It is in showers, behind walls, and deep in linen closets. It is black and slimy, white and fuzzy, or green and spongy. It is making homeowners struggle to breathe and costing them millions in repairs and healthcare bills, they say. Mold is driving Americans crazy, and they want their home builders to pay.

Modern energy efficiency codes have lowered the margin of error for builders when it comes to moisture management. And construction defects can cause mold growth, which thousands of Americans believe is making them sick.

Mold has been linked with exacerbating asthma and other respiratory issues, medical researchers say. Meanwhile, online communities and functional wellness doctors have connected it to symptoms ranging from emotional disturbance to heart problems.

Those claims helped spark a rise in the number of lawsuits against home builders. Homeowners say their builders have made mistakes as they attempt to put up homes as quickly and cheaply as possible, installing improperly sized HVACs, putting in windows incorrectly, or poorly ventilating attics.

"There's a frenzy of building activity, and everyone is in a hurry," said Ernest Freeman, a Houston-based lawyer who represents homeowners suing builders.

Builders say the fears are overblown. Many of these lawsuits engage vastly different opinions on the health impacts of mold, how it should be measured and what is required to remove it.

Mold can cause different reactions in different people, and how it has the impacts it does isn't completely understood, said Mark Mendell, former air pollution research specialist at the California Department of Public Health.

TikTok and Reddit channels discuss "Toxic Mold Syndrome," a series of vague symptoms including sudden fits of anger and brain fog stemming from systemic mold in the body. Health Secretary Robert F. Kennedy Jr. has suggested that mold may be a cause of autism.

"People will often attribute things they don't understand to dampness and mold," Mendell said.

What is more clear is that these lawsuits are long, unpredictable and exorbitantly expensive for both sides. For America's largest home builders, they are adding to their growing legal costs and weighing on their profits as they cope with a stagnant housing market.

"The mold litigation isn't going away," said Tom Curran, a partner at law firm Duane Morris.

Many lawsuits pursue damages for more than the total value of the home, and in high-profile litigation involving a number of homes plaintiff lawyers are looking for hundreds of millions of dollars from home builders.

D.R. Horton, the country's largest home builder, faces litigation involving thousands of Louisiana homeowners who say shoddy building practices failed to keep out moisture, leading to widespread mold growth in their homes.

Its reserves for legal claims, which are based on expectations for future claims including those involving mold, rose 57% to $1.1 billion from the end of fiscal 2022 to the end of fiscal 2025.

Meanwhile, the Seminole Tribe of Florida is alleging in litigation that Lennar, a home builder, built more than 450 defective homes with improperly installed roofs and mold-ridden walls, causing health problems.

Lennar's self-insurance reserve, earmarked for liabilities that insurance won't cover, rose 21% in fiscal 2025 to $336.9 million.

D.R. Horton didn't respond to requests for comment. Lennar declined to comment. But builders have asserted that contracts and warranties signed with homeowners shield them from liability for the claims and that often mold problems are the fault of poor maintenance.

New energy-efficiency regulation may be making builder mistakes more noticeable. Standards for keeping homes "tight" and insulated to prevent air from escaping are critical to keep homes comfortable, but also limit their ability to dry, increasing the likelihood of mold growth if builders make mistakes, especially in the humid Southeast.

"The industry standard of care needs to evolve to be better," said Christine Williamson, an architecture professor at Virginia Tech. "We really need to be attentive about how we're installing windows."

Home builder lawyers say that plaintiff lawyers are relying on flawed testing methods and mold assessors with questionable qualifications, while taking advantage of a growing hysteria across the country about mold's health effects.

"Some of these claims are really tangential at best," said Ian Faria, a construction defect defense attorney in Texas who is currently defending against about 20 mold lawsuits. "Litigation like this is making [houses] really unaffordable because builders are having to spend a lot of money on experts and on lawyers."

Lawyers on both sides often use tests that measure the number of mold spores in the air. But those tests can produce vastly different results depending on how they are performed and there are also no standards for how many mold spores are too many.

Meanwhile, the mold experts hired by plaintiffs and defendants often come to vastly different conclusions about what is required to remediate the problem. "One will say it will cost a million dollars, the other will say it costs $50,000," said Clay McCullough, a lawyer in South Carolina.

Ashley Frazier is suing Lennar over mold growth in her home, which she bought in 2021. Less than two years in, she started having daily asthma attacks, as well as headaches, dizziness, and rashes while her walls were sweating and windows were fogging up because of the humidity.

She brought in a mold assessment company that told her that her home was uninhabitable. They conducted tests of the house, finding mold levels as high as 2.2 million spores per cubic meter. Just days before, Lennar had come in with its own mold assessor that found levels of around 500 spores per cubic meter.

By the end of the case, Frazier will likely have spent $75,000 on experts, while the problem has also devastated the value of her home, which was recently assessed at $117,000, down from $318,700. In Texas, as in many states, sellers have to disclose the presence of mold to potential buyers.

"It's a total nightmare," she said.