. In spring, the sump pump ran almost continuously as the spring water flowed down the dimple sheeting, through the gravel bed, and into the plastic drain tile. In spite of the volume of water, the basement remained dry. There wasn’t even a hint of moisture. It was really impressive.

  • Waterproofing: Given all I know about mold and what we’ve discussed about moisture and foundations, my advice would be to wear a belt and suspenders when it comes to finishing the outside of concrete walls. As mentioned, use a rubberized or peel-and-stick membrane followed by dimple sheeting. My feeling is that you want to make the outside as watertight as possible with a membrane. However, all membranes eventually leak. Having the dimple sheeting will make this a non-issue.
  • Moisture Break: Make sure to use plastic between the concrete footings and the concrete walls that rest on top of them. This prevents moisture from wicking up into the walls. To keep the walls in place during backfilling, the mason should form a groove (keyway) into the top of the footings. The keyway locks the wall and footing together. Likewise, make sure to separate the concrete floor from the top of the footings and walls with plastic. Plastic should always go directly underneath concrete floors. Do not allow the mason to add a layer of gravel or sand on top of the plastic to aid drainage of “bleed” water when the concrete is poured.

    By the way, many believe the plastic shouldn’t be perforated. This is wrong. It’s perfectly fine if the mason wants to cut slits in the plastic to allow the bleed water to drain into the soil below.

  • Foam on Concrete: Always use closed-cell XPS foam board against concrete walls and below concrete floors. Select the thickness based up the perm rating and insulating value you desire. In so doing, moisture that transpires through finish basement walls will hit its dew point inside the foam where it can’t do any damage. If you want to add additional insulation to walls, this is fine. Just make sure that the any finish walls that go up are breathable.

    Take the time to study the details for your specific location in the Builder’s Guides. By the way, if you take my advice and use foam under the concrete floors, it can “eat up” precious headroom. To compensate, ask the mason to add and extra course of block, or to use taller forms for poured walls.

  • Breathable Finish Walls: Finish basement walls that cover over concrete walls must be breathable. As mentioned, tightly fitted foam always goes directly against the concrete. My first choice to fill the stud bays is AirKrete with rock wool coming in second. For wall board, I use fiberglass faced sheetrock like DensArmor Plus. It’s readily available, breathable, and nearly impervious to mold. On the down side, you can plan on having to apply two coats of plaster to cover over the rough surface composed of all those glass strands. This is no big deal for guys that sling “mud” for a living but it does add to the cost. Alternatively, I’m impressed with other boards too such as magnesium oxide wallboard but it seems like overkill for a non-structural wallboard, is harder to find, and costs more.
  • Sump Pit: For those with basements and sump pits, make sure to get a cover that seals the pit. Many of the cheaper covers are loosely fitting. In addition, it makes good sense to install a battery powered backup pump along with setting a couple of those moisture alarms next to the sump. If the pumps fails during a rainstorm, that is going to be a lot of water flooding into the basement.
  • Gravel: Use a lot of “poorly graded” gravel – gravel that is all one size to allow water to flow through. Ideally, you want a 4” bed of gravel underneath your concrete floors. The gravel under floors allows any water to flow into the drain tile running around the inside perimeter of the foundation. Additionally, by installing a vent pipe into this gravel bed, it’s easy to off-gas radon safely. On the outside, use lots of gravel over the outer drain tile. Cover this stone with a filter fabric to keep it from getting plugged up with dirt over time.
  • Cleanouts: Drain tile at the base of the footings can get plugged up over the years. To allow for cleaning it out, install a Tee into the outer drain tile and run pipe up to the surface. Do this in a least a couple places. I would often run this clean out pipe up into window wells that were filled with gravel. I’d fit a grate over the end of the pipe. In so doing, the pipe serves double duty. In the event the drain tile becomes plugged, this pipe makes flushing out the tile easier. It also acts as a fail-safe against the window wells from filling up with rainwater.

Admittedly, I did all my building up north where we have over 8,000 heating degree days. It gets cold up here in winter. As such, my focus is on construction for cold weather. Nonetheless, if you live in a warmer region, much of what I said still holds. However, please make sure to do your homework and read the Builder’s Guide and Building America Climate-Specific Guidance for your specific location.

My last bit of advice is to not assume the builder or mason knows about good water management practices. In fact, they probably don’t. One useful measure of a good contractor is their willingness to learn, or at least accommodate changes in how they do things. As such, when a contractor walks away because they won’t hear about other ways to do their job, consider this a good thing. He just saved you both a lot of headaches.

2 thoughts on “How to Find and Prevent Mold: Basements, Crawl Spaces, & Concrete Floors

  1. Hello, sorry i write in the wrong article, but i have a huge Problem. I have since many years PND, wasnt sure before its infectious, i had the feeling it is, but now i know for sure because i infected my 4 years old daughter. Please help me, i read your marcons article very closely and have some questions. For example you say to give symbioflor 2 to the nose, but i know this product and you only give symbioflor 1 to the nose. I dont know if it was a mistake and have some other questions because i’m from Germany and don’t know if they even make such Tests as you described so im thinking about just doing the Natural stuff. Im so depressed, its one thing to have this sickness but infecting my little Baby girl is really killing me. I See this Place as my best chance because its very hard to get information for this problem. And sorry for my spelling, writing from my phone with a German layout. Best regards Martin

    • Hi Martin,

      Really sorry to hear about your little girl. I think you’re doing right by trying to look into this a bit. I don’t take many questions these days because I simply don’t have the time. Here are a few thoughts.

      MARCoNS is only one piece of Chronic Inflammatory Response Syndrome (CIRS). You didn’t mention if you’re symptomatic besides having Post-Nasal Drip (PND). If you read about the types of bacteria that live in people’s noses, many of them are quite contagious. Conventional medicine considers most of these to be “commensal” – meaning they’re not really a problem. I’m going to write a new article on MARCoNS soon that gets into why this may not always be the case.

      If it’s more than just PND and you think you have CIRS, this will be a challenge to get proper testing. I have heard of people outside the U.S. getting the MARCoNS swab so I think this is possible for you in Germany. Make sure to call and use MicrobiologyDX. See MARCoNS Testing.

      In the new article on MARCoNS I’m writing, I’m going to discuss how I’ve found most alternative treatments to be helpful but ultimately not successful in treating MARCoNS. Anyway, this is getting ahead of the situation for you. The first step may be to do a nasal swab and look into CIRS more closely. You could also try posting questions at Mold Avoiders.

      Regarding the probiotic Symbioflor2, this was a remedy mentioned by Dr. Klinghardt that I included mostly as a way of being through-all in discussing the topic. I can’t really speak to it much more beyond the notes I made on the MARCoNS page about my experience with using probiotics.

      Lastly, try to take deep breaths. We all do the best we can. This is where you’re at right now and you’re working with the situation. You love your daughter and I’m sure the feeling is mutual. This is wonderful. Over time, I’m sure you’ll find a good solution for the two of you.

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