Learning Center

Mold test at home kits: the new snake oil

Moldy Fan

Over the past decade concern about the health risks associated with exposure to mold has increased. With this increased concern a new market for test at home mold kits now exists. Home Depot and many other stores now carry test at home mold kits that are supposed to be a cheap way to identify a mold problem in the home. The kit consists of a petri dish with a nutrient solution in it. Though the specific instructions for each kit may vary, the general instructions are to open the petri dish and let it sit out in your home for a period of time before covering it. If mold is growing in the petri dish a few days later than there is mold in the house, if it’s not than the house is clean. That’s the theory anyway; the test would lead you to believe it’s that simple to tell if there is mold in your home. In truth results from this type of test don’t mean anything.

The problems with this testing procedure are numerous, the biggest problem being that mold spores are a natural component of the air. Mold spores are everywhere, they’re on your clothes, they’re in your hair, on your keyboard, everywhere. If a petri dish is left open on the counter for an hour it’s almost guaranteed that at least one viable mold spore will settle into it. Even if you took the dish into a completely sterile room a spore could float of your shirt and settle into the dish. There a probably even mold spores on the packaging for this test, just opening the petri dish near the box it came in could get a mold spore in the dish. And once that spore settles into the petri dish its surrounded by a nutrient rich solution.


A nutrient rich solution sets up the perfect conditions for mold growth to occur. So not only is it very likely that a spore will settle in the petri dish but once it does it’s very likely to grow. In fact, getting mold to grow in a nutrient rich solution is about as hard as getting a fat kid to eat a second piece of cake. In truth anyone could perform this exact same test without buying an expensive test kit. Just take a piece of bread, wet it slightly, and set it out on the counter for a couple days. If the bread is moldy after a few days than a test kit would also have mold in it.

The point here is you would expect a wet piece of bread to be moldy after a couple days and you should expect this test kit to have some mold in it after a couple days as well. What does it mean? Nothing. This test yields no valuable data whatsoever. The only real way to know if you have a mold problem in your house is to have an inspection performed by a certified mold professional. This inspection should primarily be a visual assessment of the home, attic, basement, and crawlspace looking for visible signs of mold. Testing of some sort may be a part of that inspection but testing should only be one tool of many in diagnosing the environmental conditions in a building. And that test should never be a petri dish or settling plate left open to the environment to see what grows.

Mold and Building Products: Then and Now

10 years ago mold was largely considered a mild nuisance, but over the last decade health effects and insurance claims related to mold have increased dramatically. There are many contributors to the increase in mold awareness, media coverage of “toxic black mold” being a big one, but perhaps the biggest contributor is the change in building products over the last fifty years. Fifty years ago a new home would have plaster walls and the framing was mostly heartwood. These products are far more moisture friendly than some of the building products we use today.

Before we delve into the difference in building products we need to understand a few things about what mold needs to grow. Mold has the same basic requirements for life that we do. It needs oxygen, food, and water just like we do. The oxygen it needs will be easily obtained anywhere people are because we share the same need. The food mold needs is acquired from cellulose based material (dead plants), specifically the glucose in this material. Houses are built with a lot of cellulose material so mold can find food in most of our housing. Lastly for mold to grow it needs water, and of the three requirements listed this is the only one we have any real control over. Now that we know these requirements for mold growth lets examine how different building products handle mold.

Mold on Wall

Plaster can hold a lot of water before there is enough moisture available to support fungal growth. When every wall in a house is covered by an inch of plaster the total amount of water it can hold before you have a moisture problem is really astounding. Unfortunately plaster is a painstaking slow process to install and that is why gypsum drywall, which is today’s most common interior sheathing, is far more common in new construction. Gypsum drywall is essentially just two pieces of paper sandwiched around a sheet of chalk (gypsum). Gypsum drywall can hold less than 1% of its weight in water. The water capacity of plaster is exponentially greater and this is one reason that newer homes with gypsum drywall are more likely to have mold related issues than older construction.

Another reason new building products are more likely to suffer from mold growth is the actual difference in the wood we frame our houses with. The outer layers of wood on a tree are called sapwood. This wood is very porous and carries water and nutrients from the soil up the tree. As a tree ages and grows larger in diameter the sap wood near the center becomes heartwood. Heartwood is very hard and dense, it acts as a structural base for the tree to grow around. Heartwood is also very resistant to insects and decay. Older homes were framed with a lot more heartwood than you find today. The lumber we use today is primarily sapwood as it grows faster and is thus cheaper to produce in mass than heartwood. Old houses framed with heartwood have a resistance to fungal growth because the glucose in heartwood is so much harder for mold to get to. Sapwood, by contrast, is soft and the glucose is more easily acquired.

Thermal Image

New construction also includes a lot of processed cellulose materials that were not available in houses built over 50 years ago. The more heavily processed the cellulose material the easier the glucose is to get to. The easier the glucose is to get to the quicker mold will grow. Some examples of processed cellulose material are plywood, OSB, Particle board, MDF, and paper. Paper is the most heavily processed cellulose material and will likely be the first thing mold grows on. Unfortunately we now sheath the interior of our homes with gypsum drywall, which has a paper covering. So we quite literally line the insides of our homes with paper now.

The new building products are superior to the old in a number of ways and that’s why we use them. Unfortunately a new and improved product doesn’t mean new and improved in every way. The use of more heavily processed cellulose materials and the very low water capacity of gypsum drywall make new construction more likely to suffer from fungal growth than older homes. This is one of the big reasons the mold is more of an issue now than it was in the past.

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