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Fighting Mold: The Homeowners’ Guide

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Understanding mold

What are molds?

Molds are microscopic fungi, a group

of organisms which also includes

mushrooms and yeasts. Fungi are

highly adapted to grow and reproduce

rapidly, producing spores and mycelia

in the process.

You encounter mold every day. Foods

spoil because of mold. Leaves decay

and pieces of wood lying on the ground

rot due to mold. That fuzzy black

growth on wet window sills is mold.

Paper or fabrics stored in a damp

place get a musty smell that is due

to the action of molds.

Molds can be useful to people. The

drug Penicillin is obtained from a

specific type of mold. Some foods

and beverages are made by the

actions of molds. The good kinds of

molds are selected and grown in a

controlled fashion.

Molds are undesirable when they grow

where we don't want them, such as in

homes. Over 270 species of mold have

been identified in Canadian homes.

Molds that grow inside may be

different from the ones found outdoors.

What makes molds grow?

Molds will grow if we provide them

with moisture and nutrients. If we

keep things dry, molds do not grow.

High moisture levels can be the result

of water coming in from the outside,

through the floor, walls or roof; or from

plumbing leaks; or 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 molds grow on

different materials. Certain kinds of

molds like an extremely wet

environment. Other kinds of molds

may be growing even if no water can

be seen. Dampness inside the material

can be enough to allow them to grow.

Why are molds a concern?

Damage to materials is one concern.

Materials get stained or discolored,

and over time they are ruined. Moldy

paper and cardboard disintegrate over

time. Fabrics are damaged. Continued

mold growth can be indicative of

moisture conditions favorable for

growth of fungi that cause wood rot

and structural damage.

When molds are growing inside the

home, there may be health concerns.

Molds release chemicals and spores.

Health experts indicate that, depending

on the type of mold present in a home,

the amount and degree of exposure, and

the health condition of the occupant,

the health effects of mold can range

from being insignificant to causing

allergic reactions and illness.


Fighting Mold: The Homeowners’ Guide CE 8













Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

Pregnant women, infants, the elderly

and those with health problems, such

as respiratory disease or a weakened

immune system, are more at risk when

exposed to mold. Consult your family

physician if you believe there is

someone who may be at risk.

Is there a mold problem?

Molds are always found in the air

outside and in all buildings. They

come into the home in many ways

—through open windows or doors,

on clothing, pets, food or furniture.

The problem starts when mold grows

inside the home.

Some mold growing, for example on

the window sill but not elsewhere, is

not a cause of concern. You can clean

the mold yourself. The presence of

mold is a sign that there is too much

moisture in your home–a situation

which must be corrected.

Inspect your home to find the

extent of the mold.

How much mold is growing?

One way is to estimate the area of

the mold.

Mold is considered to cover a

“small area” if the patch is no larger

than a square meter. There should be

no more than three patches, each patch

smaller than a square meter. Clean up

small areas yourself using a detergent

solution, household rubber gloves and

a dust mask for protection. Refer to

page 3 for the procedure.

Small moldy areas in homes may

become larger over time, if ignored,

so it's important to clean up and

remove even small patches of mold.

The mold area is considered “moderate”

if there are more than three patches,

each patch smaller than a square meter,

or there is one or more isolated patches

larger than a square meter but smaller

than 3 square metres (size of a 4 x 8 foot

sheet of plywood). Assessment by a

professional is recommended. You

can clean up moderate amounts of

mold but you must follow the proper

procedures and use the proper

protective equipment (see page 3).

A mold area is considered “extensive”

if a single patch of mold is larger in

area than a sheet of plywood. Being

exposed to this much mold is not a

good idea. Do not attempt to clean

up large areas of mold yourself. You

need professional help to determine

why the mold is there in the first place

and how to clean it up.










How can you tell if it is mold?


Discoloration is a sign of mold. However, all discoloration is not due to

mold. 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.

Mold may be any colour: black, white, red, orange, yellow, blue or violet.

Dab a drop of household bleach onto a suspected spot. If the stain

loses its colour or disappears, it may be mold. If there is no change, it

probably isn’t mold.


Sometimes molds are hidden and cannot be seen. A musty or earthy

smell often indicates the presence of molds. But a smell may not be

present for all molds. Even when you don’t notice a smell, wet spots,

dampness or evidence of a water leak are indications of moisture

problems and mold may follow.

Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

When should you seek

professional help?

