Condensation is the change of water vapor in the air to liquid water on a cold surface. It occurs because air holds only a limited amount of water at a given temperature. The lower the temperature, the less water vapor the air can hold. If the temperature of moist air is lowered by a cool surface (beyond its maximum water holding capacity) water will form on that surface - this is water condensation.

Surface Condensation

Surface condensation is a visible indication of a high moisture content in the air or a cold surface. It is a common problem in homes and usually requires only minor action to prevent. It appears as condensation on windows, inner wall surfaces, toilet tanks and cold water pipes or around electrical outlets. Surface condensation is not necessarily serious but can become a problem if it persists for many days on many

surfaces or to the point where it results in staining, mould growth, or finish deterioration.

Concealed Condensation

Concealed condensation usually occurs when warm moist air goes out through cracks, gaps or holes in the building shell (walls, floors or ceilings). It may also result when water vapour diffuses through building materials. Diffusion, the molecular movement of water vapour through solid materials, is caused by differences in vapour pressure. Diffusion of water vapour accounts for a small portion of moisture movement through the walls of homes. The greatest problems with concealed condensation result from the leakage of warm moist air through the building shell. If warm moist air leaks out through a gap in the building shell during cold weather, at some point in the passage air will cool to the point where it reaches 100 percent relative humidity. Condensation occurs on the closest non-porous surface, often the inside face of the exterior sheathing. If the outside temperature is quite low, water vapour may deposit directly as ice or frost. If the amount is small, it may change right back into water vapour and be carried away by natural air movement with little damage to the structure. But large deposits of ice will melt and can soak insulation materials, destroy interior or exterior finishes, or lead to structural deterioration.

Fortunately, concealed condensation is not generally a major problem in our climate. Air leakage in most homes involves warm air moving out through vents, ducts and chimneys, while cold air is moving in through cracks and joints. The cold air drawn in to replace it carries little total moisture - the initial relative humidity may be quite high, but as the air warms to room temperature the relative humidity may be reduced to less than 10 percent. The resulting dry indoor environment, often requiring humidification, is usually a sign that a great deal of outside air is leaking in. This causes drafts, reduces comfort, and increases heating costs.

Even in our climate concealed condensation can result when vapour diffusion is reduced on the outside of a wall when re-siding. Moisture that normally moved through the wall is now trapped. Resulting condensation could cause deterioration of the insulation or the structure. When re-siding you should seal all leakage areas - such as behind the baseboard and around electrical outlets - from the inside to reduce moisture movement into the wall.

1.1. Indication: Condensation on most inside surfaces of inner pane.

Possible Causes:

- Abnormally high humidity.

- Lower thermostat setting at night.

- Sudden change to colder weather.

- Poor air circulation.

1.2. Indication: Condensation on upper story windows only.

Possible Causes:

- Cool air leaking into lower level.

- Warm air leaking out upper level.

1.3. Indication: Condensation on windows on one side of house only.

Possible Causes:

- Cold air on windward side.

- Warm air out on downwind side.

- Prevailing winds will impact negatively.

1.4. Indication: Condensation on north windows only.

Possible Causes:

- Inner surfaces cooler than south windows in daylight.

1.5. Indication: Condensation on windows, 1-2 rooms.

Possible Causes:

- Room(s) cooler than rest of house.

- High humidity rooms: laundry room w/humidifier.

1.6. Indication: Condensation between layers of glazing.

Possible Causes:

- Air leakage from inside into space between glazing.

- Outer storm window sealed.

- Broken insulated seal.

1.7. Indication: Condensation on window frame.

Possible Causes:

- Air leakage: rough opening space and/or poor weather stripping.

- Air leakage: frame, sash meeting rails.

- Poor quality metal or plastic frames.

1.8. Suggested Solutions:

- Control humidity sources.

- Add layer of glass or plastic on inside to increase interior surface temperature.

- Condensation should disappear as house warms up (adjust thermostat setback slightly higher if condensation is severe).

- Ventilate home to lower humidity level, control sources of moisture (humidifier, showers, etc)

- Improve circulation by opening drapes, moving furniture or objects blocking registers, running furnace fan continuously.

- Seal inner pane between glass layers with caulking or replace weather-stripping between sliding units.

- Seal inner unit, allow outer unit to breathe to exterior.

- Replace window with superior new sealed unit capable of withstanding higher relative humidity levels.

- Improve caulking, weather-stripping to reduce cold air infiltration, ensure there are adequate fresh air and combustion air supplies.

- Improve circulation by opening drapes, leaving room doors open, run furnace fan continuously.

- Exhaust dryer to outside, shut off humidifier.

- Seal between frame and rough opening space, improve caulking and weather-stripping.

- Install sealed interior plastic or Plexiglas storm or shutter.

- Replace with higher quality wood or PVC units with an improved thermal break.

