Moisture Problems
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Why You Should Worry About Moisture Problems
Moisture problems in Canadian housing are detrimental to our health and to
the durability and resale value of our homes. Here's how to solve moisture problems,
improve energy efficiency and reduce greenhouse gas emissions that contribute
to climate change.
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Does Your House Have a Problem?
Start with a few basic questions:
- Is there persistent mould growth?
- Are there musty smells, particularly in the basement?
- Are windows fogged up or frosted up through much of the winter?
- Is there evidence of moisture deterioration – rotting wood, soft drywall,
buckled siding or spalling bricks?
- Does frost build up in the attic?
- Has basement flooding ever occurred?
- Are humidity levels often above 55 percent in the winter?
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Why You Should Control Moisture Flow
We must control moisture in all its forms to keep our homes durable and comfortable.
Building components and practices such as flashing, roofing and basement damp-proofing
successfully protect the home from liquid water.
It is equally important to control the movement of water vapour, providing
added protection for the house structure and helping to maintain indoor humidity
at a comfortable level.
Controlling moisture involves three strategies:
- using construction techniques that keep moisture away from the structure
- producing less moisture
- exhausting excess moisture
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Sources of Moisture in the Home
Even if your house has no leaks in the basement or roof and is apparently dry,
it can have moisture problems. Where does all the moisture come from? There
are a number of major sources that are not always obvious:
- Occupants and their activities: An average family of four
will generate about 63 litres (20 gallons) of water a week through normal
household activities.
- Wind-blown rain in walls: Where basement damp-proofing
is inadequate, ground water in the soil can migrate through the foundation
by capillary action and evaporate on the surface of the wall or floor.
- Damp basements
- Moisture stored in building materials and furnishings:
Building materials and furnishings absorb moisture from the air during damp,
humid weather and then expel it during the heating season.
Despite all this water produced each day, most older houses have "dry" air
in winter to the point where they have to have humidifiers installed. Why?
Cold outdoor air cannot carry much water vapour. In older homes, uncontrolled
airflow brings colder, drier air indoors and forces the warm, moist household
air out through openings in the upper walls and attic. The air quickly escapes
through the un-insulated envelope without cooling down enough to cause condensation.
When insulation is added, the building exterior becomes much colder. Unless
additional protection is provided, water can condense in the building structure.
How? Remember that cold air is able to hold much less moisture than warm air.
As the warm, moist air cools in the cold outer layers of the building, the water
vapour it holds may condense as liquid or, if it is cold enough, as frost. This
can reduce the effectiveness of insulation and even cause rot, peeling paint,
buckled siding, mould growth and other problems.
Quantity of Moisture Added to the Air Through Various Household Activities
| Activity (for a family of four) |
Moisture
(litres per week) |
| Cooking – three meals daily for one week |
6.3 |
| Dishwashing – three times daily for one week |
3.2 |
Bathing
– 0.2 litre per shower
– 0.05 litre per bath |
2.4 |
| Clothes washing (per week) |
1.8 |
| Clothes drying indoors or using an un-vented dryer (per week) |
10.0 |
| Floor mopping per 9.3 m² (100 sq.
ft.) |
1.3 |
| Normal respiration and skin evaporation from occupants |
38.0 |
| Total moisture production per week |
63.0 |
More
information about moisture
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How Much Humidity?
Humidity levels above 20 percent help prevent dry, sore throats and make
the air feel warmer and more comfortable. Moist air also eliminates static electricity
in the house and helps to protect plants and preserve your furniture.
On the other hand, humidity levels over 40 percent can cause frosting
and fogging of windows, staining of walls and ceilings, peeling paint, mould
growth and odors. When relative humidity is over 50 percent, airborne diseases
become more difficult to control. Condensation on your windows can provide a
good indication of the relative humidity. You may, however, want to install
a humidity sensor or humidistat to keep more accurate measurements of humidity
levels.
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Keeping the Structure Dry
Use four strategies to keep the structure dry:
- Provide exterior weather and moisture protection. Use building
paper, siding, flashing, gutters and other construction techniques to shed
water and repel wind-driven rain. Pay attention as well to below-grade measures.
