Condensation Tips

Steps for Solving Humidity Problems

See signs of humidity? Follow these steps:

Note the type of problem

1. A light fog on windows
2. Dampness or frost on walls
3. Stains on ceilings

Establish whether the problem is isolated or widespread

1. Just a few windows fogged or a small stain on the ceiling
2. Many windows or walls with condensation
3. Only one or two areas of the home are affected

Determine the problem

1. Is the humidity level too high?
2. Has there been a change to colder weather?
3. Is there a water leak or other new source of moisture?
4. Is your home new or has a large addition been constructed?
5. Have there been any equipment changes?
6. Has the chimney been checked recently for leaks or blockages?
7. Have you taken measures to stop air leakage?
See Table 2

Take action to rectify the situation

1. Control the sources of humidity first
2. If condensation is isolated, remedy the problem area (air leak, water leak, missing insulation, etc)
3. If condensation is widespread, add ventilation to lower the humidity level

By using the tables and other information here you should be able to understand relative humidity and to control condensation problems.

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What is Condensation?

Condensation is visible evidence of excessive moisture in the air. It may appear as water, frost, or ice on the interior surface of windows and doors. The warmer the air, the more water the air can hold, which means that the air in the center of any given room will hold more water than the air adjacent to the window or door walls since this area is always cooler.

When the warm, moisture-laden air moves toward the cooler window or door wall it becomes cooler and cannot hold the moisture it held when it was warmer. Therefore, the moisture is dropped and appears as water on the glass and frames of windows and doors. This occurs more frequently during the winter months because of the extreme difference during the winter months, when the average outdoor temperature drops to –15 or colder. For more information, please see the Better Business Bureau.

Surface Condensation

Surface Condensation is a visible indication of high moisture content in the air or a cold surface. It is a common problem in Canadian homes and usually requires only minor action to prevent. It appears as condensation on windows, inner wall surfaces, toilet tanks, 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, mold growth, or finish deterioration. Table 1 lists outdoor temperatures and the indoor humidity levels at which condensation appears on inner glass surfaces usually the coolest surface in the room. The table shows that at cold exterior temperatures it is difficult to maintain the 40 to 50 percent interior humidity level recommended by health professionals without some condensation on windows surfaces.

Table 1 shows triple glazed windows (three layers of glass) will allow higher interior relative humidity without condensation than double glazed units. The extra glass and trapped air between help keep the inner surface warmer. When combined with triple-glazing superior condensation reduction results will be achieved. Any increase in RSI (R) value of the windows allows it to perform better than a unit of lower RSI (R) value. RSI value (R) value refers to the resistance to heat flow – the higher the better.

Table 1: Relative Humidity

When the outside temperature is: Condensation will occur on the inner glass surface when the interior relative humidity is near or beyond:
Celsius (Fahrenheit) Double Glazed Relative Humidity( %) Triple Glazed Relative Humidity (%)
-40 (-40) 15 36
-34 (-35) 15 39
-28 (-20) 20 44
-23 (-12) 25 48
-18 (0) 25 53
-12 (10) 30 63
-7 (20) 35 67
-1 (30) 40 72

**** Room temperature is 21 degree Celsius (70 Fahrenheit)
Note: Prevailing winds can have a dramatic effect on the cooling of glass surfaces and as a result relative levels of inside humidity may be impacted by up to 10%.

Interior surface condensation can occur on any cool surface. Cool water pipes, toilet tanks, poorly insulated walls or floors, metal hinges or locks on exterior doors – not just on window surfaces. High relative humidity with cold surface temperatures creates the most problems. This type of condensation can occur no matter how well built a house is. Covering the window with curtains can restrict air movement and result in a cooler glass surface, which increases condensation.

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 into the wall.

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Outdoor Condensation on Glass

Condensation forms on outdoor surfaces of glass when the outdoor glass temperature drops below the outdoor dewpoint temperature. This does not indicate there is a problem with your glass or windows. It is actually just the opposite. If insulating glass, it is performing just the way it should.

Here’s what happens. Insulating units that use high-performance glass, such as PPG Solarban® 60, are designed to reduce the amount of heat that flows from inside the building through the glass to the outside during the cold weather.

Since the insulating unit keeps more heat inside, the outside glass surface stays colder. If the outdoor surface stays colder, if the outdoor surface temperature actually drops below the outdoor dewpoint temperature, condensation may form.

If the outdoor humidity is low and if wind blows against the glass, condensation will usually not appear. But on cool, clear nights, when there is no wind, and high relative humidity, condensation often forms on outdoor glass surfaces.

If the condensation disappears when the sun comes up and warms the glass, you can relax. Your insulating glass is working properly.

This document is intended to inform and assist the reader in the application, use, and maintenance of PPG Flat Glass products. Actual performance and results can vary depending on circumstances. PPG makes no warranty or guarantee as to the results to be obtained from the use of all or any portion of the information provided herein, and hereby disclaims any liability for personal injury, property damage, product insufficiency, or any other damages of any kind or nature arising from the reader’s use of the information contained herein.