You may need professional help


• There is a lot of mold

• The home is very damp and moist

• Mold comes back after repeated


• A family member suffers from

asthma or respiratory problems or

other health problems that appear

to be aggravated inside the home

How do you get

professional help?

Contact your local CMHC office for

a list of individuals who have completed

the CMHC Residential Indoor Air

Quality Investigator program. A trained

IAQ investigator, who operates a private

business and sells his/her services,

examines the indoor air quality of your

home and documents your concerns.

He/she identifies the problems, finds

their sources and suggests solutions in

a written report. Recommendations

are provided to you in an action plan

that consists of various options to

improve the indoor air quality in

your home.

How to clean up small

mold problems

“Small area” clean-up

You can clean up “small areas” of mold

(fewer than three patches, each smaller

than a square meter) yourself. There’s

the minimum protective wear needed:

• safety glasses or goggles;

• a disposable dust mask (3M 8210

or equivalent); and

• household rubber gloves.

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.

Steps to follow in cleaning

up “small” mold areas

Washable surfaces

Scrub 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

moldy odours.

Moldy 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.

Mold that comes back after cleaning

is usually an indication that a source

of moisture has not been removed.

Seek professional help from a trained

IAQ investigator.












The presence of organic (humic)

materials, the pH (acidity/alkalinity)

of the water, the surface material

and contact time affect the

effectiveness of bleach for disinfection.

Since these factors are not generally

controlled, bleach cannot be relied

upon for disinfection.The most

compelling reason for advising against

bleach is that cancer-causing

substances can be formed by the

reaction of bleach with organic materials.

In addition, the fumes are harmful.

About Your House

Fighting Mold: The Homeowners’ Guide

How to clean up “moderate”

mold problems

If you follow the proper procedures and

use the proper protective equipment,

you can clean up “moderate areas” of

mold. “Moderate” means more than 3

patches of mold, each smaller than one

square meter, or one or more isolated

patches larger than one square meter

but smaller than 3 square metres (size

of a 4 x 8 ft. sheet of plywood).

a) Safety precautions

• Wear a disposable dust mask (for

example, 3M 8210 or equivalent),

glasses or safety goggles and

household rubber gloves.

• Isolate the area to be cleaned

with plastic sheeting, taped to

walls and ceiling.

• Infants and other family members

suffering from asthma, allergies

or other health problems should

not be in the work area or adjacent

room during the cleaning.

A small clean up should take minutes

(not hours) to finish. When the clean

up takes hours to a day to finish, it

is suggested that you upgrade to a

better filter, such as a half-face respirator

with charcoal cartridges. An exhaust

fan installed in a window in the

room being cleaned would prevent

contamination of other areas of the

house as well as provide ventilation.

b) General cleaning

Vacuum surfaces with a vacuum cleaner

which has a High Efficiency Particulate

Air (HEPA) filter or is externally

exhausted. Scrub or brush the moldy

area with a mild unscented detergent

solution. Rinse by sponging with a

clean, wet rag. Repeat. Dry quickly.

HEPA vacuum the surfaces that were

cleaned as well as surrounding areas.

c) Cleaning wood surfaces

Vacuum loose mold from wood surfaces

using a HEPA or externally exhausted

vacuum. Try cleaning the surface of the

wood with detergent and water. Rinse

with a clean, damp rag and dry quickly.

If the staining does not come off, sand

and vacuum the surface of the wood

with a vacuum/sander combination. It

is important to vacuum at the same

time to prevent mold spores from being

dispersed into the air. Note that wood

affected by rot may need to be replaced.

d) Cleaning concrete surfaces

Vacuum the concrete surfaces to be

cleaned with a HEPA or externally

exhausted vacuum cleaner. Clean up

surfaces with detergent and water. If

the surfaces are still visibly moldy,

use TSP (trisodium phosphate).

Dissolve one cup of TSP in two

gallons of warm water. Stir for two

minutes. Note: TSP must not be

allowed to come in contact with

skin or eyes. Saturate the moldy

concrete surface with the TSP

solution using a sponge or rag. Keep

the surface wetted for at least 15

minutes. Rinse the concrete surface

twice with clean water. Dry

thoroughly, as quickly as possible.

e) Moldy drywall

The paper facings of gypsum wallboard

(drywall) grow mold when they get

wet or repeatedly wet and don't dry

quickly. Cleaning with water containing

detergent not only adds moisture to

the paper but also can eventually

damage the facing. If the mold is

located only on top of the painted

surface, remove it by general cleaning

(above). If the mold is underneath the

paint, the moldy patch and other moldy

material behind it are best cut out and

the surrounding areas also cleaned.