- Replace with insulated door and add storm door.

2.0. Door Areas

2.1. Indication: Condensation on door frame/threshold.

Possible Causes:

- Air leakage around door.

2.2. Indication: Condensation on lock, knob, hinges.

Possible Causes:

- Air infiltration due to negative pressure inside.

2.3. Suggested Solutions:

- Replace or adjust weather-stripping at threshold, add storm door.

- Replace weather-stripping, add fresh air duct to heating system, add storm door.

3.0. Wall Areas

3.1. Indication: Condensation on closet walls.

Possible Causes:

- Poor air circulation around clothes.

3.2. Indication: Condensation and staining below window.

Possible Causes:

- Melted window condensation.

- Window in humid area.

3.3. Indication: Condensation on many walls.

Possible Causes:

- Poor air circulation.

3.4. Indication: Condensation on closet walls.

Possible Causes:

- Abnormally high humidity.

- Poor air circulation.

- Missing insulation.

3.5. Indication: Mold on wall.

Possible Causes:

- Abnormally high humidity.

- Poor ventilation.

- Poor air circulation.

3.6. Indication: Frost on basement walls.

Possible Causes:

- Abnormally high humidity levels.

- Air leakage at sill plate.

- Uninsulated walls.

3.7. Indication: Condensation around electrical outlets.

Possible Causes:

- Incoming cold air.

3.8. Indication: Water on basement floor and walls.

Possible Causes:

- External sources like runoff, rain or ground water.

- Abnormally high moisture level.

3.9. Indication: Stains on ceiling.

Possible Causes:

- Frost build-up melting in attic when weather warms.

- Leaking roof.

3.10. Indication: Frost on exposed nails in attic.

Possible Causes:

- Small amount of warm air leaking into attic space.

3.11. Suggested Solutions:

- Control humidity sources.

- Improve air circulation, open doors, add louvers, grilles, undercut doors, leave space by outside wall.

- Improve windows as above.

- Eliminate window unit, insulate opening.

- Add insulation to wall cavity.

- Run furnace fan continuously.

- Add insulation to wall cavities.

- Caulk siding corners, window brick-moulds, siding, parging joint, etc.

- Add insulation in wall or on outside.

- Use fans, add mechanical ventilation.

- Aid circulation by move furniture/objects blocking registers, run furnace fan continuously.

- Seal and insulate basement walls.

- Seal under plates.

- Seal wall, install weeping tile, direct gutter drains away from wall, maintain ground slope away from wall, check that existing weeping tiles are open.

- Add vents to overcome inadequate attic ventilation, seal air leaks into attic space from below.

4.0. Ceiling/Attic Areas

4.1. Indication: Large isolated frost deposit in attic.

Possible Causes:

- Indicates large volume of warm and moist air leaking into attic space.

- Inadequate attic space ventilation.

4.2. Suggested Solutions:

- Identify source, caulk or seal leaks at ceiling level (large amounts usually occur around stacks or from poorly sealed exhaust ducts, make sure all exhausts vent to exterior and not into attic space.

- Add attic vents.

5.0. Other Areas

5.1. Indication: Condensation on cold surfaces such as pipes, toilet tank etc.

Possible Causes:

- Abnormally high humidity level.

- Cold water supply.

5.2. Indication: Exterior paint peeling.

Possible Causes:

- Lack of air/vapor barrier.

- Rain penetration behind siding.

5.3. Indication: Structural damage to studs, joists or rafters.

Possible Causes:

- Bacterial action due to warm, moist and dark environment.

5.4. Suggested Solutions:

- Control Humidity sources

- Insulate pipes or toilet tank.

- Add air/vapor barrier to interior or paint with vapor barrier paint on interior surfaces, seal around electrical outlets.

- Identify and caulk any leaks or gaps on exterior, repair flashings.

- Identify cause of leakage (external or concealed condensation), replace damaged members' seal wall, floor or ceiling assembly to prevent recurrence.

Normal Home Operation

In an average home there are many sources and causes of humidity, some evaporating periodically, some constantly, others adding moisture directly to the air. Typical sources of moisture are identified below.

1.0. Type: Showers (8 per week).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 18-29 (7-9).

2.0. Type: Drying clothes.

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 12 (2.6).

3.0. Type: Cooking (steam).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 9 (2).

4.0. Type: Gas cook stove.

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 9 (2).

5.0. Type: Refrigerator (frost free).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 9 (2).

6.0. Type: Dishwashing (sink).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 3 (0.7).

7.0. Type: Large house plant (over one meter).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 3 (0.7).

8.0. Type: Bathing (8 per week).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 3 (0.7).

9.0. Type: Laundry (2 per week).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 2 (0.4).

10.0. Type: Summer moisture, trapped when windows closed in fall, may last 3 to 4 weeks (seasonal).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 40 (9).

