The question we get most
A cold plunge in my bathroom. Won't that cause mold?
One owner answered it by accident. He replaced his bathtub after a year of daily plunging, and photographed what was underneath before the new one went in.
Brad Hunter. Texas. Four photos, nothing cropped.
The short answer
One HomePlunge owner ran an H3 in a built-in bathtub at 48 degrees, around the clock, for more than a year. When he removed that tub during a remodel he photographed the subfloor and the wall framing underneath. He found dust, no evidence of moisture, and no mold. He ran his bath vent all day and kept the room under 40 percent humidity, in Texas. That is one bathroom rather than a laboratory result, and the conditions he ran it in are part of the answer.
Mold needs moisture and time. The thing worth managing is whether your bathroom stays wet and unventilated, not whether the water in the tub is cold. Below are all four of his photographs, his account, the conditions he ran it in, and the parts we could have cropped out.
Verified owner
Brad Hunter has been in our H3 testimonials since 2025.
That is him alongside, in the same testimonial that runs on the H3 product page. Same customer, a year before any of this came apart.
He did not shoot the photographs for us. He was replacing a built-in corner tub with a freestanding one, opened up the floor and the wall, and emailed to say he thought it might be useful.
His HomePlunge ran in that built-in tub, held at 48 degrees around the clock, for more than a year.
What he found
"There was nothing but dust underneath. No evidence of moisture. No mold of any kind."
Brad Hunter, HomePlunge owner
He checked before the new tub went in, which is the only time anybody ever gets to check. A year of cold water in a built-in tub, and what came out from under it was dust.
He also made the point that he lives in Texas. If anyone was going to have a humidity problem, he figured it would have been him.
The teardown in 35 seconds. No sound needed.
The part we are not leaving out
His result came with a routine.
The vent
Runs all day, every day.
A dehumidifier
Keeps the room under 40% humidity.
The tub
Held at 48 degrees, around the clock, for over a year.
This is one bathroom in Texas, not a laboratory, and we are not going to pretend it is a guarantee for yours.
Run your bath fan. Keep the room dry. That advice is free and it is good advice whether or not you ever buy anything from us.
Built for a bathroom
What we ship for exactly this.
Insulator
A cover that stays on the tub between plunges. Keeps the water clean and adds insulation.
Bath Stone
Natural diatomaceous earth. Absorbs water on contact, dries itself, resists mold.
Built-in filtration
Keeps your water circulating and clean between sessions. The filter is reusable, rinsed every four to six weeks. There are no replacement filters to buy.
A weekly reset
Drain the tub, refill it, start fresh. Most owners make it a Sunday.
Questions people actually ask
Does a cold plunge in your bathtub cause mold?
One HomePlunge owner ran an H3 in a built-in bathtub at 48 degrees, around the clock, for more than a year. When he removed that tub during a remodel he photographed the subfloor and the wall framing underneath. He found dust, no evidence of moisture, and no mold. He ran his bath vent all day and kept the room under 40 percent humidity, in Texas. That is one bathroom rather than a laboratory result, and the conditions he ran it in are part of the answer.
What did he actually find under the tub?
In his words, there was nothing but dust underneath, no evidence of moisture, and no mold of any kind. He also explained the two things in the photographs that look like something and are not. The mark on the green board is splash from inside the tub. The open section of wall was left that way when the original tub went in, so the motor and pipes could be reached from the closet on the other side.
What keeps a bathroom dry if you cold plunge in it?
Run the bath fan while you plunge and for a while afterward, and keep the room's relative humidity down. The owner in these photographs ran his vent all day and used a dehumidifier to hold the room under 40 percent. Mold needs moisture and time, so the thing to manage is how wet the room stays, not how cold the water is.
Does HomePlunge include anything that helps?
The Insulator is a cover that stays on the tub between plunges, which keeps the water clean and adds insulation. The Bath Stone is a natural diatomaceous earth mat that absorbs water on contact, dries itself and resists mold. The H3 has built-in filtration that keeps the water circulating and clean between sessions, with a reusable filter rinsed every four to six weeks. Drain and refill the tub weekly.
The bathtub you already own
No dedicated tub. No plumbing. No bags of ice.
The HomePlunge H3 turns the bathtub in your bathroom into a cold plunge, and rolls into a closet when you are done.
4.95 stars from 94 verified reviews. CES Innovation Award winner. Named Best Indoor Cold Plunge by Garage Gym Reviews.
