Why New Build Homes in London, Ontario Get Mold

A brand-new house can grow mold in its first or second winter, and it surprises a lot of owners. The usual cause is not bad luck. New homes are built tight to save energy, and they start out holding a large amount of moisture from construction. If that moisture has nowhere to go, it ends up on the coldest surfaces in the house: window frames, attic sheathing, closet corners and basement walls.

Where the moisture comes from

• Concrete. A new foundation and basement slab release moisture for many months after they are poured.
• Lumber. Framing often gets rained on during construction and dries out inside the finished house.
• Drywall mud, paint and flooring adhesives, all of which give off moisture as they cure.
• Day-to-day living: showers, cooking and laundry in a house that was built to leak very little air.

Where new homes tend to show mold first

• Attic sheathing, often near bathroom fan ducts that came loose or were never connected.
• Window frames and sills, from heavy condensation in the first winter.
• Closet corners and the bottoms of outside walls in bedrooms.
• Basement framing and the back of insulation, where finished basements were closed in before the concrete dried.
• Under sinks and around tubs where a fitting was not fully tightened.

The ventilation system matters most

New Ontario homes have mechanical ventilation, usually an HRV or ERV, because they are too airtight to rely on leaks for fresh air. Many owners never touch it, set it to its lowest setting or turn it off because of the noise. In a new, tight house that is often the single biggest reason for condensation and mold. Learn where the controls are, run it the way the manufacturer intends, and clean or replace its filters on schedule. Run bathroom fans long enough to clear the steam, not just while the shower is on.

What's normal and what isn't

Some window condensation on the coldest mornings in the first winter is common. Steady water running down the glass every day, black spotting on the frames, a musty smell, mold in the attic or water on the basement floor are not normal and should be looked into. Attic mold in a new home is very often traced to a bathroom fan duct that was never attached to its roof or wall vent.

Your builder and Tarion

New homes in Ontario come with a warranty administered by Tarion. In broad terms, the first-year warranty covers work and materials, and the two-year warranty includes water getting into the basement or through the foundation walls, and defects in work or materials that let water into the building envelope, such as around windows and doors. Read your warranty documents for the exact terms and deadlines, which are strict. If you find mold or water in a new home, report it to your builder in writing right away, take dated photos, and keep copies of everything you send.

An independent moisture inspection can help you describe the problem accurately. A report with photos, moisture readings and thermal images shows where the water is coming from, which makes the warranty conversation clearer. Our inspections run $249 to $549.

What to do now

1. Check the attic hatch for frost or dark staining on the sheathing near bathroom fan vents.
2. Make sure the HRV or ERV is running and its filters are clean.
3. Run bathroom fans for 20 to 30 minutes after showers.
4. Pull furniture a few centimetres off outside walls.
5. Report any mold, leaks or water in the basement to your builder in writing, with photos.

We work across London and Middlesex County, including the newer subdivisions in the north, west and south ends of the city. If you are seeing mold or heavy condensation in a newer home, send photos and we will tell you what we think it is and whether it looks like a warranty issue.

Common questions