MOISTURE
PROBLEMS

 

Moisture problems in are detrimental to our health and to the durability and resale value of our homes. This fact sheet shows you how to solve moisture problems.

 

Why Should I Worry about
Moisture Problems

 

Does My House Have a Problem?
 Start with a few basic questions:


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.

Checking your home's humidity levels
Knowing the level of relative humidity (R.H.) in your home is very useful. Buy or borrow a hygrometer and watch the changes in R.H. that occur throughout a typical day in different rooms of the house and over the heating season.

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. This section includes some examples. For further details, refer to one of the publications listed on the last page of this fact sheet.


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
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:

Add insulation to cold surfaces—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/vapor barrier (usually polyethylene) on the room side of the insulation to prevent hidden condensation behind the insulation.

Basic Facts About Moisture
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 vapor. The warmer the air is, the more water vapor 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% 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% R.H. and -10°C will have a relative humidity of less than 10% once it is warmed to 20°C. That's why leaky houses can be excessively dry indoors in winter.

As air is cooled, its relative humidity will increase. For example, warm, moist air at 50% R.H. and 20°C leaking out of a house and into an attic in winter will reach 100% R.H. by the time it has cooled to 9°C. At 100% R.H., condensation occurs, with water or frost forming on the nearest surface. This is the reason why air leakage outwards 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% and condensation occurs.



Air leakage into a house

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 below).


Insulating a thermal bridge
  1. thermal bridge at exterior corner in closet
  2. add insulation (plus polyethylene and drywall)

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% R.H. and 25°C will condensate 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.