Proper drainage, grade slope and damp-proofing can protect the foundation
from ground-water leaks or from moisture movement by capillary action.
- Reduce moisture at the source. This means producing less
moisture in the first place and exhausting moist air and bringing
in drier air.
- Prevent moist indoor air from getting into the envelope.
A vapour barrier will reduce moisture movement by diffusion, and an air barrier
can prevent moisture movement by air leakage.
Although less moisture can be moved into the envelope by vapour diffusion than
by air leakage, it
is still important to provide a vapour barrier. An effective vapour barrier
must be the following:
- resistant to vapour diffusion
- durable
- installed on the warm side of the insulation
A number of building materials resist vapour diffusion well enough to be used
as vapour barriers. These include polyethylene, oil-based paints and special
vapour-barrier paints, some insulation materials and exterior-grade plywood.
Different materials may act as the vapour barrier in different parts of the
house.
The same material may work as both an air barrier and a vapour barrier, provided
it meets both requirements and is properly installed. Polyethylene sheets and
foil-backed gypsum drywall can both combine these functions. To avoid confusion
of terms, we refer to a material doing both jobs as an air and vapour barrier.
As a general rule, the vapour barrier should be on the warm side of the insulation.
In some cases, however, the vapour barrier can be located within the wall or
ceiling assembly, provided that at least two thirds of the insulation value
of the wall is on the cold side of the vapour barrier. Because this ratio should
be adjusted for houses with high interior humidity or for homes in extremely
cold climates, it is recommended that you consult a professional builder-renovator,
who will apply the specifications outlined in the National Building Code
of Canada.
- Let the envelope "breathe" to the outside. This will allow
the house to deal with seasonal fluctuations in humidity and to release any
moisture that does penetrate the envelope from the interior or exterior. The
materials of the envelope are layered, with those most resistant to vapour
diffusion located on the warm side of the envelope and the least resistant
(such as building paper) located on the outside. In this way, any vapour that
penetrates the envelope can escape to the outside.
Some wall systems work well with a relatively impermeable insulated sheathing
because the interior wall-cavity temperatures are kept high. As a precaution,
when retrofitting a wall, always ensure that the interior surfaces are vapour-resistant.
Some siding applications have an air space immediately behind the exterior
finish to promote drying out of materials that have been soaked by rain or dampness.
This air space also provides an escape route for any moisture that has penetrated
the wall cavity from the indoors. This type of installation should not be used
with insulated siding, as convection in the air space will negate the effect
of the insulated backer board on the siding.
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What to Look For and Where
Put on some old clothes and a detective's cap, grab a flashlight and some simple
tools, and go through the entire house, both inside and outside, searching for
moisture damage and mould growth and their potential causes. Winter is the best
time for this inspection, although basements should also be inspected in the
summer. You can also do this type of inspection when you're looking to buy a
home, particularly if it is an older house.
Typical Moisture Problems
Here are some of the key locations to check:
- Mould, frost or wet insulation or wood in attic
- Mould in the bathroom
- Frost on door sills
- Damp basement walls
- Water on basement floor
- Mould behind furniture and stored items
- Frost on window frames
- Peeling paint, deteriorating siding or efflorescence on bricks
- Mould in corners
- Wall damage and wet carpets below windows
- Condensation or frost on windows
- Mould in closets
- Sagging or stained ceilings
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Checking Your Home's Humidity Levels
Knowing the level of relative humidity (RH) in your home is very useful. Buy
or borrow a hygrometer and watch the changes in RH that occur throughout a typical day
in different rooms of the house and over the heating season.
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How to Solve Moisture Problems and Save Energy
The good news is that many of the measures that you can undertake to solve
moisture problems can also save energy, thereby helping to pay for themselves.
A number of examples follow.
More information
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Reduce Moisture Sources Inside the Home
Getting at the source of excess moisture makes more sense than having to rely
on a lot of ventilation. Ventilation removes moisture, but it also removes heat.
