How to Vent an Attic to Prevent Mold Growth
Venting an attic is simple, right?
Sounds easy – follow the code calculations, install a few RVOs or a ridge vent and make sure you’ve got decent soffit vents. Yet many attics with ventilation far superior to the code requirements suffer from mold issues. And many homes with terrible attic ventilation are completely mold free.
Nope – venting an attic is actually pretty hard
Here’s how ventilation is supposed to work
Attic ventilation is a two part process. First, enters through the soffit vents. These are often round holes with wire mesh called ‘bird blocks’. Other common soffit vent styles include the perforated panels or slots on underside of the soffit. Technically the air is drawn in through the soffits via suction, rather than entering in on its own accord.
Once the air enters the soffits, it moves along the underside of the roof sheathing. As it flows across the sheathing, moisture is pulled away along with the air movement. This prevents condensation (and mold growth) from occurring. Eventually the airflow reaches the top and exits through the upper roof / ridge vents.
On breezy days, the process is aided by the wind. However, the wind is not reliable enough and therefore the process depends on the upward movement of warm air. As you recall from physics class, warm air rises. When a home is heated, a significant portion of that heat escapes upward into the attic. As the air in the attic warms, it begins to rise upward. When an attic is working correctly, this rising air finds an opening in the roof (either a ridge vent or RVO) and exhaust to the outside. As air moves out the upper roof vents a mild suction is created, which pulls air in through the soffits.
These air movements and pressure differentials are very slow. Imperceptibly slow. Even a minor impediment can stop the process completely. A bit of framing in the way is enough to stop the process. This is when mold growth occurs. The moisture in the air lingers a bit and the roof sheathing begins absorbing the moisture.
NOTE: Technically, we’d much prefer to see all attics converted to conditioned spaces and the ventilation completely removed. But that’s not likely to at a large scale any time soon. So for now, we’re stuck with attempting to improve a fundamentally flawed approach.
First, the obvious attic ventilation mistakes
Soffits blocked by insulation batts
Ventilating an attic properly can be very tricky. But some problems are simple. Like stuffing the soffit vents full of insulation… Unless a roof has a very steep pitch, the fiberglass batts must be tapered down as they reach the soffit venting. The venting problem can also be solved by pulling the batts back from the edge, but this leaves a dead spot which can lead to mold on the sheet rock corner in the room below.
Baffles installed in soffits, but crushed by insulation.
Insulation baffles work great. When they’re installed correctly… Baffles are typically made out of flimsy cardboard. This is more than sufficient to keep blown-in insulation away from the soffits. But they are not nearly strong enough to withstand the expansion force of a compressed fiberglass batt. In many of these situations, the batts looked great when they were first installed. However, in the days and weeks after the project was completed, the force of the insulation eventually crumpled the batts and blocked the soffits.
Poor roof designs
These are very challenging. This style of roof severely limits the available space to install upper roof venting. Because the roof is essentially a pyramid, the ratio of upper roof space to lower roof space is very small, leading to wildly disproportionate ventilation.
It gets worse. Hips are built with a tall rafter running along the length of the ridge/hip. This framing is usually quite stout (i.e. tall) and therefore it impedes the movement of air up the roof line. You can have perfect soffit venting, but it won’t accomplish much because the air has nowhere to go.
Though they’re not cheap, solutions do exist for hip roofs. Roof valleys are even trickier. A “valley” occurs where two roof lines intersect. This is the inverse of the issue found in a hip roof. Because the valley collects water and directs it down the roof, no ventilation in the immediate vicinity is possible. Solutions exist, but they are imperfect and not aesthetically pleasing.
Mechanical ventilation options
Sometimes passive ventilation is insufficient or impractical. Low angle roofs, for example, struggle to produce sufficient air flow and often rely on mechanical assistance. Heavily gabled roofs, with a wide footprint and minimal ridge area, also suffer from poor passive ventilation and may require an active system. In a commercial setting, mechanical ventilation is often achieved through wind powered turbine vents. These work especially well in areas with consistent wind. However, they are unsightly for residential applications and most homeowners elect to install electrically powered roof vents instead. These are mounted on either the upper portion of the roof or on a gable end.
Remember, a powered roof vent still requires sufficient intake air if it has any hope of properly venting the attic space. In fact, if proper intake air is not provided, the fan will pull warm, humid air from the home rather than from the soffit vent. This would only compound the condensation and mold problems.
Perfect ventilation, but still mold growth? Yep.
Often attics perform poorly because someone made a clear mistake. The venting was installed incorrectly, the quantity was insufficient, the insulation is blocking the vents, etc. These problems are very common. But there are also many attics that are genuinely hard to vent right. Look at the photo below. Despite perfect soffit ventilation, mold growth is covering the roof sheathing.
In the image below, mold growth is occurring right next to a well installed ridge vent. The photo at the very top even shows condensation occurring directly on the ridge vent. Why is mold growing on well ventilated attics?
Project Report > Inspection to Refit Soffits
Work #: 106033
Inspection of attic revealed moderate mold growth throughout significant areas of attic sheathing. At this time, the mold growth has not lead to significant structural damage of the underlying materials.
Elevated moisture content was noted in few areas of gable sheathing where insulation has contacted sheathing. The affected areas are extremely small and should dry without any special equipment or techniques.
Insufficient ridge area ventilation installed throughout attic. This limits the exhaustion of humid air through the roof assembly. The current ridge ventilation is partially obstructed by a thick mesh that should be removed to promote better air flow.
Insufficient soffit ventilation throughout the attic. The existing soffit ventilation is not consistent (some bays are open, and some bays have a single 3” diameter hole). The soffit venting is also severely restricted by a perforated vinyl covering.
The property is heated primarily by electric wall heaters or radiant hydronic heating.
Homes that are not equipped with a forced air heating system must have special care taken to ensure appropriate ventilation, and prevent the buildup of humidity and other pollutants.