Mold growth in the attic occurs in a wide variety of colors and growth patterns, some of which are often misdiagnosed. Fuzzy/three-dimensional mold growth is almost always identified correctly. When the growth pattern is very flat and looks like a staining, it can cause confusion.

Roof Leakage vs. Condensation – Determining the Cause of Attic Mold

Despite what your roofer would have you believe, roof leaks rarely cause significant mold problems. What for? Leaks on the roof are obvious. It’s pretty hard to miss the water flowing down your ceiling. For this reason, people usually address their roof leaks quickly – long before mold growth can occur. Also, roof leaks occur in an area (again, despite what your roofer will tell you). They do not flee en masse. Even if a roof leak is not detected long enough to cause mold growth, the problem will be located in a small area directly around the leak. 95% + of mold growth problems are due to condensation.

Why the confusion? Heavy condensation can look a lot like a roof leak

During cold periods, condensation can become so strong that moisture flows out of the coating. This can often trick inspectors and homeowners into thinking they have a roof leak. Unfortunately, this is a costly mistake. Once a roofer is called upon to investigate, you can rest assured that they will determine that you need a new roof…

Mold growth in attics

Below is an image of mold growth in a valley (where two roof sections meet). This often causes confusion, as these sections of the roof often leak. However, they are also areas of poor circulation and therefore condensation-based mold. The best way to determine the source is to examine the attic during a period of cool, dry weather. A roof leak will stop but the condensation will get worse.

What causes condensation in an attic?

Condensation occurs when the temperature of a surface of a material reaches the dew point. This means that the air is 100% saturated and can no longer contain water molecules. Once this happens, the water molecules begin to condense on anything that is less saturated. In an attic, this is usually your roof covering or frame. Usually, you cannot see the individual droplets unless there is an area of exposed metal or the outside temperature is particularly cold.

If condensation persists long enough, mold spores begin to grow. Many people think that mold growth is prevented by removing the mold spores themselves. It would be great if it were true. But mold spores are ubiquitous. Even if you managed to kill all the mold spores in an attic, ten thousand new spores would drift in a few hours and settle on your beautiful clean surface.

Determine the solution

That’s where it gets tricky. Because the majority of mold cases are caused by condensation, this means you’ll rarely find a steaming gun. Typically, condensation is caused by several mixing factors. There are two main factors that cause condensation.

Excessive moisture entering the attic of the attic space below.

Insufficient ventilation in this attic.

If one of these factors malfunctions, an attic could prevent mold growth by doing well in the other factor. For example, if a house has poor attic ventilation, but the ceiling is well airtight and the interior of the house has excellent ventilation, mold growth probably won’t occur. The opposite is true, but also to a lesser extent. Homes with excellent roof ventilation can overcome poor airtightness, but if indoor RH is high, mold growth will outweigh ventilation and mold growth will occur.

PROBLEM #1.) Excess moisture transport from the house to the attic

In most cases, the moisture that causes condensation on the roof covering comes from the activity of the occupants in the house below. Taking a shower, cooking, doing laundry and even breathing adds water vapor to the air. Due to the chimney effect (rise of hot air), this moisture slowly migrates to the attic. That’s why you rarely find mold growth in the attic above a garage or a rarely used part of the house.

Some of the movement of moisture occurs directly through building materials, such as rock sheets. This is why building codes often require a vapour barrier on the ceiling. In the past, this has been accomplished through tar on fiberglass mats. Many builders now use paint/primer with low permeability to act as a steam retardant. While this is a good idea, the vast majority of moisture transport is through the movement of air. This happens when air circulates through penetrations into your ceiling (box lights, fans, etc.) and top plates (holes for electrical wires). As air circulates through these holes in the attic, it also attracts moisture.

The result? The vast majority of condensation is due to the movement of air carrying water vapor rather than water vapor moving directly through the ceiling of the sheet rock. Here is a dramatic photo of this principle in action. The blue piping is unsealed in the house, allowing warm, moist air to move through the attic space. Once it hit the cold gable wall of the attic, the moisture immediately condensed on the surface, leading to mold growth. Although it is rarely so dramatic, this same principle is behind the majority of mold problems in the attic.

Air leakage around sky lights

This is another example of an area that commonly suffers from actual roof leaks (flashing around the skylight). And yet, because these skylights are often located in an empty space of air, air leaks in the attic can be serious. The problem is also exacerbated when skylights are located in a bathroom, which suffers from significantly higher humidity loads.

PROBLEM #2.) Poor ventilation of the entire attic

The need for ventilation in the attic is directly related to the amount of moisture coming from the occupied spaces below. If the house has an extremely well sealed ceiling/attic joint, very little moisture will enter the attic of the house. Minimal ventilation will be required to prevent condensation formation. Alternatively, if the ceiling is waterproof and full of unsealed pipe lights, penetrations, etc. the roof will need much more ventilation.

Internally disconcerted crest vents

These vents basically look like multiple layers of a corrugated cardboard box (although they are made of plastic). They have one thing to do for them – they don’t cost much. On the negative side – they do not seem to pass much air through them. As you can see in the photo below, the channels are very small. The airflow in an attic is very slow, so any obstacle will significantly reduce the movement of air.

Ventilation from the ridge to the confusing exterior

As the name suggests, external baffle ventilation relies on outdoor barriers to prevent water intrusion. This allows for much larger openings in the actual ventilation part of the ridge vent. These ridge vents are also larger than a typical internal chicane model. This also allows for good airflow.

Many models with external baffles are built with a plastic fin on the outer edge. This is designed to create an area of low pressure on the vent when the wind moves over the ridge. In the Pacific Northwest our wind is quite irregular, so this characteristic should not be invoked.

Yes, they can be clogged with needles, but I would still be on an externally clogged crest vent on an internally bewildered model.

Treat and ventilate the flat roof section would be impossible due to limited access. Therefore, we recommended reducing indoor humidity levels and waiting for the roof to be replaced. At this time, the ventilation must be completely removed, and the insulation must be installed directly on the roof covering. This is called a conditioned attic and will completely eliminate condensation and mold growth.