Here are some tips on reducing moisture sources:
- Avoid drying clothes indoors. Vent dryers to the outside.
- Cover any exposed earth in the crawl space or basement with heavy polyethylene,
sealed and weighted down.
- Slope soil away from foundations to keep the basement walls and slab dry.
Patch any foundation leaks.
- Don't use humidifiers unless humidity levels are below 30 percent RH.
- Avoid drying firewood indoors.
- Operate bathroom exhaust fans during a bath or shower. Use a range hood
exhaust when cooking. Make sure fans are vented to the outdoors.
- Avoid steam cleaning carpets in winter.
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Insulating a thermal bridge
- Thermal bridge at exterior corner in closet
- Add insulation (plus polyethylene and drywall)
Sealing air leakage into attic
- Plumbing stack
- Attic hatch
- Electric wiring
- Tops of partition walls
- Electrical boxes
- Chimney
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Add Insulation to Cold Surfaces
Areas that are un-insulated or poorly insulated – such as exterior corners
or foundation walls – can be improved with additional insulation. This may be
expensive by itself but can be incorporated as part of a renovation. Don't forget
to install an air/vapour barrier (usually polyethylene) on the room side of
the insulation to prevent hidden condensation behind the insulation.
| Warning: Some vermiculite insulation, which can be found
in attics and walls of homes, may contain asbestos fibres. If you find vermiculite
insulation in your home, do not disturb it and consult the free Health Canada
publication "It's
Your Health – Vermiculite Insulation Containing Asbestos. " To order
a free copy, call 1 800 443-0395. |
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Reduce Uncontrolled Air Leakage
In the upper part of your house, air leakage outward carries moisture into
the attic and wall cavities, causing long-term deterioration. In the lower part
of your house, air leakage inward can chill window frames and door sills, causing
frost to form. Air leaking in through foundation walls and the floor can be
a source of moisture. Seal all cracks, joints and openings for services and
around sump pump lids. It's a good strategy to reduce this uncontrolled air
leakage and rely more on controlled mechanical ventilation for fresh air. Seal
hidden openings into the attic, tighten the attic hatch, weatherstrip and caulk
around windows and doors, install gaskets on electrical outlets, caulk baseboards
and seal the top of foundations.
More information
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Install Effective Ventilation Systems
If indoor humidity levels are high and moisture sources cannot be reduced any
further, increase ventilation rates. Ensuring adequate mechanical ventilation
is especially important if you are air-sealing your home. Ventilation has the
added benefit of improving your home's indoor air quality.
Schematic of a ventilation system with a heat recovery ventilator (HRV)
- Kitchen
- Living room
- Bedroom
- Cold-air return
- Fresh air
- Exhaust air
- Heat recovery ventilator
- Furnace
- Bathroom
- Laundry
Many older exhaust fans make a lot of noise but don't move much air. Replace
these with more efficient units and use them to remove moisture from bathrooms
and kitchens. You'll find that they are quieter too. They produce less than
53 decibels (2 sones) when working. Make sure the air is being exhausted to
the outdoors and not into your attic. Remember to check the ductwork from the
exhaust fan – it's usually hidden in the attic. The best approach is to provide
a balanced ventilation
system with a heat recovery ventilator (HRV). An HRV provides the opportunity
for continuous ventilation without wasting a lot of energy by transferring heat
from the exhaust air to preheat the fresh air.
Three additional points about ventilation are worth noting:
- When outdoor conditions are mild and damp, such as in coastal areas or on
a rainy day in the spring, ventilation will not be very effective at removing
moisture because the outdoor air is saturated and has little drying potential.
- A new house or newly renovated house needs lots of extra ventilation for
the first year because construction materials – particularly wood and concrete
– will be drying out.
- Homes soak up moisture in the summer and gradually release it back into
the air in the fall. To help dry out your house before the cold weather arrives,
provide more ventilation and open up the windows on warm, dry fall days.
Using an air conditioner on muggy summer days can also help.
