What Causes Roof Condensation?

The short answer to the question of what causes roof condensation is the same thing that causes all condensation.

Forgive us if that isn’t quite the answer you’re looking for, but to understand the causes of roof condensation, and the condensation problems it can trigger, it helps to understand what causes all condensation.

How Condensation Forms

The best way to understand condensation is to look at some simple facts.

1. Water is always in the air all around us, but we can’t see it because it’s in the form of water vapour. 
2. The air has a limit to how much water vapour it can hold, and hot air can hold more water vapour than cold air.
3. When there is excessive moisture in the air, or the air is too cold to hold the moisture, the tiny water molecules in water vapour stop floating freely in the air and start to cling to each other. In other words, they “condense”.
4. Enough condensation produces water drops. You can see them on the side of a glass of cold water when the air around the glass is cooled as it comes into contact with the glass. Outside, that condensation is known as rain. In your attic space, it’s known as roof condensation.

How Roof Condensation Happens

Now that we understand how condensation happens, let’s look at how conditions in your attic can make condensation issues a real problem, especially in winter months. Again, the facts speak for themselves.

1. The air in the living areas of your home can have a lot of moisture, especially from the bathroom and kitchen, where it’s produced by things like cooking and showering.
2. That warm, moist air can get into your attic, especially if you don’t have enough attic insulation and/or proper attic ventilation. The correct R-value of insulation for your home will prevent the moist air from getting into your attic and a proper attic ventilation system would get rid of it before it does any damage.
3. When the warm air from below contacts the cold underside of your roof deck, condensation happens, just like it does on the side of a glass.
4. The moisture build-up might not be an immediate problem. Very often, roof condensation that happens in winter will freeze until springtime rolls around.

Roof condensation can cause as much moisture damage throughout your home as if there was a leak in your roof. Our local weather conditions create the perfect environment for condensation in your home’s roof. That’s why it’s critical to have proper attic ventilation and attic insulation in Toronto homes. This is also why sometimes water dripping from above is the result of condensation rather than a roof leak.

If you enjoyed this post, check out our recent article “The Winter Tales Your Roof Can Tell”.