Ventilation and mold
Attic ventilation, once thought to be boring and well understood, has recently received
a much needed reassessment. The need for new techniques and even an entirely new
way to think about attics has come from two primary sources: Energy efficiency concerns
& mold growth problems. Thankfully, in an attic, these issues work hand in hand;
solving one problem will often address the other.
Attic ventilation attempts to accomplish issues.
- Reduce condensation (and mold) on the underside of roof sheathing in the winter
months.
- Reduce the solar heat gain within the home during the summer months.
- Provide an avenue for drying out unexpected water intrusions, roof leaks, etc.
The first is important in cold climates, where ice damming and condensation can
occur if the sheathing temperatures reach the dew point. The second is critical
in hot climates, where extreme attic temperatures heat can radiate into the conditioned
space and increase the load on the A/C.
Active attic ventilation is a fairly new invention, necessary only after the advent
of tighter construction techniques and lower quality materials. The former occurred
when houses converted from skip sheeting and cedar shake to plywood and composition
roofs. Skip sheeting with shakes allowed a significant amount of heat and moisture
to escape without the need for specific ventilation. This is why in older, turn
of the century homes, you'll very rarely find mold problems in the attic. Compounding
the problem in recent decades was the move from real wood to manufactured materials
such as OSB and plywood. These materials have a much lower resistance to mold growth.
The worst problems typically occur in homes built in the 60's, 70's and 80's. Houses
built in this era often contain manufactured wood materials, composition shingles
and poor airflow management. Additionally, these homes tend to have no air sealing
on the ceiling, allowing a large amount of warm, damp air to enter the attic space.
The need for attic ventilation varies greatly from one home to another. I've observed
a number of homes with the exact same layout, roof structure and ventilation, yet
one is covered and mold and the other is spotless. Though frustrating at first,
after a bit of digging, the compounding factors can often be identified. Perhaps
the most common variable is the number of penetrations in the ceiling. Items like
can lights, access hatches, etc. provide a very efficient route for warm, humid
indoor air to migrate into the attic space and can have a dramatic effect on condensation
and mold growth. Unseen penetrations exist as well; everywhere an electrical wire
penetrates the top plate of a wall, heat and humidity can escape into the attic.
How does attic ventilation work?
Most residential home rely on passive ventilation to encourage air flow. Passive
ventilation requires intake (typically soffit ventilation) and exhaust (ridge vent
or roof jacks). Due to the stack effect, as warm air rises, fresh air is drawn in
through the soffit vents and out through the ridge area. Occasionally, houses rely
on gable end vents to achieve passive ventilation. Problems can occur if any of
the ventilation components are not working properly.
Common issues include:
- Soffit vents blocked by insulation
- Gable end vents competing for airflow
- Missing ridge are vents
- Ridge vents with insufficient airflow (caused by poor design)
- Ridge vents with insufficient cutback of roof sheathing.
- Hip roofs with limited ridge area.
- Low angle roof with insufficient space between the top of the insulation and the
roof sheathing.
Air Sealing - the surprising solution.
Though attic ventilation is important, the latest research has identified an equal,
if not superior solution. Air sealing. The concept is very simple. We can either
attempt to remove the heat and humidity from the attic after it arrives, or we can
prevent it from entering in the first place. This is accomplished through air sealing
the top plates, can lights, access hatches, electrical penetrations, etc. In addition
to greatly reducing the chance of condensation and mold growth, a home will typically
see major energy efficiency gains. Air sealing a home is relatively inexpensive,
often cheaper than increasing ventilation or adding a powered roof vent.
When is mechanical ventilation necessary?
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.
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