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Clean and Maintain Mechanical Equipment
Humidifiers, dehumidifiers, air-conditioning units and filtration systems can
be a source of mould growth. Regular cleaning has the added advantage of improving
the equipment's efficiency.
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Upgrade or Replace Windows
Windows are usually the first place where you notice condensation because they
are the coldest surfaces inside the home. New window technology offers improved
insulating value and can greatly reduce condensation. But replacing your windows
is not your first solution to a moisture problem. Reduce moisture sources before
performing costly measures, such as replacing windows.
Basic Facts About Mould
Characteristics
- Moulds (also called mildew or fungi) are parasitic micro-organisms
that appear as a black, white or multi-coloured stain or "fuzz."
- Most mould spores need "free water" (in other words, condensation
or damp materials) to germinate. However, once mould colonies are established,
many generate their own moisture and can continue to survive even under
dry conditions. Moulds also need mild temperatures and a source of food,
such as house dust or drywall paper.
Sources
- Musty odors are a symptom of dampness and mould growth. A flood, sewer
backup or burst plumbing can lead to many hidden moisture problems,
even after the mess has been cleaned up.
Health Effects
- Large-scale health surveys in Canada and other countries have confirmed
a strong correlation between dampness and mould and respiratory disease
in children.
- Mould spores can cause asthma and other allergies. Some moulds also
release complex chemicals into the air called mycotoxins that can cause
serious health problems. There are tens of thousands of varieties of
moulds, and identifying them is a difficult and expensive task, even
for experts. Therefore, health officials recommend eliminating all moulds
from inside your home.
Potential Damage to House Structure and Contents
- Besides being annoying, winter-long condensation and frost on windows
can damage the window frame and wall below.
- Stains from mould growth in closets, cupboards and basements can foul
clothing, books, carpets, furniture and other possessions.
- Warm, moist air leaking out can condense inside walls. Symptoms of
deterioration can include peeling paint, spalling bricks and buckled
or rotting siding. Repairs can be costly and ongoing.
- Warm, moist air commonly leaks into the attic, condensing and causing
problems. This can lead to rotting wood framing, wet insulation and
sagging or stained ceilings.
- Moisture problems are often associated with cold surfaces and air
leakage, both of which mean excessive heat loss and high energy bills.
- Home buyers will shy away from a house with moisture problems, even
if they are minor. Resale value will be lower.
|
If you are thinking about replacing deteriorated windows, consider upgrading
to high-performance windows, which have special "low-e" coatings and are filled
with inert gases such as argon or krypton. The additional cost is usually less
than 10 percent, and the energy savings are considerable.
More information
If your home's windows are still in good condition, consider installing some
type of interior storm-window kit over the entire window and frame during the
heating season. This effectively adds a layer of glazing, prevents air leakage,
and brings the inner surface of the window closer to warm air currents. Condensation
will be reduced or even eliminated, comfort will be improved, and energy bills
and drafts will also be reduced. Thin plastic film ("shrink-wrap") kits, which
are available at hardware stores, are inexpensive but must be replaced each
year. Other systems, such as rigid plastic sheets, are more expensive but can
be re-used for several years.
If condensation is occurring between the panes of older unsealed windows (for
example, a permanent window plus a storm window or two single-glazed slider
windows), try sealing the interior unit with weatherstripping. If condensation
is occurring between the panes of a sealed double-glazed unit, the unit will
eventually have to be replaced.
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Check for Moisture Entry From Other Sources
You can trace many moisture sources leading to condensation to factors inside
the home. But there are also more obvious sources of water entry. Key areas
to check include the following:
- roof leaks (especially at chimneys, flashings, skylights and eavestroughing)
- wall leaks (especially at window and door flashings and sills)
- foundation leaks (especially where the ground slopes toward the foundation)
- plumbing leaks (especially at toilet bases and under sink drains)
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Keep the Air Circulating
Condensation and mould growth often occur in out-of-the-way areas such as closets,
corners, walls behind furniture, and unused rooms. Increasing air circulation
to these areas will warm the cold surfaces and lower local humidity levels,
reducing the potential for condensation.
