John Maher:  Hi, I’m John Maher, and I’m here today with Robert Weitz of RTK Environmental, an environmental testing and consulting firm. Robert is a certified microbial investigator, and today we’ll be talking about preventing mold in your house. Welcome, Robert.

Robert Weitz:  Hi, John. Thank you.

Where does mold grow in a house?

John:  Robert, what are the typical areas of the house where mold grows?

Robert:  I’ll preface this by saying that mold can grow pretty much anywhere in the house, particularly if there’s a pipe break or some unusual occurrence like that. Mold will grow pretty much wherever there’s moisture.

But the most common areas that we see at RTK would be the basement. That would be the number one place where inspectors spend a good part of their time. Attics, bathrooms, kitchens, anywhere where there is water, moisture, humidity levels, lack of ventilation. Those are going to be the most common areas where mold would be found.

Prevent Mold in the Bathroom

John:  What can I do in the bathroom, say, to help prevent mold from growing?

Robert:  In a bathroom, one of the main reasons that mold will grow in a bathroom is from people taking that hot shower. Either they don’t have an exhaust fan, usually a ceiling fan, they don’t have one, or they don’t use it. That would be a primary cause.

What happens is the steam from the shower is just little droplets of water, and those water droplets will rise, and they get on the ceiling. Then typically the ceiling and the upper walls just gives the opportunity once it has water and moisture, that’s going to give that opportunity for the mold to grow. That’s the most common area.

Another common area is around the toilet. When a toilet is installed, it’s seated down on to the floor with this ugly little wax ring. And if you don’t seat that toilet exactly properly, little leaks will occur around it, and they will spread out on to the floor.

You may have a tile or any other type of a floor and underneath you could have mold growing and never even know it until it starts to come to the surface. Also, pipe leaks, pipe breaks, around a sink or any other faucets, these would all be common places for mold to grow.

Preventing Mold in the Kitchen

John:  What about the kitchen? What can I do in the kitchen to help prevent mold growth?

Robert:  A kitchen is similar in some ways to a bathroom. There’s some steam that can be generated from boiling water and what not, but typically that would not be a main cause.

The primary area that we see in a kitchen where mold would be would be under the sink. Very often a leak will occur in the drain or the cold or the hot water line. Very often it’s a very slow leak that we don’t see, and little by little that water will drip down into the bottom of the cabinet. I don’t know about yours, but my kitchen cabinets are usually pretty full.

John:  Right.

Robert:  You don’t see it, and then you start to get that odor, that musty odor. You open the cabinet and go, “Woof, that’s where that smell is coming from.” You take a few a things out of there, and over time as that leak has gone on, it’s grown mold not only on the shelf that’s at the bottom of that base cabinet, its also now got underneath and possibly down into whatever space is below, either a basement, a crawl space, or a finished room that might be below.

That’s the most common place that we would see mold grow in a kitchen, other than if a pipe were to burst or something like that, which would occur all at one time. Then mold will grow within 24 to 48 hours. If that’s not dried up completely right away, including under cabinets and in areas that we can’t see, that mold will grow very quickly.

John:  Would you recommend checking under my kitchen sink fairly often to make sure there are no leaks occurring there that I’m not aware of?

Robert:  Absolutely. It’s obviously a very handy place to keep things.

John:  Sure.

Robert:  But not to keep it packed with things, so that when you do open it, if there is a potential leak or there is something leaking, you’d be inclined to see it fairly quickly, and then you could do something about it before mold really has the opportunity to take hold.

Basements and Mold Growth

John:  You mentioned the basement as well, and the obvious reason that a basement would be a problem for mold would be if your basement floods, which I know happens with a fair number of homes. Is that the main reason, or are there other reasons that basements are a problem area for mold growth?

Robert:  Actually, certainly when water seeps in through the foundation, heavy rainstorm, gutters and downspouts aren’t functioning properly. The grade of the soil may be wrong, allowing groundwater to come in. Those are very common ways.

Equally as much as that is high humidity levels. Obviously a basement is a subterranean area, so it’s almost always completely underground. We have a concrete foundation typically, or stone in an older house. That will hold humidity levels in.