11.0. Type: Damp basement or crawl space.

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 14 (3).

12.0. Type: Un-vented gas heaters (depends on use).

Typical Sources: Four occupants.

Litres (Gallons) Added Weekly to Interior Air: 5 (1.1).

Homes typically quickly eliminate the extra moisture in their atmosphere and condensation is not a problem. Many have such a low relative humidity in winter that humidifiers are used to add water vapour to the air. Air leakage and the effect of the chimney continually exhausting warm air results in outside air being drawn in. Regardless of its initial relative humidity, when that cold air is heated to room temperature it is quite dry and readily absorbs the normal moisture production of the home. There may be situations when water vapour production is greater than loss. Increased caulking and weather-stripping to reduce air leakage, the installation of a chimneyless furnace, a serious water leak or

a blocked chimney are but a few things, which can change the moisture balance. Cold air being continually drawn in (a negative pressure) can result in warm air being forced out (a positive pressure)

Changes to Normal Home Operation

Excessive humidity and surface condensation become problems when previous routes for moisture escape are closed or when more moisture is produced than can escape. The average home needs about 55 litres per second (110 cubic feet per minute) of ventilation to control humidity and maintain air quality. Higher insulation levels and lower air leakage rates, the result of extensive retrofitting or the building of a new energy efficient house, can cause higher interior humidity levels. The humidity can be controlled using a ventilation system.

Some Unusual Causes or Sources of High Humidity

1.0. Cause/Sourced Humidity: Faulty or plugged chimney serving any fuel-fired appliance (furnace, water heater, etc.).

Comments: Water vapor is a major by-product of combustion so unusually high moisture levels can be an indication of a plugged or leaking chimney vent and must be corrected immediately (have heating contractor or utility company test system)

2.0. Cause/Sourced Humidity: New home or large addition.

Comments: As much as 36 litres (8 gallons) of moisture a week is released from building and finishing materials during the first 18 months.

3.0. Cause/Sourced Humidity: New, energy-efficient home.

Comments: Low air leakage rate and moisture from building materials means ventilation must be used to prevent the buildup of moisture from day to day activities.

4.0. Cause/Sourced Humidity: Extensive renovation for energy efficiency.

Comments: Plugging air leaks means moisture that formerly leaked out must now be ventilated.

5.0. Cause/Sourced Humidity: Installation of a high-efficiency (chimneyless) furnace.

Comments: Eliminating the chimney stops a major source of air leakage. Additional ventilation may be needed.

6.0. Cause/Sourced Humidity: Flooded basement.

Comments: A major water source such as a broken water pipe, spring run-off or a high water table can bring up to 900 litres (200 gallons) of moisture into the home each week. Proper ground slope, weeping tile and gutters directed away from basement walls are essential for controlling water movement.

7.0. Cause/Sourced Humidity: Minor leaks and water sources.

Comments: Leaking roofs, water pipe leaks, badly dripping taps, carpet steam cleaning; mopping floors or storage of green wood can introduce a lot of moisture, recognize and control the source.

8.0. Cause/Sourced Humidity: Other.

Comments: Operating an indoor hot tub or pool, maintaining a tropical atmosphere for plants or throwing a large party can overload the home with moisture.

Controlling Moisture

Outlined steps below can be taken to lower home humidity levels. The initial steps are simple and may provide easy solutions. Working further through the steps leads to more complex and detailed solutions but in case of high humidity and low air leakage, these may be required.

Controlling Humidity

1.0. Action Required: Personal actions.

Comments: Cover cooking pots (eliminates steam, conserves energy), do not hang wet clothes inside to dry, take shorter showers.

2.0. Action Required: Turn humidifiers down or off.

Comments: May require shutting off water supply to furnace humidifier. Use individual room humidifiers sparingly.

3.0. Action Required: Check that clothes dryer is vented to exterior.

Comments: Electric and gas dryers must be vented to the exterior.

4.0. Action Required: Operate venting fans.

Comments: Use individually to control humidity from showers, cooking, etc. Run a few minutes after the activity to ensure moisture is removed.

5.0. Action Required: Operate furnace fan continuously.

Comments: Set switch for summer operation (fan runs constantly). Use two speed fans (runs continually at low speed, switches to high speed for heating cycle). Improved circulation will help reduce localized condensation.

6.0. Action Required: Isolate moisture-producing areas.

Comments: Close windows and doors to green houses, indoor pools and hot tub spaces. Do not draw air into the heating system from humid areas. Add separate exhaust venting system.

7.0. Action Required: Cover exposed earth in basement or crawlspace.

Comments: Must have a ground cover like heavy polyethylene, overlapped by 100mm (four inches) and weighed down or protected by sand layer.

