Mould Smell in Your House: Causes, Source and What to Do

A musty, earthy smell in your house usually means moisture is present, and often mould. Health Canada lists musty or earthy odours among the signs to check for when you inspect a home for mould and excess moisture. The smell is a prompt to find the damp source, not something to cover up with air fresheners. Here is how to trace the smell, what you can clean yourself, and when the job belongs to a professional.

TL;DR

• A musty smell means moisture. Mould needs moisture and a food source such as wood, drywall or carpet, so find the water first.
• Walk through every room, the attic, the basement and any crawl space. Look for stains, leaks, condensation and warped or peeling materials.
• Small areas (under 1 m² in total) can be cleaned with unscented soap and water, wearing an N95 respirator, gloves and safety glasses. Do not use bleach.
• Call a professional for a single patch over 3 m², for mould that returns after cleaning, or when a strong smell has no visible source.
• Keep indoor humidity between 30% and 50% to stop the smell coming back.

Table of Contents

• What a mould or mildew smell is like
• Common causes of a musty smell
• How to find where the smell comes from
• The musty basement: what to check first
• How to remove the smell safely
• When the smell means you need a professional
• Keeping the smell from coming back
• Our perspective
• What mould remediation costs in Ottawa
• Get the smell checked
• FAQ
• Sources

What a mould or mildew smell is like

People describe it as damp, earthy, stale, like wet socks, old cardboard or rotting wood. It is strongest in closed spaces such as basements, closets and under sinks. It gets worse after rain, after a spring thaw and on humid summer days.

The smell comes from mould growing on a damp surface. You do not need to see mould for it to be there. Health Canada notes that mould can grow behind walls or above ceiling tiles. If you smell strong musty odours but see no water damage or mould, consider speaking to a professional about hidden mould in wall, ceiling and floor cavities.

The words “mildew” and “mould” get used interchangeably at home. For your purposes the response is the same: treat a persistent mildew smell as a sign of moisture and look for the cause.

Common causes of a musty smell

Mould grows indoors where moisture is present. Health Canada’s guide says it can begin growing in a wet area within 48 hours, and that unless the cause of the moisture is found and fixed, it will come back. The usual sources in Ottawa-area homes are:

• Past or present leaks. Plumbing, roof, window and foundation leaks. Check around pipes, windows, doors and any openings for wires.
• Flooding or sewer backup. The Government of Québec advises that water damage be cleaned and dried completely within 24 to 48 hours, or mould may develop.
• Summer condensation in the basement. The Québec government explains that warm, moist air condenses on cooler basement surfaces in summer.
• Unprotected concrete under flooring. When concrete under wood flooring or carpet has no waterproof membrane, odours can come from decaying wood or moisture trapped in the carpet. A poor or badly installed vapour barrier also lets water vapour migrate through walls.
• Poor ventilation. Bathrooms, kitchens and laundry areas without working exhaust fans, or dryers that do not vent outside.
• Condensation on cold surfaces. Windows, exterior walls and uninsulated corners are typical spots.

Why older homes smell more often

Older houses tend to have older foundations, less effective vapour barriers and past renovations that hid moisture problems. That is not a reason to worry on its own. It is a reason to inspect more carefully, especially in the basement, attic and any crawl space.

How to find where the smell comes from

Start with a systematic walk-through. Health Canada recommends inspecting all rooms, attics, basements, crawl spaces and storage spaces and looking for these signs:

1. Stains and discolouration on walls, ceilings and floors
2. Peeling paint and wrinkled wallpaper
3. Cracks in plaster
4. Warped wood
5. Efflorescence, the white powdery deposit on concrete or masonry
6. Leaks, condensation or past flooding
7. Musty or earthy odours

Then narrow it down:

• Follow your nose. Walk room to room with the doors closed for a few minutes first. The smell is usually strongest closest to the source.
• Check behind and under things. Furniture against exterior walls, stored boxes, carpets, the back of cabinets under sinks and the inside of closets.
• Look at the ceilings below bathrooms and above basements. Staining there points to leaks.
• Measure humidity. A hygrometer is inexpensive. The Québec government recommends using one to track indoor humidity. Health Canada advises keeping relative humidity between 30% and 50%.
• Check the attic and crawl space. Frost, roof leaks and bathroom fans venting into the attic can all create moisture. We cover these spaces in our guides to attic mould remediation and crawl space mould removal.

