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Avoiding Mould

Indoor moulds can trigger a host of respiratory symptoms, including increased susceptibility to respiratory infections and exacerbation of asthma. Other acute symptoms can include headache and fatigue, and longer-term exposure may result in environmental sensitivities. This page describes what mould is, how to recognize it and how to mould-proof your home.

What Is Mould?

Moulds are microscopic fungi, a group of organisms that includes mushrooms and yeasts. Fungi grow and reproduce rapidly.

Moulds can be useful. For example, the drug penicillin is obtained from a specific type of mould. But moulds are undesirable when they grow in our homes, and some types are very toxic. Over 270 species of mould have been identified as living in Canadian homes.

What Makes Moulds Grow?

Moulds will grow if we provide them with moisture and nutrients. If we keep things dry, moulds do not grow. High moisture levels can result from water coming in from the outside, through the floor, walls or roof; from plumbing leaks; or from moisture produced by the people living in the home, through daily activities, like bathing, washing clothes or cooking. Water enters the building when there is a weakness or failure in the structure. Moisture accumulates within the home when there is not enough ventilation to expel that moisture.

Different kinds of moulds grow on different materials. Certain kinds of mould like an extremely wet environment. Other kinds grow even when no water can be seen – dampness inside the material can provide enough moisture to allow them to grow.

Why Are Moulds a Concern?

In addition to the damage mould can do to walls, fabrics and other materials, moulds growing inside the home can cause health problems. Moulds release spores and chemicals that can be irritating or toxic. Depending on the type of mould present, the amount and degree of exposure, and the health condition of the occupant(s), the health effects of mould can range from being insignificant to causing localized irritation, allergic reactions and even chronic illness, such as environmental sensitivities. Any sign of mould in the home should be taken seriously and the mould should be removed as quickly as possible.

Pregnant women, infants, older people and people with health problems, such as respiratory disease or a weakened immune system, are at greater risk of developing health problems when exposed to mould.

How Can You Tell if it Is Mould?

Discolouration can be a sign of mould; however, not all discolouration is due to mould. Carpeting near baseboards, for example, can be stained by outdoor pollution entering the home. Stains or soot may also be caused by the smoke from burning candles or cigarettes.

Mould may be any colour: black, white, red, orange, yellow, blue or violet. If you notice white powdery stains or other discolourations, particularly if there is also a musty smell, it may be mould.

Sometimes moulds are hidden and cannot be seen. A musty or earthy smell often indicates the presence of moulds. But some moulds don’t smell. Even when you don’t notice a smell, wet spots, dampness or evidence of a water leak are indications of moisture problems and mould may follow.

Cleaning Up Small Mould Problems

Small Areas
Small areas of mould can be cleaned with a detergent solution. Wear a mask, safety goggles and rubber gloves. If there is a lot of mould or if mould comes back after cleaning, seek professional help.

Washable Surfaces
Scrub washable surfaces with an unscented detergent solution. Then sponge with a clean, wet rag and dry quickly.

Using an unscented detergent will make it easier for you to detect residual mouldy odours.

Mouldy Drywall
Clean the surface with a damp rag, using baking soda or a bit of detergent. Do not allow the drywall to get too wet.

When Should You Seek Professional Help?

Mould that comes back after you have cleaned the area is usually an indication that a source of moisture has not been removed. Seek professional help from a trained indoor air quality (IAQ) investigator. Infants and other family members with asthma, allergies or other health problems should not be in the work area or adjacent room during the cleaning.

You may need help from a trained professional when:

  • there is a lot of mould
  • your home is very damp and moist
  • mould comes back after repeated cleaning
  • a family member suffers from respiratory or other health problems that appear to be aggravated inside the home

Dealing with an Ongoing Problem

Repair to the building envelope is required if moisture is entering the home from the outside. At the same time, steps should be taken inside the home to reduce your exposure to mould.

  1. Discard mouldy or damaged materials. Wear a dust mask and gloves. Furnishings, such as mattresses, carpets, or sofas that got wet or have been stored in damp conditions, should be discarded.
  2. Proper vacuuming reduces the amount of mould spores. All surfaces in the home (floors, walls, ceilings, shelves) and non-washable furnishings (such as sofas, chairs) must be vacuumed thoroughly.
  3. Pull carpets and furnishings away from walls that get wet. Carpets and underpads that are mouldy should be cut out and discarded.

4.   Take steps to dry areas that get wet. Monitor the relative humidity of the air. Use a portable dehumidifier, if necessary. Ensure that the condensate drain pan of the dehumidifier is emptied regularly and clean the dehumidifier with a mixture of white vinegar and water.