8.0. Action Required: Increase outside air supply to heating system.

Comments: Fresh air duct with variable damper allows a controlled amount of dry outside air into the home.

9.0. Action Required: Add mechanical venting.

Comments: Install fans in moisture producing areas such as the bathrooms, laundry, and kitchen. Install a central fan controlled by a humidistat, (operates only when the humidity is above preset level).

10.0. Action Required: Install air-to-air Heat exchanger into venting system.

Comments: Can recapture part of the heat from the outgoing air, (costs and

benefits must be considered).

Note: The installation of a dehumidifier is not a solution to overall home humidity problems because it is only capable of lowering the relative humidity to 50 or 60 percent and is not suitable for colder basements.

Increasing the Fresh Air Supply

One method that may be required to control humidity build-up is to install a controlled source of fresh air. An insulated duct is installed from outside to the return air plenum of the forced air heating system. Although required by the building code in new homes, a fresh air supply source is often lacking in older homes. Installing a duct introduces a new source of dry outside air in homes renovated to cut down air

leakage heat loss. For correct operation, the damper is adjusted so that a small amount of condensation shows on the inside of window glass. It is then opened or closed as necessary to maintain the slight condensation as the outside temperature varies. This duct provides fresh air for humidity control while a separate duct should be installed to supply combustion air for fuel-burning appliance like the furnace, water heater or boiler. It should not have a manual damper. Approved automatic dampers are available. They automatically close and open

according to the requirements of the heating unit.

Adding Mechanical Ventilation

Older homes often do not have exhaust fans in moisture-producing areas like bathrooms, laundry room or the kitchen. If a fan is required to control humidity, it is best to install it on an interior wall, venting down the wall cavity and out through the joist space. This eliminates potential problems in the attic. Make sure exhaust fans are powerful enough to move the air the distances required. If vents must be installed through the attic and roof or soffit, be sure they have automatically closing dampers. They must be sealed and insulated so that warm, moist air cannot leak into the attic space. If two or three exhaust locations are required, they can be joined and operated with a single, large variable-speed fan. Controlling the single exhaust fan with a humidistat allows the homeowner to set the desired humidity level and allow the fan to operate as needed at high speed when someone is showering or many people are in the home, at a slower speed during normal activities or shut off at night when little moisture is being produced. The desired humidity level will vary (high during warm weather, lower during cold weather) to keep window condensation at a minimum. Operate the exhaust system at a minimal rate because it is heated air that is being blown out. The exhausted air must be replaced. If the house is airtight the air leakage will not provide enough make-up air. Replacement air can then be brought in through a fresh air vent. The ease of installation will depend on the accessibility to walls and the basement ceiling. The benefits in terms of efficiency and controlled ventilation, however, far outweigh those of standard ceiling fans units.

Incorporating a Heat Recovery Ventilator

As mentioned, operating exhaust systems can control the buildup of excessive humidity but remember

the exhaust is also removing heated air-air that you have paid to heat! If little air needs to be exhausted

to control humidity, nothing can be done to recapture the heat. However, if large volumes of air are

continually being exhausted, then a heat recovery ventilator can be installed to recapture a portion of the

heat from the outgoing air.

Heat recovery ventilators are devices in which warm air is exhausted past cold incoming air. The two

streams are separated by a thin membrane, which allows heat, but not odors, to be transferred. Although

expensive, these devices can recapture part of the heat from exhaust air. They must be installed into a

ventilation system from a centralized exhaust point. Heat exchangers are generally considered

economically justifiable only in airtight, energy efficient homes.

Summary

If there are signs the humidity level in your home is too high, try the following steps.

1.0. Note the Type of Problem:

1.1. A light fog on windows.

1.2. Dampness or frost on walls.

1.3. Stains on ceilings.

2.0. Establish Whether the Problem is Isolated or Widespread:

2.1. Just a few windows fogged or a small stain on the ceiling.

2.2. Many windows or walls with condensation.

2.3. Only one or two areas of the home affected.

3.0. Determine the Problem:

3.1. Is the humidity level too high?

3.2. Has there been a change to colder weather?

3.3. Is there a water leak or other new source of moisture?

3.4. Is your home new or has a large addition been constructed?

3.5. Have there been any equipment changes?

3.6. Has the chimney been checked recently for leaks or blockages

3.7. Have you taken measures to stop air leakage?

4.0. Take Action to Rectify the Situation:

4.1. Control the sources of humidity first.

4.2. If condensation is isolated, remedy the problem area (air leak, water leak, missing insulation etc).

4.3. If condensation is widespread, add ventilation to lower the humidity level.

By using the information herein, you should be able to understand relative humidity and to control condensation problems within your home.

For more in-depth information on condensation, please reference several accredited sources available on the Web as outlined below.

National Research Council – http://www.nc.ca

Natural Resources Canada – http://oee.nrcan.gc.ca