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Insulating a thermal bridge 1) thermal bridge at exterior corner in closet |
Areas that are uninsulated 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.
| Advisory |
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 Health Canada publication It's Your Health - Vermiculite Insulation Containing Asbestos by calling 1 800 443-0395, or visit www.hc-sc.gc.ca. |
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. (Natural Resources Canada has published another fact sheet of this series entitled Air-Leakage Control. See the end of this fact sheet to find out how to order your free copy.)
Sealing air leakage into attic 1) plumbing stack |
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 airtightening your home. Ventilation has the added benefit of improving your home's indoor air quality.
Many older exhaust fans make a lot of noise but don't move much air. Replace these with more efficient (and quieter! – that is, they produce less than 53 decibels or 2 sones when working) units and use them to remove moisture from bathrooms and kitchens. 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.
Schematic of a ventilation 1) kitchen |
Three additional points about ventilation are worth noting:
The use of air conditioners on muggy summer days can also help.
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.
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 Moulds |
Moulds (also called mildew or fungi) are parasitic micro-organisms that appear as a black, white or multi-coloured stain or “fuzz.” 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. 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. |
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. For more information on energy-efficient windows, refer to Natural Resources Canada's publication entitled Consumer's Guide to Buying Energy-Efficient Windows and Doors. See the end of this fact sheet to find out how to order your free copy.
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.
This fact sheet has focused on moisture sources leading to condensation inside the home. However, don't forget to inspect for more obvious sources of water entry. Key areas to check include the following:
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.
Even if you eliminate condensation and lower humidity levels in your home, existing moulds will likely 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. |
Air contains moisture in the form of water vapour. The warmer the
air,
the more water vapour it is able to hold. Relative humidity (R.H.) 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
R.H. holds half the moisture it is capable of holding.
As air is warmed, its relative humidity will decrease. For example, air leaking into a house from the outdoors in winter at 80 percent R.H. and at -10°C will have a relative humidity of less than 10 percent once it is warmed to 20°C. That's why leaky houses can be excessively dry indoors in winter.
Air leakage into a house |
As air is cooled, its relative humidity will increase. For example, warm, moist air at 50 percent R.H. and at 20°C leaking out of a house and into an attic in winter will reach 100 percent R.H. by the time it has cooled to 9°C. At 100 percent R.H., 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 R.H. 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, windows have the coldest surface, which explains 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. (See diagram)
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 R.H. and 25°C will condense on basement walls and floors that are cooler than 19°C. 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:
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.
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.
 Relative humidity and health effects Decrease in bar width indicates decrease in effect. |
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.
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.
If your house has high humidity levels and no obvious moisture sources, it is essential to check any fuel-burning equipment – furnaces, water heaters, boilers, fireplaces and wood stoves – to ensure that they are venting properly. 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.
Also, 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.
Technical InformationThis fact sheet is meant only to give you an introduction to the importance of moisture problems and how to solve them. The following publications contain more detailed information: Air-Leakage Control To obtain additional copies of this or other free publications on energy efficiency, contact
About Your House: Hiring a Contractor
|
| 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.” |
Cat. No. M144-59/2004E
ISBN 0-662-38771-6
Aussi disponible en français sous le titre :
Les problèmes d'humidité
The One-Tonne Challenge asks Canadians
to reduce their annual greenhouse gas
emissions by 20% or about one tonne.
www.climatechange.gc.ca
Leading Canadians to Energy Efficiency at Home, at Work and on the Road
The Office of Energy Efficiency of Natural Resources
Canada
strengthens and expands Canada's commitment to energy efficiency
in order to help address the challenges of climate change.
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