Condensation

A problem we regularly encounter when checking for the cause of apparent roof leaks is moisture caused by condensation. This condensation is usually a result of humidity within the home rather than leakage from an exterior source. A large amount of water can be generated through this condensation process. The solution for this problem is usually not quick or easy to find. It is often complex, normally involving multiple steps all approaching the problem from different directions.

HOW DOES CONDENSATION OCCUR?

Humidity is water that has passed into the vapor phase and is contained in the air. The amount of water vapor in the air is typically defined by the term “relative humidity.” Relative humidity is the amount of water vapor in the air relative to the amount or water vapor the air is able to hold. Warmer air can hold significantly more water vapor than cold air. Condensation occurs when water vapor is cooled and the vapor moves into the liquid phase. This usually occurs when the water vapor comes into contact with a surface that is at or below the dew point temperature. Often condensation occurs on surfaces that are significantly below the dew point temperature and the water vapor moves directly into a solid phase and it appears to us as frost. The most severe problems occur when there has been an extended period of very cold temperature. The frost build up gets very heavy and when the first warm day arrives, the frost melts. The timing of this can make it look like an ice back up problem or even a roof leak. Typically the condensation leakage will then stop until the frost has a chance to build up again. This process can repeat itself over and over. The length of cold spells and the amount of humidity that is allowed to condense will determine the amount of water. This process can even be occurring to a small degree regularly and the water absorbed into the structure as it melts.

WHAT IS DAMAGED?

The damage that can be caused by condensation may be simply water staining of plaster and paint, but it can also cause delamination of plywood decking and in some cases we see rotting of structural members. In some cases the moisture can cause mold and mildew to occur. We see the water from condensation show up on both ceilings and walls. It is also very common to see skylights and windows wet and dripping as well as water dripping from kitchen and bathroom exhaust vents.

WHERE DOES IT COME FROM?

There are many sources of the moisture that raise the level of humidity in the house. Some common sources are bathing, showering, cooking, washing and drying clothes, plants, aquariums, in house pools, improper venting of exhaust ducts, leaking plumbing pipes or radiators, vaporizers and mechanical humidification units. In addition, many aspects of the construction process can contribute large amounts of moisture inside a building such as painting, plastering, concrete or tile work and even new lumber that has a high moisture content. Sometimes basements and crawl spaces can allow significant ground moisture up and into the home.

HOW CAN THIS HAPPEN?

Humidity gets to a place where it can condense in two ways. One of these ways is through diffusion. In this process, humidity passes through a barrier. The second way humidity moves is through infiltration. Infiltration involves humidity moving through cracks and openings in a barrier that are called “bypasses”. The amount of moisture that can reach the dew point temperature is normally many times more severe through infiltration than through diffusion.

Materials have a perm rating which measures their ability to allow water vapor to pass through them. Humidity is prevented from reaching the dew point temperature through the use of vapor retarders installed on the warm side of insulation. If a material has a zero perm rating it means that no water vapor will pass through it. Typical vapor retarders have low perm ratings but are rarely at zero.

Bypasses that exist occur in many different places, some as simple as cracks in walls and ceilings or openings in electrical switches, outlets and light fixtures. Recessed light fixtures in particular can allow large amounts of water vapor to move through them. Incomplete or damaged vapor retarders are another common bypass. Openings in framing members where plumbing pipes or wires pass through or even where cracks, knots or splits occur can allow humidity to pass through as well. We have often seen where exhaust ducts are not properly connected to vents or they stop short and empty out into the attic – again allowing the potential for severe condensation.

THE PROBLEM GETS WORSE

Humidity is actually forced through materials and bypasses in a house by both vapor pressure and positively pressurized heating systems.

Vapor pressure increases as the temperature gets warmer and as the air contains more water vapor. The pressure will try to equalize itself across a barrier forcing humidity on one side to the other.

Many heating systems draw in outside air to positively pressurize a house with the intent that the house will have less cold air drafts. This will effectively force warm humid air out rather than letting cold air drafts in, which magnifies a condensation problem.

WHAT CAN I DO?

The solution to a condensation problem involves two principals; one is to stop the humidity from getting to the dew point temperature by sealing bypasses and installing vapor retarders. The second is to minimize the humidity so there is very little that can condense. Both of these require that a thorough evaluation of the building be accomplished and then decisions concerning which options are practical. The cost may be prohibitive to solve all the problems. You will need to evaluate how far you are willing to go and take care of as many aspects or conditions as you can to minimize the amount of condensation.

VENTILATION’S ROLE

Some, as a cure for condensation, have recommended ventilation. Ventilation can help a condensation problem but it will not eliminate it all by itself. Ventilation can remove some of the humidity that gets into an attic space before it condenses but it will not remove it all. Furthermore, ventilation can help to dry out a space after condensation has occurred. We would recommend ventilation as an aid but not as a cure by itself. Other things should be done to minimize the amount of humidity that can condense at the same time. Ventilation can also change which surface is at the dew point temperature by lowering the temperature in the attic cavity.