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I frequently get asked if my inspections include mold testing.  With this blog post I’m going to answer questions about mold and mold testing.  Much of the information here is taken from this document from the U.S. Centers for Disease Control (CDC) and it’s a great place to go for more detailed information.

Another great resource is the U.S. Environmental Protection Agency and their website:

The first thing to understand is that there is almost certainly mold in your house – in the house you live in now, in the house where you lived as a kid, and in the house you’re going to buy.  There’s also mold outside.  There’s mold pretty much everywhere in our environment.  It’s very common.  From the CDC document:  “There is always a little mold everywhere – in the air and on many surfaces.

So to answer the question: I don’t do mold testing, and the CDC (and really nobody at all) recommends routine mold testing.  From the CDC:  “CDC does not recommend or perform routine sampling for molds.

There’s really no test that can tell you definitively if there’s a mold problem in your house.  There’s mold, for sure, but is it a problem?  You can hire a specialist to take air samples, but what do those results tell you?  Nothing, really.  Again from the CDC:  “Standards for judging what is an acceptable, tolerable or normal quantity of mold have not been established.  Sampling for mold can be expensive, and standards for judging what is and what is not an acceptable quantity of mold have not been set.

So you can have air sampling done, and get back a report with some numbers on it, but there’s no authoritative answer as to whether those numbers are high or low.  So there’s really no point.

So how do you know if there’s a mold problem in your house?  Again from the CDC:  “Large mold infestations can usually be seen or smelled.”  If we can see it, it’s a problem.  Or if we can smell it, then it’s a problem.  It’s pretty much as simple as that.

Mold needs food and water to grow.  It can get food from many different sources, including paper products (like the paper facing of drywall), cardboard, ceiling tiles, and wood.  Mold can also grow in dust, paints, wallpaper, insulation, carpet, fabric, and upholstery.  Mold gets water from the environment, and that’s really where the problem is.  If there’s enough water for mold to grow then there’s too much water and you need to stop the water.  From the CDC:  “Mold growing in homes and buildings indicates that there is a problem with water or moisture.  This is the first problem to address.

Any kind of water problem is at the top of my priority list for a home inspection, be it a roof leak, a plumbing pipe leak, foundation water seepage, or a condensation problem.  Looking for water problems is the key to finding and stopping mold.  So the best mold test is really just a test for excess water, along with a very thorough inspection to look for any visual signs of mold.

But what about a mold test to determine the type of mold in your house?  Again, not necessary.  From the CDC:  “If you can see or smell mold, a health risk may be present. You do not need to know the type of mold growing in your home, and CDC does not recommend or perform routine sampling for molds. No matter what type of mold is present, you should remove it. Since the effect of mold on people can vary greatly, either because of the amount or type of mold, you cannot rely on sampling and culturing to know your health risk.

Some people refer to “black mold” as if that’s the real problem and a serious health risk.  But again that’s vastly overstating the issue.  From the CDC:  “Mold growth, which often looks like spots, can be many different colors, and can smell musty.  Color is not an indication of how dangerous a mold may be.  Any mold should be removed and the moisture source that helped it grow should be removed.

So it’s clear that the key to answering the question of mold isn’t a mold test, it’s just a very thorough inspection with an eye towards current and potential water problems.  And of course experience helps in knowing where to look.  Based on my experience, here are some important places to look.

Closets — Most closets have at least one outside wall, and that’s where water can leak in to help grow mold.  And that’s especially true if the closet is in the basement.  Plus there are usually a lot of things stored in a closet, so it’s especially important to move those stored items to try to get a look at the wall behind.  I’ve seen several basement closets that had water seepage behind the wall that was leading to mold growth, and the only way to find it was to be vigilant and move the stored items to be able to see the wall.

Mold on a below-grade closet wall.


Basements — Basements often have water problems.  Sometimes it’s seepage through the foundation or up through the floor slab.  Sometimes it’s sewer pipes backing up, or one of many other problems.  I’ve seen lots of drywall in basements that had mold around the bottom.  It’s important to look for this.

Condensation — Condensation is one source of water that can feed mold growth.  Here’s one example that I saw a while ago.

I was inspecting a rather large house and it had a separate pool house, with a family room, a small kitchenette, some small loft areas for sleeping, and a bathroom.  Being a pool house, it wasn’t heated very well.  The only heat source was in the family room, and there was no heat source in the bathroom – strike one for the bathroom being cold.  Plus the bathroom was in the corner of the building, and that’s often the coldest area because a corner room has more exterior wall than other rooms – strike two for the bathroom being cold.  Now look down at the baseboard around the floor, and that’s usually the coldest part of a room because warm air rises – strike three.  Now look in the corner, which is usually the coldest part of any room because warm air can’t circulate well there – strike four.

There was mold on the baseboard in the very corner of this bathroom.  That space got very cold because of all the strikes against it, and that allowed condensation to form, and that allowed mold to grow.  From experience I know to double check areas that are likely to be quite cold and allow water from condensation.

Mold on the coldest surface of the house due to condensation.

That’s mold.

Plumbing leaks — I use an infrared camera to look for water problems below all the sinks, tubs, and showers after I’ve run a lot of water.  It’s not very common, but I have occasionally seen some pretty dramatic leaks that were only visible with infrared.  Part of the repair process for these types of leaks is to check for mold above the ceiling and behind the wall, and I always make sure my clients are aware how important that is.

So when might mold testing be a good idea?  If you suspect that there’s hidden mold because you can smell it or feel its effects, then it might be time to call in an expert, form a hypothesis about what might be going on, and do some testing to try to confirm that hypothesis.