How to Stop Mould From Growing in Your Basement

If there’s one thing homeowners across Toronto and the GTA know, it’s that basements are magnets for mould. Between spring thaws, heavy rainfall, and humid summers, our basements are the perfect place for mould to thrive. Once it shows up, it’s not just an eyesore, it can damage your foundation, ruin furniture, and even make your family sick.

The good news? You don’t have to put up with it. With the right mould prevention and removal strategies, products, and expert services, you can keep mould out of your basement for good.

Let’s break down exactly why mould grows in the first place, what you can do about it yourself, and when to call in the professionals at DGI Waterproofing.

What Causes Mould To Grow In Your Basement

Mould spores are always around, they’re a natural part of the environment. The problem starts when they find the right conditions to grow: moisture, warmth, and organic material.

Why Are Toronto & GTA Basements So Vulnerable To Mould?

Moisture from our climate: According to Environment Canada, Southern Ontario averages around 800–1,000 mm of precipitation annually, much of it concentrated in spring and summer. Pair that with spring snowmelt, and you’ve got basements that stay damp all year round.
High humidity: In July, Toronto’s average humidity hovers between 70–75%, which is far above the ideal level for basements (under 60%).
Foundation cracks: Freeze-thaw cycles put enormous pressure on concrete foundations. Tiny cracks let water seep in, creating damp walls and floors. Even worse, when this water freezes, it expands, making the cracks bigger over time.
Flood risk: Toronto and cities throughout the GTA have seen an increase in heavy rainfall and storms.

Add in porous building materials (drywall, wood, cardboard boxes, carpet) and temperatures between 20–30 degrees Celsius, and you’ve got a perfect breeding ground for mould.

DIY Tips To Prevent Mould

Not all mould prevention requires a contractor. There are plenty of steps homeowners can take themselves to reduce the chance of mould and keep their basement dry all year. Here’s your step-by-step playbook:

1. Control Humidity with a Dehumidifier

Mould loves humidity. Keep your basement below 60% humidity.

Best brands: Honeywell 70-Pint Dehumidifier, Aprilaire 1850 Pro, or Frigidaire 50-Pint Dehumidifier.
Pro tip: Place the dehumidifier in the middle of your basement and clean the filter monthly.
Bonus: Connect your humidifier to your sump pump. This will automatically drain all of the excess water, so you don’t have to!

2. Improve Ventilation

It might sound simple, but circulating fresh air throughout your basement can make a world of difference.

Install exhaust fans in your basement laundry room.
Use small circulating fans in corners where the air feels stale and stagnant.
When it is dry outside, crack basement windows for a few hours.

3. Seal Foundation Cracks

Don’t let water sneak in.

For hairline cracks, use Drylok Fast Plug or Sikaflex Crack Flex Sealant.
For larger cracks, call DGI for emergency crack repairs.

4. Use Mould-Resistant Products

If you’re renovating, make your basement mould-resistant:

Drylok Extreme Masonry Waterproofer for walls.
Concrobium Mould Control spray (non-toxic, Health Canada approved).
Switch carpet to tile or luxury vinyl, which won’t trap moisture.
Choose mould-resistant drywall like CertainTeed M2Tech.

5. Stay on Top of Cleaning & Clutter

Dust and vacuum once a week to remove spores. Use a HEPA filter vacuum to make sure all spores are removed.
Store items in plastic bins and avoid cardboard.
Wash and dry seasonal items before storing.

These might seem like small changes, but when it comes to mould, it is the simple things that can prevent it from even growing in the first place.

Professional Services To Book To Prevent Mould Growth

Some basement issues are too big for DIY. That’s where expert waterproofing services come in. At DGI Waterproofing, we recommend:

Interior Basement Waterproofing

What it is: We install interior drainage systems, sump pumps, and vapour barriers inside your basement.
Cost: $50 – $200 per linear foot.
Why it works: It redirects water away from the basement walls and floors, channelling all extra water towards the sump pump to be removed. This is great for properties that don’t have access to the outside of the foundation.

Learn more about our interior basement waterproofing services here!

Exterior Basement Waterproofing

What it is: We will excavate around your property’s foundation, sealing walls with waterproof membranes, and installing new weeping tiles around the perimeter.
Cost: $50 – $300+ per linear ft. (Cost depends on depth and access)
Why it works: Stops water before it ever touches your basement. This is the gold-standard solution for long-term protection.

Read more about our exterior basement waterproofing services here!

Foundation Crack Repairs

What it is: Epoxy or polyurethane injections to permanently seal cracks.
Cost: This can depend on size. Book our FREE inspections for accurate quotes.
Why it works: Seals any cracks that water might enter through, preventing leaks that lead to damp spots where mould will grow.

Learn more and book our emergency crack repair service here!

Sump Pumps

What it is: Sump pumps collect excess water and channel it out of your home and away from your property. If your basement floods often, then this is essential.
Cost: This depends on the brand, size of basement and additional features, such as backup battery.
Why it works: Pumps water out before it can pool.

Learn why installing sump pumps is essential for most basements here!

Booking one of these services means you’re not just managing mould – you’re eliminating the source of the problem entirely.

What If You Already Have Mould?

Sometimes, even with the best prevention, mould sneaks in. Here’s what to do:

Small Areas (DIY-Safe)

If the mould patch is smaller than 10 sq. ft. (about the size of a doorframe):

Spray non-porous surfaces with Concrobium Mould Control or a similar product. Remove any infected porous materials, such as carpet, pillows, clothes, etc.
Scrub with a stiff brush.
Dry the area completely with fans or a dehumidifier.
Wear gloves, goggles, and an N95 mask.

Larger Infestations (Call the Pros)

If the mould covers more than 10 sq. ft., or you notice it spreading inside walls, carpets, or HVAC systems, call for professional remediation.
At DGI Waterproofing, our mould removal team uses HEPA vacuums, air scrubbers, and antimicrobial treatments to remove spores safely.
Cost: Professional remediation usually ranges from $1,000 – $100,000, depending on severity and area size.

When To Call Immediately

If someone in your home has asthma, allergies, or a weakened immune system.
If there’s a strong musty smell, but you can’t find the source.
If the mould keeps coming back even after cleaning.

Mould in your basement doesn’t have to be a never-ending battle. With the right balance of DIY prevention and professional waterproofing services, you can create a dry, healthy, mould-free home.

At DGI Waterproofing, we’ve been helping homeowners across Toronto and Southern Ontario stop mould at the source with interior and exterior waterproofing, foundation crack repairs, and professional mould remediation.

If you’ve spotted mould or want to protect your basement before it becomes a problem, contact DGI Waterproofing today for a free consultation. Your home – and your health – will thank you.