The Practical Guide
to Mold & Moisture

Learn how moisture problems start, how to investigate them room by room, and what to do about what you find.

A look inside

Plain-language explanations, starting with how mold grows.

Spot 2 of 8

Tub, shower, and ceiling above

Grout lines, caulk joints, corners, any shelf or niche in the shower, and the ceiling above the tub.

Why it happens here

Every shower soaks the grout and caulk, and steam rises and condenses on the ceiling above the tub, which is one of the most common spots for growth in a bathroom. If the room doesn't dry out between uses, growth starts on the surface, and cracked grout or failed caulk lets water get behind the tile.

Signs you might notice

Dark spots in grout or caulk that come back after cleaning
Caulk that's pulling away, cracked, or discolored underneath
Loose, hollow-sounding, or shifted tiles
Spotting in corners and around the niche or shelf
Dark spotting or peeling paint on the ceiling above the tub

What you can safely look at

Look closely at the caulk along the tub and in the corners, then look up at the ceiling above the tub. Spotting only on the surface of the grout is common. Spotting under the caulk, or tiles that move when pressed, points to water getting behind the surface.

How to prevent it

Run the exhaust fan, or open the window and door, during and after showers
Squeegee or towel off the walls after showering
Leave the curtain or door open so the shower can dry
Recaulk as soon as you see cracks or gaps

Call a professional if

Tiles are loose, or the wall behind them feels soft
You see staining on the ceiling of the room below

Who to call: Contact a tile or shower contractor for failed caulk, grout, or waterproofing, or a plumber if the source is a plumbing leak.

Explore 12 areas of the home with interactive floor plans. Tap a spot to see why moisture collects there, what to check, and who to call.

The kinds of tests

Air sample

Spore trap

How it works: A pump pulls a measured amount of air through a cassette with a sticky slide, and a lab counts and identifies the spores. Outdoor samples are taken at the same time for comparison.
What it tells you: Which types and amounts of spores were in the indoor air at that moment, compared with outdoors and with other rooms.
What it can't: It doesn't capture gases like MVOCs, it's a snapshot of one moment, and it can't tell you where growth is.

Surface sample

Swab or tape lift

How it works: Material from a visible spot is collected on a swab or clear tape, and a lab examines it under a microscope.
What it tells you: Whether what you're seeing is mold and what kind, to the genus level (for example Aspergillus, Penicillium, Cladosporium, or Stachybotrys), plus indicators such as hyphae or spore-producing structures, which can support that fungal growth occurred at the sampled surface rather than the sample containing only settled spores.
What it can't: It doesn't tell you whether growth is active now, the exact species, how far growth has spread, or where the moisture is coming from. It only describes the spot sampled.

Settle plate

Petri dish kits

How it works: A dish of growth gel is left open, and whatever spores land on it grow into colonies.
What it tells you: Not much. Spores are everywhere, so nearly every dish grows something, a lot like leaving bread out on the counter.
What it can't: It can't tell you whether mold is growing in your home, where, or how much. A full dish doesn't mean you have a problem, and a clean one doesn't mean you don't.

Honest about testing: the different types, what each one analyzes, and what the results can and can't tell you.

Before you hire

Ask

Questions for a consultant

Do you also do remediation, or get paid by contractors you recommend?
Will I get a written report and remediation plan?
What will you inspect or sample, and why?
Will you check the work afterward if the plan calls for it?
Are you licensed or certified where required?

Ask

Questions for a contractor

Will you bid on this written scope as it is?
How will you contain the area and protect the rest of the home?
What standard do you follow? (Many follow IICRC S520.)
Are you licensed and insured for mold work?
How do you handle anything outside the scope?

Watch for

Red flags

A free inspection from a company that also sells the remediation
Pressure to sign the same day, or scare tactics about your health
A quote with no written scope behind it
A contractor testing its own work
Spraying or "fogging" as the whole solution, with the moisture left unfixed

What to ask before you hire, and the red flags that mean keep looking.

Growth, or dirt?

Dust, soot, and grime can look a lot like growth, especially around vents, and you can't always reliably tell them apart by color, texture, or whether they wipe away. What matters more is moisture: spotting that keeps coming back, especially alongside condensation or damp readings, is a reason to investigate further. If you need to know what it is, explains when a lab test helps and when it doesn't.

Clear cautions about what you can and can't conclude, so a stain or a color doesn't turn into a scare.

What's in the guide

Part 1: What is mold?
How mold grows, and why moisture is the key.
Part 2: Readings and tools
How to use hygrometers, thermal cameras, and moisture meters, and what their readings can and can't tell you.
Part 3: Early clues
The signs that can point to a moisture problem, and where to start investigating.
Part 4: Room-by-room inspection
12 areas of the home, each with an interactive floor plan and a checklist.
Part 5: Have a leak?
What to do in the first day or two after finding water, and who should handle what.
Part 6: Found growth?
How mold behaves on different materials, and when to clean it yourself or get help.
Part 7: Understanding mold tests
What each kind of test can and can't tell you, and when one is worth it.
Part 8: Hiring and remediation
How to hire the right professionals, and what a properly done job looks like.
Part 9: Keeping it from coming back
Everyday habits, simple tools, and a seasonal routine to help keep your home dry.

Built by people who work in mold every day

Working in the mold industry, we kept running into the same problem: a severe lack of real education about mold. What most homeowners find instead is fear-mongering, and quick-fix sales built around tests that offer reassurance more than answers.

We built Mold HQ to help change that: a straightforward, practical guide to understanding moisture and mold in your own home, so you can make decisions based on facts instead of fear.

Mold is a normal part of the world around us. What matters is knowing where it shouldn't be growing, why, and what to do about it.

The Practical Guide
to Mold & Moisture

A look inside

What is mold?

Tub, shower, and ceiling above

Why it happens here

Signs you might notice

What you can safely look at

How to prevent it

Call a professional if

The kinds of tests

Spore trap

Swab or tape lift

Petri dish kits

Before you hire

Questions for a consultant

Questions for a contractor

Red flags

Growth, or dirt?

What's in the guide

Part 1: What is mold?

Part 2: Readings and tools

Part 3: Early clues

Part 4: Room-by-room inspection

Part 5: Have a leak?

Part 6: Found growth?

Part 7: Understanding mold tests

Part 8: Hiring and remediation

Part 9: Keeping it from coming back

Built by people who work in mold every day

Know What to Look For — and What to Do Next

Get the Guide

You already have the guide

The Practical Guide to Mold & Moisture

A look inside

What's in the guide

Part 1: What is mold?

Part 2: Readings and tools

Part 3: Early clues

Part 4: Room-by-room inspection

Part 5: Have a leak?

Part 6: Found growth?

Part 7: Understanding mold tests

Part 8: Hiring and remediation

Part 9: Keeping it from coming back

Built by people who work in mold every day

Know What to Look For — and What to Do Next

The Practical Guide
to Mold & Moisture