What is condensation?

If you listen to some people, you’d think that “condensation” is the satanic miasma that descends upon the undeserving, punishing them for sins committed in prior lives (or capriciously,  for no reason whatsoever).  In the field of roofing, roofers will look upon an afflicted home and suggest various remedies, many of which are no better than burning chicken entrails under a full moon, or uttering incantations banishing the spirits from the crevices in which they took refuge.  It is also quite common for all kinds of building mischief to be blamed on “Condensation” hence informing the suitably-impressed home-owner that they are in the presence of a superior intelligence.


Pour yourself a cold one on a warm and humid day, and the liquid forming on the outside of the glass is condensation.  Walk through the dewy grass in the morning, and you’re walking through condensation.  Watch the clouds moving across the sky, and you’re looking at condensation.  Exhale on a cold day, and watch the condensation of your breath dissipate in the cold air.  Doesn’t look all that evil, does it?

Since we live on a water planet, and we depend on the water for our lives, our sense of well-being, our health, our pleasure, and food, we might as well learn to live with it, as it isn’t going away, and we don’t want it to.  And yet, we all know that water, in the wrong place, in the wrong amounts, at the wrong time, can kill and damage.  Water is the main reason we need roofs on our homes.

If you have ever gone camping in a single-walled tent, you know the condensation that soaks the walls of the tent and makes your morning miserable.  Compare that experience with what you get in a modern tent, designed with an mesh inner tent, and an exterior cover – you wake up reasonably dry, despite the condensation that has formed on the outside fly cover.  We need to apply some of the same principles used by good tent-makers, to our own homes.

So what is this “condensation”?  It is nothing more than water vapour in the air that surrounds us.  The amount of moisture that air can hold increases with both temperature and pressure.  The maximum quantity of moisture that air can contain at a specific temperature (and pressure) is defined as the 100% humidity mark (or dew point).  The air at this point is said to be “saturated” with water.

If air at 100% humidity (at a certain temperature) is warmed, then the air is no longer fully saturated with moisture as it has the capacity to hold more vapour.  The converse is also true, that air containing moisture to the 70% mark, will become saturated (100% humidity) as the temperature falls (see the table below).  Condensation happens when the air cools to the dew point, and the moisture in the air reaches saturation point, so the excess moisture then condenses.

Dew-point Table

In the above table, assume you have air at 70% humidity, and it is 70 degrees F.  If the temperature of the air falls to 59 degrees F, the air will be fully saturated and be at the dew point.  Any further lowering of temperature will cause the moisture to condense.

So where will this condensation happen?  Air next to cooler surfaces reaches the dew point earlier, so it is often said that the cooler surface “attracts” condensation.  Materials that lose energy faster (either by conduction or radiation – think infrared heat) will tend to cool faster.  Metal will usually cool faster than wood or plastic, and therefore metal “attracts” condensation.

In cold climates, the moisture content of external air is usually quite low.  However, it is common for interior air to have moisture content in excess of 40%.  If this warm, moist air escapes into a cold area (for example, an attic as in the picture below), then the air will cool and soon reach the dew point, at which point moisture will condense on the nearest cold surface.

Sheathing damaged by condensation, which allowed rot to occur, which then weakened the wood. Dark wood shows the initial appearance of fungus, with white splotches revealing recurring growth.

Sheathing damaged by condensation, which allowed rot to occur, which then weakened the wood.
Dark wood shows the initial appearance of fungus, with white splotches revealing recurring growth.

There are two techniques used to prevent condensation:

1. use of a vapour barrier, which prevents moist air from the living space to escape to an area which is cooler, and

2.  sufficient ventilation, which brings in comparatively dry air from the outside and “dilutes” the vapour content of the moist air so that it never reaches the dew point even as it cools.

 Another source of condensation is trapped moisture originating from a leak.  Depending on where the leak is, the moisture may become trapped between the shingles and the underlayment, or between the underlayment and the sheathing.

This trapped moisture may evaporate during the day (as the sun heats the roof surface), only to condense in a different area, once the temperature drops.  The solution to this problem is to find the originating leak and fix it.

Occasionally, one sees condensation forming in attics due to poorly-insulated air ducts that are distributing air conditioned air.  The ducts themselves can be quite cold, and may cause condensation of moisture contained in ambient summer air, especially if the humidity is high.  The solution here is to properly insulate the air ducts.

What harm can condensation do?

Since all life depends on a certain minimum amount of water, regulating the amount of water in a house will also regulate where the living things can grow.  Wood can rot if there’s enough moisture.  Rotting wood can attract carpenter ants and termites.  Mold can grow if there’s enough moisture.  If you have enough concentration of mold spores, you can get into all kinds of health issues.  So in roofing, one of our main challenges is to ensure that the cold spaces remain dry.  It’s not enough to stick a vent on a roof and call it good.  You need to ensure that your vapour barrier is working, and that the ventilation reaches all the spaces where condensation could form.

The good news is that if you have “condensation” in your home, you’re not afflicted by malevolent spirits. it’s not contagious, and you’re not a bad person.  You just need to arrange the physics in your home to work for you.

(c) 2014 Paul Grizenko

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