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Causes of Condensation and Solutions

Identify the Problem

Every homeowner should know how to recognize condensation problems, how to evaluate their seriousness and how to solve them. A sudden increase in humidity may indicate a basement leak, a broken water pipe or a faulty chimney – all harmful problems to be remedied immediately. Common locations of condensation and moisture buildup, possible causes and suggested solutions are outlined in table 2. It shows that most problems are a result of high humidity levels and such causes as air leakage, a cold surface or sudden temperature drop.

Table 2: Condensation Problems

Indication Possible Causes


Condensation on most inside surfaces of inner pane – Abnormally high humidity
– Lower thermostat setting at night
– Sudden change to colder weather
– Poor air circulation
Condensation on upper story windows only – Cool air leaking into lower level, warm air leaking out upper level.
Condensation on windows on one side of house only – Cold air on windward side, warm air out on downside
– Prevailing winds will impact negatively
Condensation on north windows only – Inner surfaces cooler than south windows in daylight
Condensation on windows, 1-2 rooms – Room(s) cooler than the rest of the house
– High humidity rooms: laundry room w/ humidifier
Condensation on door frame/threshold – Air leakage around door Condensation on lock, knob and hinges
– Air infiltration due to negative pressure inside


Condensation on closet walls – Poor air circulation around clothing
Condensation and staining below window – Melted window condensation
– Window in humid area
Condensation on many walls – Abnormally high humidity
– Poor air circulation
– Missing insulation
Mold on wall – Abnormally high humidity
– Poor ventilation
– Poor air circulation
Frost on basement walls – Abnormally high humidity levels
– Air leakage at sill plate, un-insulated walls
Condensation around electrical outlets – Incoming cold air
– High Humidity
Water on basement floor and walls – External sources like runoff, rain or ground water
– Abnormally high moisture level
Stains on ceiling – Frost build-up melting in attic when weather warms
– Leaking roof


Large isolated frost deposit in attic – Indicates large volume of warm water and moist air leakage into attic space
– Inadequate attic space ventilation


Condensation on cold surfaces such as pipes, toilet, etc – Abnormally high humidity level
– Cold water supply
Exterior paint peeling – Lack of air. Vapor barrier
Structural damage to studs, joists and or rafters – Bacterial action due to warm, moist and dark environment

Suggested Solutions

1. Control humidity sources (Table 5)
2. Condensation should disappear as the house warms up (adjust thermostat setback slightly higher is condensation is severe
3. Ventilate home to lower humidity level; control sources of moisture (humidifier, showers, etc)
4. Improve circulation by opening drapes, moving furniture or objects blocking registers, running furnace fan continuously
5. Replace window with a new unit with a superior seal capable of withstanding higher relative humidity levels
6. Improve caulking, weather-stripping to reduce cold air infiltration, ensure there are adequate fresh air and combustion air supplies (Figure 5)
7. Exhaust dryer to outside
8. Seal between the frame and rough opening space, improve caulking and weatherstripping
9. Shut off humidifier
10. Replace with insulated door, add a storm door


1. Replace, adjust weather-stripping, threshold, add a storm door
2. Replace weather-stripping, add fresh air duct to the heating system, add a storm door


1. Control humidity sources (Table 5)
2. Improve air circulation, open doors, add louvers, grills, undercut doors, leave space by the outside wall
3. Add insulation to the wall cavity
4. Run furnace fan continuously
5. Caulk siding corners, brick molds, siding – parging joint, etc
7. Use fans or add mechanical ventilation
8. Aid circulation by moving furniture/objects blocking registers or running furnace fan continuously
9. Seal and insulate basement walls
10. Seal under plates
11. Seal wall, install weeping tile, direct gutter drains away from wall, maintain ground slope away from wall, check that existing weeping tiles are open
12. Add vents to overcome inadequate attic ventilation, seal air leaks into attic space from below


1. Identify source, caulk or seal leak at ceiling level (large amounts usually occur around stacks or from poorly sealed exhaust ducts – make sure all exhausts vent to exterior not into attic space
2. Add attic vents


1. Control humidity sources (Table 5)
2. Insulate pipes or toilet tank
3. Add air/vapor barrier to the interior or paint with vapor barrier elastomeric paint on interior surfaces, seal around electrical outlets
4. Identify and caulk any leaks or gaps on the exterior, repair flashings
5. Identify the cause of leakage, replace damaged member’s, seal wall, floor, or ceiling assembly to prevent recurrence.

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Moisture Sources and Causes

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. Table 3 identifies typical sources of moisture.

Table 3: Moisture Sources

Summer Moisture (trapped when windows closed in fall
– may last 3 or 4 weeks)40 (9)

A Typical Source Liters (Gallons) Added Weekly To Interior Air


Occupants (4) 30-40 (7-9)
Showers (8/wk) 18-29 (3-5)
Drying Clothes 12 (2.6)
Cooking (steam) 9 (2)
Gas Cook Stove 9 (2)
Refrigerator 9 (2)
Dishwashing (sink) 3 (0.7)
Large House Plant (over 1 meter) 3 (0.7)
Bathing (8/wk) 3 (0.7)
Laundry 2 (0.4)


Damp basement or crawlspace 14 (3)
Un-vented kerosene heaters (depends on use) 5 (1.1)

Canadian 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 vapor to the air. Air leakage and the effect of the chimney and continually exhausting warm air result in the 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 and will affect the production of the home.

