Indoor Mould Growth And What You Can Do to Prevent It

Mould can form anywhere due to the omnipresence of unseen spores that litter the natural world. No amount of closed windows, doors, or cleaning can technically remove 100% of mould spores from an ordinary environment. There are spores in the air you breathe at this current moment. 

If this is true, then why aren’t there mould patches covering every inch of the world? The answer to this is found in the three growth conditions that mould spores require to germinate. 

What Causes Mould Growth?

Mould spores require three conditions to be valid to form into the fuzzy colonies we know and smell. These conditions are Moist or humid air, a source of heat, and some kind of cellulose-based food source. 


Mould spores grow by attaching to a moist surface, like other microbes. Moisture is one of the first and most important conditions required for growth, as it is typically the factor relevant to indoor mould growth. Familiar sources of moisture inside the home come from showers, flooding, leaking pipes, or a humid climate. In essence, anything which produces water vapour can contribute to the total humidity levels indoors. Mould requires above 60% humidity level to grow.

The resulting water vapour is capable of condensing on surfaces like walls, ceilings, carpet, concrete, and even become absorbed behind wallpaper. Anywhere water can travel, is where mould will grow. 


Not unlike most life, mould requires a specific temperature to sustain growth. Specifically, mould grows at temperatures ranging from 77 to 86 °F (25 to 30 °C), which is about room temperature. Some sources even claim that “Molds show active growth at temperatures from 5 to 40ºC” (CWC). Either way, moulds will certainly grow indoors if the temperature is set comfortably, which is why it is so prolific. 

Food Sources

Moulds require nourishment from organic, cellulose-based materials to survive and populate into those off-coloured cottony patches. Outside sources include leaves, logs, fruits, and other organic materials. While it may seem specific, there are plenty of organic-based building materials used to build homes. Indoors, mould can feed off of, wallpaper, timber beams, concrete, and human skin cells and oils. In other words, mould will find a way to digest most materials that can be found indoors. 

Moulds do this by forming interconnected tubes called hyphae, which digest any suitable material and absorb its nutrients. The hyphae are responsible for the furry or hairy texture of mould colonies. 

Mould reproduction

When a mould colony exists, it is able to reproduce asexually by creating structures called a sporangium. This sporangium forms spores, and then shoots them out into the air as a fine powder. While travelling through the air, spores will land on surfaces and lay dormant until conditions are right for germination. 

Since there is no lack of food or heat for indoor mould, the best way to keep spores dormant is by controlling humidity levels.

Mould Will Die if It Dries Out…Right?

While moisture is the main contributor to mould growth, removing it suddenly will not kill an existing colony. At best, the colony will appear to have dried out, and ‘died.’ In reality, it is just inactive, like those invisible spores. As soon as moisture levels climb back up, the colony will re-establish itself. Only mould remediation can completely rid a spot of mould.

Does Sunlight Kill Mould?

Yes, in the right conditions, mould can be killed by exposing it to sunlight. An infested blanket or mattress can be washed or sprayed with detergents, and then left exposed in the sun. The intense UV rays will destroy mould spores, which makes sense. Mould mainly thrives in dark and wet places. There is a catch to this method, of course. 

One problem is that there isn’t a set amount of time known to remove 100% of the mould. Without knowing, there is a possibility that some spores will remain and recreate the same damages all over again. 

Another major problem for indoor mould, in particular, is that there are plenty of spaces that cannot be left out in the sun. Any mould found in basements, behind wallpaper, inside of carpets, or attached to the interior of the house cannot be moved outside. It’s also impossible to bring the sun into a home, so the sunlight will not work in this case. 

Prevention – What you Can Do to Stop Mould Before it Germinates

The best way to combat mould without the use of remediation is to utilize preventative measures. Mould is dormant everywhere, but inhibiting its growth is a good enough way to prevent any damages. 

Use a Dehumidifier 

While they will not kill mould, dehumidifiers will prevent mould growth by pulling moisture from the air. Without the 60% or above humidity levels, mould cannot grow. Likewise, dehumidifiers will also remove other allergens and mould food sources like dust, pet dander, and other pollutants. 

Identify Leaks and Dry Wet Areas Immediately

Leaks should be treated right away, as they can very quickly cause long-term potential mould growth. Water can be seen through floorboards, concrete, and drywall, which will cause unseen mould growth. Special care should also be taken in the case of a flood, though immediate help may not be available. Mould only needs 24-48 hours to grow, so time is not on your side. 


A common strategy that reduces moisture buildup is ventilation. Bathrooms and kitchens usually come equipped with an electric vent for this specific reason. Opening windows occasionally can also achieve a similar effect. 

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