Do you need a mould test? Usually not. The Québec government says it is usually not necessary to analyze air or surfaces to confirm mould. When mould is visible or signs point to it, the priority is a thorough inspection to find the cause. Alberta Health Services likewise says air testing is usually not recommended, especially when you can see mould. Our article on air tests versus swabs explains where sampling does help.

The musty basement: what to check first

The basement is the most common place for the smell, and the search is the same: moisture first. Check these in order:

• Floor drains, sump pit and the base of the foundation walls for dampness or white mineral deposits
• Carpet, underlay and wood flooring laid over bare concrete
• Stored cardboard boxes, books and fabric, which absorb moisture and hold the odour
• The bottom of finished drywall, where wicking water leaves staining
• Window wells and the laundry area
• Behind insulation and vapour barriers along exterior walls

If the basement has had a flood or sewer backup, treat the smell as a sign of wet materials still in place. The Québec government advises discarding porous materials heavily damaged by water: drywall, carpets, cardboard tiles, insulation, mattresses and upholstered furniture. Our basement mould remediation guide walks through the safety steps.

How to remove the smell safely

Removing the smell means removing the mould and the moisture behind it. Deodorizers, scented sprays and ozone gadgets only mask it for a while. Health Canada sorts mould areas by size, and the size decides who does the work.

CMHC uses a similar threshold: areas with more than three patches or over 1 square metre call for a trained mould remediation professional to assess the situation.

Cleaning a small area

Health Canada’s guidance for small and medium areas is:

1. Wear safety glasses, an N95 respirator and disposable gloves.
2. Clean washable surfaces with an unscented soap solution. Health Canada says not to use bleach. CMHC also does not recommend bleach or fungicides, and says all mould residue should be removed because it can cause allergies or illness.
3. On walls, use a damp cloth with baking soda or a small amount of unscented soap solution, without soaking the drywall.
4. Discard porous items that cannot be cleaned. Seal them in plastic bags and take them outside.
5. Dry the area with fans, and use a dehumidifier if needed.

If you are unsure whether the area is small, or you have a health condition that mould could aggravate, do not clean it yourself. Ask a mould remediation contractor to assess it, and ask your doctor about your own risk.

Drying the space and cutting humidity

A dehumidifier is the most effective single tool for a damp basement. Health Canada advises running one in damp areas such as basements, especially if moisture is condensing on cold surfaces or humidity is above 50%. In summer, the Québec government says that when outdoor relative humidity is above 70%, use a dehumidifier instead of opening windows to bring in outdoor air.

Also:

• Use bathroom and kitchen exhaust fans that vent outside, and vent the dryer outside. Alberta Health Services recommends keeping relative humidity below 50% with these steps.
• Fix leaks and the source of any water accumulation. Cleaning without fixing the cause means the smell returns.
• Keep stored items off the floor and away from exterior walls.

When the smell means you need a professional

Call a mould professional when:

• A single patch is larger than 3 m². Health Canada says professional assistance is required in this case.
• Mould returns after you have cleaned it.
• You suspect hidden mould, such as a strong smell with no visible source, or damp patches behind finished walls.
• A flood or sewer backup has soaked drywall, insulation or flooring.
• The moisture source is unclear or you cannot reach it.

There is also a building reason to act promptly. CMHC explains that mould itself does not damage structure, but it signals moisture that may eventually rot wood framing and corrode structural steel. The Québec public health institute (INSPQ) adds that mould in unusual places can conceal water infiltration or major condensation, so identifying the cause is essential before choosing a remedy.

If you are weighing a quick surface treatment against a full job, our article on remediation versus removal explains the difference for areas over 1 m². If a leak caused the damage, see whether your policy responds in our guide to home insurance and mould.

