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How to Vent an Attic to Prevent Mold Growth

Attic Ventilation & 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.

Preventing mold growth in an attic is highly dependent upon consistent ventilation.  Attic ventilation is a two part process.  Air is drawn in through the soffit vents, carried along the underside of the roof sheathing, and exits through the upper roof / ridge vents.  This process depends on the upward movement of warm air.  When a home is heated, a significant portion of that heat moves upward into the attic.  This heat transfer, whether through convection or conduction, warms the air in the attic.

As we all recall from physics class, warm air rises.  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.  The amount of air leaving the attic must be replaced by an equal quantity of air from somewhere else.   This creates a mild suction effect which in turn, pulls air through the soffits.  Wind along the upper roof can increase this effect by pulling the air out of the attic more quickly.

After the air is pulled through the soffits, it moves along the underside of the roof sheathing, which keeps moisture from accumulating on the plywood/OSB.  These air movements and pressure differentials are very slow.

Mold growth occurs when the moisture in the air lingers a bit and the roof sheathing begins absorbing the moisture.   Anything that impedes this intake of fresh air will dramatically increase the likelihood of mold growth in an attic.

NOTE:  Actually, 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.

Why Venting an Attic Can Be Challenging

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.

Hip Roofs

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.

Venting an attic with hip roofs.

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.

Hip roof with mold growth

Notice how the hip framing creates a dead end for ventilation path

Roof Valleys

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.

No ventilation possible - causing mold growth

Good Attic Venting Gone Awry

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.

Soffit blocked by fiberglass insulation

Soffit blocked by fiberglass insulation

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.

soffit vent blocked by insulation 2

Soffit vent with baffling blocked by expanding insulation.

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.

Attic Fan

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.

Venting an Attic For Mold Prevention – The Right Way




Project Report > Inspection to Refit Soffits

Work #: 106033

Insulation Blocking Soffit Vent

Insulation Blocking Soffit Vent

Mold Growth:

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 Ventilation

Insufficient Ridge Ventilation

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.

Indoor Ventilation:

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.

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