HouseWorks: Winning the battle against household mold

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Winter is indoor mold season in Canada, and there are two reasons why. First, many houses get closed up tight against the cold, so there’s inadequate ventilation. And second, when deep cold penetrates windows and within wall cavities it can cause indoor wall and window surfaces to get cold enough to trigger condensation. You can see it most commonly on window glass, but the same dynamic sometimes happens on interior wall and ceiling surfaces. This is why you can have enough moisture on, say, the corner of a bedroom wall at floor level to trigger mold growth, while overall wintertime humidity in the house is low enough to be uncomfortable. All this sets the stage for the answer to a question I get often: “How can I get rid of mold in my home, ideally once and for all?”

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Household mold growth is triggered by moisture. If moisture levels stay low enough, then mold will never grow. This is why getting rid of mold permanently involves moisture control. But before that can work you need to kill the active mold that’s taking over your home. Despite widespread confidence in bleach, it’s not the best option for killing mold.

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The main thing to understand about mold is that it can go dormant for years, then spring into action when moisture levels rise enough. This is why it’s necessary to kill both the visible mold itself and the hidden roots of the mold called hyphae. Ordinary bleach is effective at killing mold on surfaces, but not very good at killing hyphae that has penetrated into porous materials. That’s one reason I don’t use bleach anymore to kill mold.

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The most effective, government-registered mold-killing fungicide I’ve discovered so far has no bleach at all. It’s called Concrobium and it’s a colourless, odourless liquid that’s also completely non-toxic to people, pets and plants. It’s only dangerous to microscopic organisms, and even then only for mechanical reasons, not chemical ones.

Oxygen-based stain removers like this one are effective at removing unwanted staining, while leaving existing colours unchanged. The best versions are also non-toxic and odour-free
Oxygen-based stain removers like this one are effective at removing unwanted staining, while leaving existing colours unchanged. The best versions are also non-toxic and odour-free Photo by Photo Steve Maxwell

Concrobium kills by mechanically crushing mold as it dries. Just spray, brush, slosh or fog it on, then walk away. It’s the drying action that’s key because that causes the Concrobium liquid to shrink, mechanically crushing not only the active mold organisms themselves but also the mold spores and hyphae that could turn into fresh mold growth later.

Your next step may be to figure out why the mold surface got moist enough to trigger mold growth in the first place, but what about the mold staining? Just because mold is dead doesn’t mean the staining goes away. Depending on the situation, you might want to make the area look mold-free again, and for this you can’t beat the action of oxygen.

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Duo Blast is an oxygen-based companion product to Concrobium, and it has done well in my tests. It’s a two-part liquid that removes mold stains but without bleaching the underlying surface. I don’t know how it can target mold stains while leaving other colours alone, but it does.

Mix two parts of the liquid Duo Blast product, then brush, spray or slosh it onto mold-stained surfaces as soon as possible. By the time it dries your dead mold stains should be gone. If not, repeat.

Now that you have mold and mold staining under control, it’s time to address the issue of moisture. The most common cause of excess indoor moisture in winter is inadequate ventilation, and this is where a heat recovery ventilator (HRV) can make all the difference. It’s a ventilation appliance that’s permanently mounted in your home. It draws fresh air indoors while also exhausting stale indoor air outside. HRVs capture and retain most of the heat from the stale air before shooting it outside, and this is why it’s better than just opening windows. To learn more about what HRVs have to offer, visit

Steve Maxwell can’t have too much fresh air. Visit him at for articles, videos and insights into the Canadian home renovation and DIY scene.

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