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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.

Water, Water Everywhere

“Water, water, everywhere” goes the old saying.  And that might be a good thing for a sailor, but not so much for a homeowner.

I tell all my clients that water is their home’s number one enemy. If you keep your house dry, it’ll last for a long long time.  If it gets wet, it goes downhill fast.

Of course, I’ve never met anybody who’d be happy going outside to the well every time they wanted a drink of water or to take a shower.  So we bring water pipes into our houses.  And we don’t (at least none of my clients) live in the desert, so rain water (snow too!) will fall on and around our houses.  We can’t stop water, we just have to control it.

Too much water in the attic. Notice that only the roof sheathing is affected, and not the rafters. That’s because the sheathing is the coldest part of the attic, so that’s where the water vapor condenses.

It’s important to keep in mind that water comes in different forms.  We have to control liquid water of course, but we also have to control water vapor.  And that’s usually the hard part.

When I see mold in an attic I know that there’s too much water in the attic.  The problem might be a roof leak – liquid water getting in.  But the problem might also be water vapor coming up from a wet basement or crawl space.  I saw quite a few cases of moldy attics last year and in all but one case the culprit was pretty clearly a wet crawlspace.


When the basement or crawlspace is wet that moisture easily evaporates into the air.  And that moist air diffuses throughout the entire house.  Water vapor wants to move from areas of high concentration and warm temperature to areas of lower concentration and colder temperatures.  The coldest and driest part of most houses is the attic (at least in the winter) and so the water vapor is going to be driven into the attic by natural forces.

Water in the crawlspace eventually ends up in the attic. Count on it.

Many people seem surprised and even a little dubious when I tell them that a wet crawlspace is the source of their attic moisture and mold problems.  But consider that in a typical house in this area you can expect anywhere from about 0.3 up to 0.7 natural air changes per hour.  This means that the total of cold air leaking in and warm air leaking out of your house will equal somewhere between about 0.3 and 0.7 times the size of your house.  So in about 1.5 hours up to maybe 3 hours the air in your house completely changes over.  Air moves, pretty fast.  And the water vapor in the air diffuses even faster than that.

So in short order that water vapor finds its way up into the attic.  When it hits the cold surface of the roof sheathing the water vapor condenses, and you get just the right conditions for mold to grow.

There are three general ways to control water: source control, ventilation, and dehumidification.  Source control involves stopping water from getting into the attic in the first place, and so stopping it from getting into the crawlspace.  Or we can ventilate the attic to bring in dry outdoor air and remove the moist air.  Or we can install a dehumidifier in the attic to remove moisture and drain it away.

Ventilation is important in an attic, and most home inspectors make a deal out of a lack of attic ventilation.  But I’ve seen so many attics with bad or no ventilation that had no problems.  And really, why let water vapor get into the attic in the first place and then be forced to deal with it.  Why not keep it out in the first place?

By far the best option is source control.  It’s always best to avoid the problem completely if you can.  Source control is the easiest, most dependable, and most robust solution.  Keep water out of the attic – and that means keeping water out of the crawlspace.

The way to keep your crawlspace dry is for another post, but here are the highlights.  There should be a vapor barrier on the ground to keep water from coming up through the ground.  (The ground is very wet – that’s why trees have their roots there!)  The gutters and downspouts should be in good condition to move rain water away from the house.  And the ground around the house should be sloped away so that rain water flows away from the house.

Stay dry my friends.  Your house will appreciate it.


Water, water, every where,

And all the boards did shrink;

Water, water, every where,

Nor any drop to drink.

“The Rime of the Ancient Mariner”  by Samuel Taylor Coleridge