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.
Why do attics have ventilation?
Attic ventilation attempts to accomplish several key goals.
- 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.
When is a professional mold inspection necessary? If visible mold growth is present on the underside of the roof sheathing, contact a mold professional. However, many mold inspectors do not understand attic ventilation well. Before hiring, make sure they understand the dynamics of attic mold, moisture, stack effect, etc. If they are simply coming out to collect a few measurements and identify the type of mold, you’re wasting your money.