- Pull furniture and stored material away from exterior walls and off basement
floors.
- Leave closet doors ajar, leave bedroom doors open as much as possible and
undercut doors.
- Don't block or deflect warm air registers.
- Open drapes, blinds and curtains.
- Set the furnace fan to run continuously. (This will use more electricity,
but you can reduce the expense if you install an energy-efficient two-speed
motor.)
- Don't close off or cut off the heating supply to unused rooms.
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Get Rid of Mould
Even if you eliminate condensation and lower humidity levels in your home,
existing moulds are likely to persist and continue to release spores. Moulds
must be physically removed. Fortunately, this is not a complex or expensive
procedure. Use chlorine bleach to clean mouldy surfaces and a mixture of one
part chlorine bleach, two parts water and a little detergent to clean nearby
surfaces. Leave for 15 minutes and then rinse well. These are harsh mixtures,
so use gloves and protective glasses and remember to ventilate well. Badly mildewed
carpets, furnishings and books will probably need to be replaced.
Technical Information
|
If you want to learn more about how moisture behaves, this section contains
technical information. If not, skip to the next section.
Basic Facts About Moisture
Air contains moisture in the form of water vapour. The warmer the air,
the more water vapour it is able to hold. Relative humidity (RH)
is a measure of how much moisture the air holds in comparison with the
maximum amount the air can hold at that temperature. For example, air
at 50 percent RH holds
half the moisture it is capable of holding.
Air Leakage Into a House
As air is warmed, its relative humidity decreases. For example, air leaking
into a house from the outdoors in winter at 80 percent RH
and at –10°C (14°F) will have a relative humidity
of less than 10 percent once it is warmed to 20°C (68°F). That's why leaky houses can
be excessively dry indoors in winter.
As air is cooled, its relative humidity increases. For example, warm,
moist air at 50 percent RH and at 20°C (68°F) leaking out of a house and into
an attic in winter will reach 100 percent RH by the time it has cooled to
9°C (48°F). At 100 percent RH, condensation occurs, with
water or frost forming on the nearest surface. This is why air leakage
outward can cause moisture problems in attics and walls.
Air doesn't need to pass into or out of the house to change its RH
or to condense. Cold surfaces can also cause condensation. Think of the
droplets of water that form on a glass of cold water on a humid summer
day. The air immediately adjacent to the glass has been chilled to the
point where its relative humidity is 100 percent, and condensation
occurs.
Where is the coldest surface in a house during the winter? Usually on
windows. That's why condensation – in the form of fogging or frost – is
common on cold days. Cold surfaces can also occur at thermal bridges,
which are locations where there is very little insulation or where structural
members extend through the insulation from inside to outside. Examples
of thermal bridges include exterior corners and wall/ceiling
intersections.
You may be surprised to learn that condensation caused by cold surfaces
can also be a problem in the summer. Usually, in late spring or early
summer, the ground has not fully warmed up from the previous winter. On
warm, humid days condensation can occur on basement walls and floors that
are in contact with the ground. For example, outside air at 70 percent
RH and 25°C (77°F) will condense on basement walls
and floors that are cooler than 19°C (66°F). Therefore, opening basement
windows won't always dry out the basement; in some cases, it can actually
increase moisture problems.
The higher the indoor relative humidity, the greater the chance of condensation
occurring on cold surfaces. There are many moisture sources that can contribute
to elevated humidity levels in winter:
- occupants (30–40 litres/week for a family of four)
- summer moisture absorbed by house and released in fall (20–40 litres/week
for about four weeks)
- drying of construction materials in a new house (25–35 litres/week
for the first 18 months)
- drying firewood indoors (15–30 litres/week for three cords/season)
- household activities: showering, cooking, bathing, washing (15–20
litres/week)
- damp basement, crawl space or sump pit (15–20 litres/week)
- drying clothes indoors (10–15 litres/week)
- humidifiers
- indoor pools, hot tubs, greenhouses, house plants and aquariums
Indoor humidity levels are also influenced by air exchange, which has
the effect of replacing moist indoor air with drier outdoor air during
winter. The rate of air exchange in a house depends on the use of mechanical
ventilation, such as bathroom and kitchen exhaust fans or a heat recovery
ventilator, and on natural air leakage.