Keeping the smell from coming back

Controlling moisture is the simplest and most effective way to prevent mould, according to the INSPQ. CMHC puts it as keeping materials dry through moisture control, ventilation and surfaces warm enough to avoid condensation. In practice:

• Keep humidity between 30% and 50% and check it with a hygrometer.
• Run exhaust fans during and after showers and cooking. Good airflow matters in every season, and why ventilating your home matters is worth understanding before you add fans or change your routine.
• Do a full walk-through at least once a year. The Québec government says regular maintenance and a complete yearly inspection help detect traces of water infiltration and mould.
• Dry any water damage within 24 to 48 hours.
• When renovating a damp area, choose materials that tolerate moisture. See our article on mould-resistant renovation materials.

Our perspective

We get calls about the smell far more often than about visible mould. By the time someone phones, they have usually tried sprays, baking soda and a dehumidifier, and the smell keeps returning. That is typical when the moisture source is still active or there is damp material inside a wall or under flooring.

Our approach is to treat the smell as evidence. We find where the moisture is coming from, define how far the damage goes, and put it in a written, line-by-line quote so you know what each part of the work costs. Abatement is our licensed specialty, and we also handle demolition and renovation, so the same team can remove affected drywall and flooring, dry the space and rebuild it. That avoids handing a job between several contractors.

Our crews work cleanly and keep you informed. We aim to give estimates within 24 hours, and after 25+ years working in Ottawa homes we know the typical problem areas: older foundations, finished basements without proper barriers, and attics with poorly vented bathroom fans.

If you can clean a small patch yourself following Health Canada’s steps, do it carefully. If the smell persists, the area is bigger than 1 m², or you suspect something behind a wall, a proper assessment saves you repeated cleaning.

Jason

What mould remediation costs in Ottawa

HMJ Contracting publishes ranges for its mould work, and every project is priced in an itemized quote after assessment:

The final price depends on the size of the affected area, the materials involved and how much needs to be rebuilt afterwards.

Get the smell checked

If you have found the damp source but not the extent of the damage, or the smell has returned after cleaning, we can assess it. HMJ Contracting is a licensed and insured Ottawa contractor offering mould remediation, abatement, demolition and renovation, with a 5.0-star Google rating. We aim to provide free estimates within 24 hours.

See our mould remediation services and request an estimate, or visit HMJ Contracting to see everything we do.

FAQ

Can you smell if your house has mould?

Yes. A musty or earthy odour is one of the signs Health Canada lists for checking mould and excess moisture. The smell can come from mould you cannot see, for example behind walls or above ceilings, so follow it with a full walk-through of the house.

Why does my basement smell musty even though it looks clean?

Cleaning the surface does not remove moisture trapped in concrete, carpet, stored boxes or wall cavities. Summer condensation is also common in basements, as warm humid air meets cool surfaces. Check humidity, run a dehumidifier and look for leaks, then see a professional if the smell remains.

Can I use bleach to get rid of mould smell?

Health Canada advises against it and recommends an unscented soap solution on washable surfaces. CMHC does not recommend bleach or fungicides either. Wear an N95 respirator, gloves and safety glasses, and dry the area thoroughly afterwards.

How much mould means I should call a professional?

Health Canada says a single patch larger than 3 m² needs professional help. CMHC suggests an assessment once you have more than three patches or any over 1 m². Also call if mould returns after cleaning or you suspect it is hidden.

Do I need a mould test to confirm the smell?

Usually no. The Québec government says analysis of air or surfaces is generally unnecessary. When mould is visible or signs point to it, a thorough inspection to find the moisture cause matters more.

Sources

• Guide to addressing moisture and mould indoors, Health Canada
• Mould, Health Canada
• Moisture and Air, a guide for understanding and fixing, CMHC
• Basement moisture, Gouvernement du Québec
• Finding and eliminating mould from your home, Gouvernement du Québec
• Health risks associated with the indoor presence of moulds, INSPQ
• Mouldy homes and buildings: causes and prevention, Alberta Health Services
• Mouldy homes and buildings: managing indoor mould, Alberta Health Services

Recommended

• Mould in HVAC Ducts: Signs, Causes and What to Do
• Mould in Insulation: Signs, Causes and What to Do
• Attic mould remediation: what to do and when to call a pro
• Basement mold remediation: your step-by-step safety guide