How to Prevent Basement Mold in Ontario's Humid Summers

You prevent basement mold in Ontario by keeping the basement air at or below about 50 percent relative humidity through the summer and by keeping liquid water out. In practice that means running a properly sized dehumidifier from late spring to early fall, keeping the windows shut on muggy days, and fixing anything that lets water in or lets it sit. Mold needs moisture more than anything else, so if you control the moisture you control the mold.

Why basements get moldy in an Ontario summer

Southwestern Ontario summers are humid. London sits between the lakes, and the warm air here carries a lot of water. Your basement is the coolest part of the house because the concrete walls and floor sit against soil that stays cool all year. When warm, damp air touches a cool surface it gives up moisture. That is why cold water pipes sweat in July and why the bottom of a basement wall can feel clammy with no leak anywhere.

Relative humidity also climbs as air cools. Air that feels comfortable upstairs can sit well above 70 percent once it settles into a cool basement. Mold can start on drywall paper, cardboard, carpet backing and dusty wood when those surfaces stay damp for a few days. Older homes in Old North, Wortley Village and Old East often have stone or block foundations with no moisture barrier on the outside, so dampness comes in from the soil side as well.

What humidity should a basement be in summer?

Aim for 50 percent or lower, and treat anything that stays above 60 percent as a problem. A digital hygrometer costs about $15 to $20 at a hardware store. Leave it at the far end of the basement, away from the dehumidifier, and check it morning and evening for a week. One reading tells you very little. A week of readings tells you whether your setup is keeping up.

How to prevent basement mold in Ontario: the summer routine

• Run a dehumidifier from roughly May to September. Set it to 45 or 50 percent and let it cycle on its own.
• Drain it through a hose to a floor drain or sump pit, or buy a unit with a built-in pump. A bucket that fills overnight and shuts the unit off is the most common reason a dehumidifier fails to do its job.
• Keep basement windows closed on humid days. Opening them feels like airing the place out, but it brings in the wet air that causes the problem.
• Run the central air if you have it. Air conditioning removes moisture as it cools. Keep the basement supply registers open so that dry air reaches the lower level.
• Leave interior doors open so air moves. Closed storage rooms, closets on outside walls and cold cellars are where growth usually shows up first.
• Put foam sleeves on cold water pipes so they stop dripping onto framing and stored items.

Storage and finishing habits that feed mold

Mold eats paper, wood, fabric and dust. Cardboard boxes sitting on a concrete floor are close to ideal food. Move storage into plastic totes and onto shelving that sits a few inches off the floor and a few inches away from the outside walls. Carpet laid straight on a slab holds dampness against the concrete and is hard to dry once it gets wet. Hard flooring made for below-grade use is a safer choice.

If you are finishing a basement, deal with moisture before the framing goes up. Wood studs and drywall tight against bare concrete tend to hide dampness until the smell arrives. Ask your contractor how the wall will handle moisture, including what goes against the concrete and what sits under the bottom plates. Leave an access point where you can look at the foundation wall later.

Signs your basement is losing the fight

• A musty smell that is stronger after the house has been closed up for a few days.
• White, powdery deposits on the concrete. This is mineral salt left behind as water evaporates, and it tells you moisture is moving through the wall.
• Dark spotting along the bottom of drywall, on baseboards or on the back of furniture pushed against an outside wall.
• Rust on the feet of metal shelving, the furnace cabinet or the water heater.
• A hygrometer that stays above 60 percent even with the dehumidifier running.

A dehumidifier handles humid air. It will not fix water coming through a crack, a dripping pipe or poor drainage outside. If the unit runs nonstop and the number will not drop, look for a water source. Health Canada's guidance is that indoor mold should be cleaned up whatever the type, and that the moisture behind it has to be fixed or it returns. A small patch on a hard surface can be washed with soap and water and dried. Larger areas, or growth on drywall that keeps coming back, should be assessed properly. Basement mold removal in this area typically runs $1,200 to $9,500, so a dehumidifier and a hygrometer are cheap by comparison.

Your next step is simple. Put a hygrometer in the basement this week, write down the readings for seven days, and adjust the dehumidifier until the number holds at 50 or below.

Common questions