This should be done by a mold

clean-up contractor. New materials

may become moldy if the moisture

entry has not been stopped. If this is

the case, replacement of the materials

should be deferred until the source

of the moisture is corrected. The

affected areas should be temporarily

covered with plastic sheeting and

sealed at the edges.

4 Canada Mortgage and Housing Corporation









Any areas that show new patches of

mold should be cleaned promptly.

Dealing with an ongoing


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 the occupants'

exposure to mold.

1. Discard moldy 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. Discard items that are

no longer needed. Use this

opportunity to reduce the amount

of furnishings–this means fewer

materials to absorb moisture and

grow mold. Clothes and other

items that have been cleaned

should be stored in sealed plastic

bags to prevent re-contamination.

2. Proper vacuuming reduces the

amount of mold spores. All

surfaces in the home (floors, walls,

ceilings, shelves) and non-washable

furnishings (such as sofas, chairs,

etc.) must be vacuumed thoroughly.

3. Keep moisture generated within

the home to a minimum by

conscientiously following the

prevention steps below.

4. Pull carpets and furnishings away

from walls that get wet. Carpets

and underpads that are moldy

should be cut out and discarded.

5. Take steps to dry up 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.

6. If the mold 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.

7. Healthy individuals can regularly

clean “small” and “moderate”

areas of mold, thus preventing

these from getting out of hand.

Make sure you follow the safety

precautions and cleaning guidelines.

8. Consider seeking professional help

from trained IAQ investigators

to identify appropriate remediation

steps inside the home. Removing

large amounts of mold will

require the services of mold

clean-up contractors.

Preventing mold

Basic steps to prevent and

reduce mold growth

Mold needs moisture to grow.

Controlling the moisture and keeping

your home dry prevents the growth

of mold.

• Check your home for signs of

moisture and molds.

• 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

your home (for example,

cooking, bathing, plant jungle).

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.

Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

• Keep your home dry

• Find and fix water leaks

• Discard clutter and excess

stored materials

• Clean and maintain your home


• Encourage lifestyle practices that

reduce moisture








• Measure how much moisture is

in the air. To find the relative

humidity in your home, you'll

need a hygrometer. You can buy

one at a hardware store or

electronics store. A hygrometer

costs from $5 to $20. 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. Molds grow on

fabrics, paper, wood and practically

anything that collects dust and

holds moisture.

Mold-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 half a

cup of bleach, 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 filter.

• If you have a heat recovery

ventilator (HRV), clean the

filter inside the HRV often.

• If you notice molds 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.

To clean a drain:

• Pour a handful of baking soda

into it.

• Add a cup of vinegar.

• Put the plug in the drain.

• Let the vinegar and baking soda

work for about 20 minutes.

• Run fresh water into the drain.

If the drain is still clogged, use a

small plumbing snake.


• If the fan over your stove exhausts

outside, use it when you cook.

• Minimize open boiling.

• Keep your drains in good shape.

Follow the steps in the Bathrooms

section, above.

Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

• 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 mold 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. (See next page.)

• Clean hard floors with a damp mop.

• Do not bring into your home

furniture, clothing, books etc. that

have been stored in a moldy place.

• Cut down the number of potted

plants in the house–soil is a good

place for mold.


• Regularly check the condition of

the roof and exterior finish for any

places where water might enter.

• Make sure that 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.

Frequently asked questions

about mold

Should I have my house air

tested for mold?

This is the question most frequently

asked by homeowners who think their

home may have a mold problem.

Testing is generally not recommended

to homeowners. Testing of moldy

materials or an air sample identifies

the types of molds that may be present

but does not identify the cause/source

of moisture. The type of mold does

not change the procedures for cleaning

up areas of mold less than 3 square

meters. You have to clean up the mold

and correct the problem irrespective

of the type of mold. The cost of

testing may be better spent hiring a

professional investigator or fixing

the problem.

Testing of a moldy material involves

sending a swab, imprint on a Scotch

tape or piece of the material to a

competent laboratory. Air sampling

requires specialized equipment. An

air sample typically captures mold

spores in a period of minutes. Since

replicate samples must be taken due

to variations in the airborne molds

over time (even hours) and compared

with outdoor samples, air testing is

both expensive and time-consuming.

Interpretation of test results may

not be very useful, since there are

no advocated "safe levels" of indoor

molds and the results will not tell

the health risks from the molds.