There may be situations when water vapor production is greater than the loss. Increased caulking and weather stripping to reduce air leakage, the installation of a chimney-less 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).

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Energy Efficient Windows and Doors

Excessive humidity and surface condensation become a problem when previous routes for moisture escape, are closed or when more moisture is produced than what escapes. The average home needs about 55 liters 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 remodeling or the building of a new energy efficient house, can cause higher interior humidity levels. The humidity can be controlled, by using a ventilation system.

Table 4: Usual Causes and Sources of High Humidity

New Home or Large Addition As much as 36 liters (8 gallons) of moisture a week are released from the building and finished materials during the first 18 months.
Faulty or plugged chimney serving any fuel-fired appliance (furnace, water heater, etc) Water vapor is a major by-product of combustion so unusually high moisture levels can be an indication of a plugged or leaky vent and must be corrected immediately. It is very important to have a heating contractor or utility company test the system
New, Energy Efficient Home Low air leakage rate and moisture from building materials mean ventilation must be used to prevent the buildup of moisture from day to day activities.
Extensive Renovation for Energy Efficiency Plugging air leaks means moisture that formerly leaked out must now be ventilated.
Installation of a High-Efficiency (chimney-less) Furnace Eliminating the chimney stops a major source of air leakage. Additional ventilation may be needed.
Flooded Basement A Major water source such as a broken water pipe, spring run-off or a high water table can bring up to 900 liters (200 gallons) of moisture into the home each week-proper ground slope, weeping tile and gutters directed away from the basement walls are essential for controlling water movement.
Minor Leaks and Water Sources Leaking roofs, water pipe leaks, badly dripping taps, carpet steam cleaning, mopping floors or even storage of green wood can introduce a lot of moisture – Recognize and control the source.
Other Operating an indoor hot tub or pool, maintaining a tropical atmosphere for plants or even throwing a large party can overload the home with moisture.

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Control Moisture

Table 5 outlines steps that can be taken to lower home humidity levels. The initial steps are simple and may provide easy solutions. Working further through the chart leads to more complex and detailed solutions – but in case of high humidity and low air leakage, these may be required.

* Always Try the Simple Steps First *

Table 5: Controlling Humidity – Actions Required

Personal Actions – Cover cooking pots to elminate steam and conserves energy
– Do not hang clothes inside to dry
– Take less time in the shower
Turn Humidifiers Down or Off – May require shutting off water supply to furnace humidifier
– Use individual room humidifiers sparingly
Make sure that dryer is vented to the exterior – Electric and gas dryers MUST BE VENTED TO EXTERIOR (electric dryers may have been vented indoors as a heat saving measure)
Operate Venting Fans – Use individually to control humidity from showers, cooking, etc. (Make sure you run it a few minutes after the activity to ensure moisture is removed)
Operate Furnace Fan Continuously – 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
Isolate Moisture-Producing Areas – Close windows and doors to greenhouses, indoor pools, and hot tub spaces.
– Do not draw air into the heating system from humid areas
– Add separate exhaust venting system
Cover Exposed Earth in Basements/Crawlspace – Must have a ground cover like heavy polyethylene or roll roofing overlapped by 100mm (four inches) and weighted down or protected by sand layer.
– Ventilate space in summer according to Local Building Codes
Increase Outside Air Supply to Heating System – Fresh air duct with variable damper allows a controlled amount of dry outside air into the home
Add Mechanical Venting – Install fans in moisture-producing areas: bathrooms, laundry areas, and kitchens
– Install a central fan controlled by a humidistat (operates only when the humidity is above preset level)
Install Air to Air Heat Exchanger into Venting System – This can re-capture part of the heat coming from the outgoing air.

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

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Increase Fresh Air Supply

One method that may be required to control humidity buildup 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.

This duct provides fresh air for humidity control while a separate duct should be installed to supply combustion air for fuel-burning appliances like furnaces, water heaters, and boilers. Approved automatic dampers are available. They automatically close and open according to the requirements of the heating unit.

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Add Mechanical Ventilation

Older homes often do not have exhaust fans in moisture-producing areas like bathrooms, laundry rooms, and kitchens. 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 the exhaust fans are powerful enough to move the air the distance required.

If the vents must be installed through the attic and roof or soffit, be sure the dampers close automatically. They must be sealed and insulated so that warm, moist air cannot leak into the attic space.

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 fan units.

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Incorporate 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 re-capture the heat. However, if large volumes of air are continually being exhausted, then a heat recovery ventilator can be installed to re-capture 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.

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In-Depth Information on Condensation

For more in-depth information on condensation, please reference several accredited sources available on the web.

National Research Council –
Natural Resource Canada –