  1. If the mould is limited to one area, isolate the area if possible. Cover the affected surfaces with plastic sheeting secured at the edges with duct tape. Note that this is only a temporary measure to minimize your exposure.
  2. Healthy individuals can regularly clean small areas of mould, thus preventing these from getting out of hand.
  3. Consider seeking professional help from trained IAQ investigators, to identify appropriate remediation steps inside the home. Removing large amounts of mould will require the services of mould clean-up contractors.

Steps to Prevent or Reduce Mould Growth

Mould needs moisture to grow. Controlling the moisture and keeping your home dry prevents the growth of mould.

  • Check your home for signs of moisture and moulds.
  • Find out if water is coming in from the outside and if substantial moisture is produced inside the home.
  • Fix any water leaks promptly.
  • Think of the different ways moisture is produced inside the home (for example, cooking, bathing). Remove the moisture as it is produced by using exhaust fans. In the absence of fans, open windows for a short time, but note that the wind can push the moisture to other parts of the home.
  • Measure how much moisture is in the air. To find the relative humidity in your home, you’ll need a hygrometer, available from hardware stores. Relative humidity in the home should be under 45 per cent in the winter (or lower to avoid condensation on windows). If necessary, use a dehumidifier to lower the relative humidity.
  • Reduce the amount of stored materials, especially items that are no longer used. Moulds grow on fabrics, paper, leather, wood and practically anything that collects dust and holds moisture.

Mould-Proofing Your Home, Room by Room

Basement or Crawl Space

  • Reduce the amount of clothes, paper and furnishings stored in the basement. Discard badly damaged materials. Eliminate clutter to improve air circulation. Only washable items should be stored.
  • Dehumidify the basement during the warm months.
  • Avoid carpets on slab-on-grade or below-grade floors.
  • Periodically clean the drain in your basement floor. Use an environmentally friendly drain cleaner; let it stand for a few minutes, then flush with plenty of water. Keep the drain trap filled with water.
  • Avoid standing water. Keep sump pits covered (you can use plywood wrapped with plastic).
  • Regularly clean and replace furnace filters. Use a pleated one-inch filter, not a coarse pleat filter.
  • If you have a heat recovery ventilator (HRV), clean the filter often.
  • If you notice mould or signs of dampness, such as water on your windows or wet spots elsewhere, do not humidify. Disconnect furnace humidifiers that are no longer used.
  • If you have electric baseboards, vacuum the units, or have a professional clean them for you.

Laundry Areas

  • Check that your clothes dryer exhausts to the outside.
  • Remove lint every time you use the dryer.
  • Don’t hang-dry laundry indoors.
  • Dry your laundry tub and washing machine after you use them.


  • Check the bathroom fan to make sure it exhausts to the outside.
  • Turn the bathroom fan on when you shower. Keep it running for a few minutes after you finish your shower.
  • Take short showers.
  • Keep surfaces that get wet, such as the walls around the bathtub and shower, clean and dry.
  • If there is a carpet in your bathroom, remove it.
  • Check for water leaks.
  • Keep drains in good shape by removing debris from them.


  • If the fan over your stove exhausts outside, use it when you cook.
  • Minimize open boiling.
  • Keep your drains in good shape.
  • There’s a drip pan at the back of the refrigerator. Pull the refrigerator out to clean the drip pan. At the same time, vacuum dust from the coils at the back of the refrigerator.
  • Check under the kitchen sink to make sure there are no leaks.
  • Take out the garbage daily to prevent odours and spoiling.

Closets and Bedrooms

  • Get rid of clothes and other stored items that you don’t use. Keeping your closets and bedrooms tidy makes it easier for air to circulate – and harder for mould to grow.

Other Parts of the Home

  • A dehumidifier helps to reduce moisture in the home during the warmer months. Close the windows when the dehumidifier is running.
  • When family and friends come into the home, have them take off their shoes.
  • Vacuum often. If you are buying a vacuum cleaner, try to get one with a HEPA filter.
  • Clean hard floors with a damp mop.
  • Do not bring furniture, clothing, books, etc. that have been stored in a mouldy place into your home.
  • Cut down the number of potted plants in the house – soil is a good place for mould.


  • Regularly check the condition of the roof and exterior finish for any places where water might enter.
  • Make sure that your eavestroughs and downspouts are connected and working properly, and that they are free of debris.
  • Install downspout extensions, to lead water away from the building.
  • Deal promptly with any problems that you find.

Adapted from the Canadian Mortgage and Housing Corporation’s booklet Fighting Mold – The Homeowners’ Guide.

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