The air feels dry. Can I humidify?

Before you add moisture to the air,

measure the relative humidity. Air

that feels dry may not be really dry.

It may be moldy. High relative

humidity (over 45 per cent)

promotes the growth of molds and

dust mites. The moisture in the air

may condense on colder exterior

walls where molds start to grow.

Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

If your physician has advised you to

use a humidifier in your child's

bedroom at night, monitor the relative

humidity. Turn the humidifier on

and off as necessary. In the morning,

take steps to make sure the room

gets dry. Empty and clean the

humidifier after each use.

What advantages do HEPA

vacuums provide?

Ordinary vacuums capture large particles

only, small mold spores pass through

the vacuum into the air. HEPA

vacuums have special filters that

capture small particles. A central

vacuum cleaner which is exhausted

to the outside also removes mold spores.

A regular portable vacuum is useful

only if its exhaust goes outside the

home. Vacuuming removes settled

dust that contains an accumulation

of mold spores over time. Reducing

the settled dust reduces molds.

Vacuuming with any vacuum

cleaner (ordinary, central or HEPA)

stirs dust and mold during the

process. Wear a dust mask so you

will not be breathing more mold.

Is vacuuming with a HEPA or

externally exhausted vacuum

cleaner recommended for

serious mold problems only?

Vacuum regularly with a HEPA or

externally exhausted vacuum cleaner

to prevent the ongoing accumulation

of dust and molds. The need for

HEPA or external exhaust

vacuuming increases with the

severity of the mold problem.

If a furnishing has been wet at some

time in the past or has been exposed

to dampness over a prolonged period

of time, vacuuming with HEPA or

externally exhausted vacuum is

unlikely to remove the mold growing

beneath the surface. It is better to

discard the item.

Where do you find a HEPA

vacuum cleaner?

Vacuum cleaner dealers carry HEPA

vacuums. Consider purchasing one as

an upgrade to what you may be using.

A HEPA vacuum is a good investment

in the long term whether you have

mold or not. A generic canister HEPA

vacuum cleaner costs approximately

$300. Brand name products of the

same type may cost more. You may

inquire if the dealer has a HEPA

vacuum cleaner to rent. Contractors

who clean up or renovate houses for

mold should also have this equipment.

Does painting over a moldy

surface take care of the mold?

Painting over mold only masks the

problem. Paint does not kill the mold

nor stop it from growing. Surfaces

that are washable should be cleaned

with a detergent solution, following

the procedure suggested on page 3,

then allowed to dry. If you are going

to paint, remove mold first.

Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

Does cleaning stop the

mold growth?

Mold will reappear until its source of

moisture is removed. High moisture

levels that are not corrected can make

the molds grow back quickly.

Cleaning is only a temporary but

essential measure.

You can help by making a conscious

effort to keep the home dry. Obviously,

water must be prevented from entering

the home. But you can help by

controlling moisture that you produce.

How does one clean

clothes that are moldy?

Non-washable clothing can be dry


Wash clothes with a detergent solution

to which a cup of bleach is added.

Make sure the detergent you use does

not contain ammonia. Repeat as

necessary until the moldy odour is gone.

Clothes and other items that have been

cleaned should be stored in sealed

plastic bags to prevent re-contamination.

Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

Canada Mortgage and Housing Corporation

About Your House


Fighting Mold: The Homeowners’ Guide

©2001, Canada Mortgage and Housing Corporation

Printed in Canada

Produced by CMHC 18-03-05

Revised 2002, 2005

Although this information product reflects housing experts’ current knowledge, it is provided for general information purposes only.

Any reliance or action taken based on the information, materials and techniques described are the responsibility of the user. Readers

are advised to consult appropriate professional resources to determine what is safe and suitable in their particular case. Canada Mortgage and

Housing Corporation assumes no responsibility for any consequence arising from use of the information, materials and techniques described.


Priced Publications

Clean-up Procedures for Mold in Houses Order No. 61091

Cleaning Up Your House After a Flood Order No. 61094

Investigating, Diagnosing and Treating Your Damp Basement Order No. 61065

Free Publications

About Your House fact sheets

Measuring Humidity in Your Home Order No. 62027

The Importance of Kitchen and Bathroom Fans Order No. 62037

The Condominium Owner’s Guide to Mold Order No. 62341

To find more About Your House fact sheets plus a wide variety of information products,

visit our Web site at www.cmhc.ca.You can also reach us by telephone at 1 800 668-2642

or by fax at 1 800 245-9274.


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