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Solving Moisture Problems

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
– 0.2 litre per shower
– 0.05 litre per bath
Clothes washing (per week) 1.8
Clothes drying indoors or using an un-vented dryer (per week) 10.0
Floor mopping per 9.3 (100 sq. ft.) 1.3
Normal respiration and skin evaporation from occupants 38.0
Total moisture production per week63.0


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:

  1. 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.
  2. Reduce moisture at the source. This means producing less moisture in the first place and exhausting moist air and bringing in drier air.
  3. 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.

  1. 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:

  1. Mould, frost or wet insulation or wood in attic
  2. Mould in the bathroom
  3. Frost on door sills
  4. Damp basement walls
  5. Water on basement floor
  6. Mould behind furniture and stored items
  7. Frost on window frames
  8. Peeling paint, deteriorating siding or efflorescence on bricks
  9. Mould in corners
  10. Wall damage and wet carpets below windows
  11. Condensation or frost on windows
  12. Mould in closets
  13. 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

  1. Thermal bridge at exterior corner in closet
  2. Add insulation (plus polyethylene and drywall)

Sealing air leakage into attic

  1. Plumbing stack
  2. Attic hatch
  3. Electric wiring
  4. Tops of partition walls
  5. Electrical boxes
  6. 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)

  1. Kitchen
  2. Living room
  3. Bedroom
  4. Cold-air return
  5. Fresh air
  6. Exhaust air
  7. Heat recovery ventilator
  8. Furnace
  9. Bathroom
  10. 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

  • 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.


  • 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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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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