We don’t have ventilation of air like we do on the upper level. We don’t have windows that are opened and closed, typically, doors that allow ventilation of air. It’s usually a fairly closed space, even if it’s finished.

When we get elevated humidity levels that in and of itself will cause mold to grow. We like to see those humidity levels below 50 percent. 50 percent humidity in the air or less usually will not foster mold to grow. Once you get up above that, and you get towards 60 and 70 percent, and up above that, mold loves that condition.

Very often basements are a dark space. You might not go down there very often, or you go down, turn the light on for a short period of time, and then it’s shut off for sometimes days or weeks at a time. That is the environment that mold really likes.

It really indicates how important it is to keep the humidity levels down below 50 percent. This is done either by ventilation or more often it’s done with dehumidifiers. A dehumidifier will keep the humidity level down, and not allow mold to grow. That’s a very common way for mold to grow in a basement, is elevated humidity.

John:  Do you pretty much recommend having a dehumidifier in your basement for about everybody?

Robert:  Pretty much, yeah. In our area particularly, in the northeastern states down into the mid‑Atlantic, we live obviously very close to a very large body water, being the Atlantic Ocean. That makes our water table very high in most areas that are within even 100 miles of the coast.

When you have that higher water table, that’s going to make these basements a damper place, and dehumidifiers are almost always recommended for any basement in our service areas up and down the coast.

Mold in Attics

John:  Are attics similar to basements? I know in terms of temperature, they can be very different. My attic gets really hot in the summer. Is that a problem for mold growth? Where can I work on some things in my attic to help prevent mold growth?

Robert:  The primary cause…it’s not so much the high temperature, because certainly attics in the summer get very hot. In the winter, they get very cold, because they’re not heated or cooled spaces.

The most typical thing that we see in attics is going to be formed by condensation. Condensation will form on usually the ceiling of the attic, which is the underside of the roof. The reason for that condensation forming is lack of ventilation.

Houses have to breathe. If we don’t allow them to breathe, then the air becomes stagnant. Then when you have that high temperature in the summer time, which could be 120 or 130 degrees, you add some moisture to that in the form of condensation from a lack of ventilation, you’re going to grow mold very, very quickly.

The wood surface, which is that ceiling, is a wood surface and mold loves that. It’s going to grab hold. Mold is naturally occurring in the environment. As it passes around through the air, those mold spores are going to land, and they are going to grow and prosper on a moist surface like that.

Getting a Mold Inspection

John:  When should I get a mold inspection versus trying to take care of the problem myself?

Robert:  From my perspective, John, I know so much and I have so much information. It’s always a good idea to have a mold inspection.

It goes back to education. Why would I do that? If you have visible mold, then you certainly want an investigation. You want to find out what kind of mold do I have? Is the air contaminated in that area? Is the air contaminated in adjacent areas where mold spores can fly through the air on air currents?

That’s certainly one circumstance, but we also a lot of clients that have us come back on a periodic basis. They have us come back every six months or every year, to do another mold inspection because they want to be sure that mold is not growing in areas that they can’t see.

We have a number of different techniques, including thermal image cameras, moisture meters, and air sampling that we can use to not only see visually what you see within a room, but to literally see what you cannot visibly see that might be inside walls, ceilings, or any other floor cavities that are hidden by finished components.

John:  Among the other things that you can do to help prevent mold, I’m hearing that the most important things would be turning on your bathroom fan or installing a bathroom fan for ventilation in your bathroom, checking the drain under your kitchen sink on a regular basis to make sure there are no leaks there, putting a dehumidifier in your basement, ventilating your attic, and then of course, calling to get a mold inspection done to make sure there’s no mold, maybe in places you can’t see.

Robert:  That’s correct. That’s a very good summary.

John:  Great. Robert, thanks very much for speaking with me today.

Robert:  Thank you for having me, John.

John:  And for more information, you can visit the RTK website at, or call 1‑800‑392‑6468.

Photo credit: Falcon_33 / Foter / CC BY-SA

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