The combination of indoor moisture sources, air exchange rates and cold
surfaces will determine how much condensation will take place in your
home.
|
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Keep Humidity Levels Within a Healthy Range
Health Canada recommends that your home's relative humidity be kept between
30 and 55 percent in winter. Lower levels aggravate skin allergies and
respiratory infections, and higher levels increase the spread of mould, bacteria
and viruses. Dust mites spread when the humidity is above 50 percent.
- Decrease in bar width indicates decrease in effect.
- Relative humidity and health effects
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Piggyback!
If you are undertaking a major renovation, such as finishing a basement or
gutting an interior, don't lose the opportunity to "piggyback" solutions to
moisture problems. Some measures – such as adding insulation, a polyethylene
air/vapour barrier or a balanced ventilation system – are much easier and less
expensive to implement during construction than afterward.
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Use Dehumidifiers Wisely
Dehumidifiers are generally not effective in winter, since they can lower humidity
levels to between 50 and 60 percent only. However, running a dehumidifier in
the basement on muggy summer days can be effective in reducing condensation
on foundation walls and floors. Exhausting moist air from the basement may help,
unless the replacement air from outside is just as moist.
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Don't Become a Statistic!
If your house has high humidity levels and no obvious moisture sources, it
is essential to check fuel-burning equipment – furnaces, water heaters, boilers,
fireplaces and wood stoves – for proper venting. A blocked chimney could mean
that combustion products, including large amounts of water vapour, are spilling
into your house. Along with that moisture come dangerous combustion gases, such
as carbon monoxide, which kill more than a dozen Canadians every year. Have
heating equipment and venting systems checked by a trained service person.
If your moisture remedial work or energy retrofit includes extensive air sealing,
ensure that all fuel-burning equipment has an adequate supply of combustion
air. Advanced equipment such as high-efficiency furnaces have their own air
supplies and exhaust fans. However, conventional equipment may rely on house
air for combustion and on "natural draft" to move combustion products up the
chimney flue. If starved for air or overpowered by a powerful exhaust fan somewhere
else in the house, such equipment can spill combustion gases indoors. Examples
of combustion spillage include stains near the vent of a gas water heater, smoke
entering the room from a wood-burning fireplace or stove, and pilot lights being
blown out.
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Where Can You Get More Information?
Energy Publications From Natural Resources Canada's Office
of Energy Efficiency
Air-Leakage
Control
Consumer's
Guide to Buying Energy-Efficient Windows and Doors
Improving
Window Energy Efficiency
Keeping
the Heat In
Heat
Recovery Ventilator
Available from:
Energy Publications
Office of Energy Efficiency
Natural Resources Canada
c/o S.J.D.S
Gatineau QC J9J 3N7
Tel.:1 800 387-2000 (toll-free)
995-2943 (National Capitol Region)
Fax: (819) 779-2833
TTY: (613) 996-4397
Web
site
Canada Mortgage and Housing Corporation (CMHC) Publications
About Your House: Hiring a Contractor
Clean-Up Procedures for Mold in Houses
Investigating, Diagnosing & Treating Your Damp Basement
Available from:
Canada Mortgage and Housing Corporation
Canadian Housing Information Centre
700 Montreal Road
Ottawa ON K1A 0P7
Tel.: 1 800 668-2642 (toll-free)
Fax: (613) 748-4069
Web
site
Renovation Contractors
| If you want to hire a contractor to help solve moisture problems, read
CMHC's
publication About Your House: Hiring a Contractor before you start.
Make sure the contractor has a sound understanding of the causes of moisture
problems. Supply a copy of this fact sheet and ask if the contractor has
read any of the publications listed above or has taken a training course
on moisture problems. If you have had basement flooding, look for a professional
in the Yellow Pages under "Water Damage." |
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