Attic Ventilation


Attics are strange place. Here is this sometimes very large space just above our heads, typically dry and protected from the elements, that spends its existence apparantly doing nothing. However, your attic is actually doing a lot more than nothing, there is a unique environment in your attic that needs to work properly to protect your home.

To explain attic ventilation, we need to first build up some of the known facts about our attic space:

Why do I have an attic?

Roofs come in two common configurations, flat and sloped. Sloped roofs make best use of gravity to assist your home in shedding rain and snow water. In addition, the triangle created by the roof peak is an extremely strong shape for home designers to use to create a strong structure capable of carrying snow loads, wind loads, and literally keep the roof over your head. This triangle shape creates an attic space. Flat roofs are less common for residential wood homes, largely for curb appeal reasons, as engineers can create very strong flat roof systems also (just look at the typical Costco warehouse roof for example).

Attic Air Flow

High school science fact: Warm air rises and cool air falls. This process when active is called a convection current. If you have ever had curtains above an electric baseboard heater and noticed them moving when the windows were closed, that is the force of the warm air circulating in the room.

In winter, the outside air may be very cold but the house is warm. The causes the air in the attic to circulate moving the warm air that higher in the attic. In summer, the sun and air temperature will create very high heat in the attic space which can radiate into the living space in the home. Ventilating the attic with cooler outside air can make it more comfortable in the interior of the home.

The Role of Attic Insulation

As we want to keep heat in the attic from transferring into the living space below in the summer, and keep as much heat in the living space from escaping into the attic in the winter, builders put a layer of insulation on the floor of the attic to slow the transfer rate of this heat. More insulation means less heat transfer and the lower the heating bills in the winter and the more comfortable the interior in the summer.

Finally, Ventilating the Attic

Ventilating the attic is critical to protecting the homes structure and to improving occupant comfort in the home. Here are a few reasons why:

  • Managing Condensation and Humidity - No matter how hard we try to seal the attic from the home, some warm, moist air from our home will enter the attic through leaks or osmosis. Because cold air can’t hold water vapour as well as warm air, this moisture will ‘condense’ on cold surfaces in the attic like the under side of the sheeting. When water, wood, and warm air exist, mold and rot can grow and cause structural damages. Nobody wants to discover mold/rot growing in their attic.
  • Protecting Roofing Materials - Summer heat and direct sunlight can cause our attic space to become extremely hot. Some roofing materials like asphalt shingles will actually shorten their life span if allowed to overheat.
  • Preventing Ice Dams - In cold weather climate, warm air in the attic can melt snow on the roof which re-freezes when it reaches cold air at the roof edges. The build-up of re-freezing water at the roof edges can create a ‘dam’ which allows water to flow back into the homes structure. 
  • Improving Occupant Comfort - High heat levels in attic spaces will radiate heat back to occupants in the home and as temperatures in the attic may remain high long into the night, it can make for a hot and uncomfortable sleep for top floor occupants, particularly children are susceptible to this effect as their bodies are not as good at regulating temperatures as adult bodies are.

In order to flush out hot or moist air from attic spaces, building designers take advantage of the natural convection currents to create air flow through the attic. This is done by allowing cooler air to enter at the lower part of the attic (typically the soffits) and to exhaust from a higher point in the attic (roof or ridge vents). This purposeful air flow creates desired attic ventilation.

Challenges for Attic Ventilation

Disconnected exhaust vents in the attic can overwhelm the attic ventilation ability.

Getting attic air flow and right is a very difficult science. There are many things that can prevent good attic ventilation:

  • ‘Dead Spots’ - Dead spots can occur in attic ventilation when structural members or other attic penetrations like chimneys block the path of air flow. This can also happen if high vents are not high enough or low vents are not low enough. Air trapped in these dead spots can cause localized water or heat damages.
  • Short-Circuits - Air will flow following the path of least resistance. In some cases, two roof vents installed opposite each other will cause air to go through the top of the roof only short circuiting the convection currents from lower in the roof.
  • Blocked Vents - Insulation blocking the soffit vents from allowing air to flow from the bottom of the attic is a common cause of poor attic ventilation.
  • Exhaust Vents - Sometimes even a well ventilated attic can be overwhelmed by an incorrectly connected bathroom/kitchen exhaust pipe in the attic or in some cases the air flow in the attic may ’suck’ exhaust air from these vents placed too close to the soffit grills outside the home.

What do Home Inspectors Look for with Attic Ventilation

Black organic growth on plywood sheeting is a common sign of condensation in the attic and may lead to structure problems.

Home inspectors are at a disadvantage for attic ventilation as a single observation tells us very little about the performance of a system, however, there are clues when an attic is not ventilated properly.

  1. Are there suitable attic vents present? - Home inspectors should be looking for both high and low sources of air flow for correct ventilation.
  2. Are vents blocked? Inspectors should be able to see light spilling into the attic or correct air baffles in place to help direct air into the attic space.
  3. Are there signs of moisture/condensation in the attic? Rusty nails or organic growth in the attic will typically indicate poor ventilation performance. Nearly all attics have some ventilation dead spots, so it is important for inspectors to interpret the severity of a problem.
  4. Are roof materials stressed? Do the asphalt shingles appear to have lost more material than normal for their age and do they look heat stressed.

Home inspectors who discover these symptoms should report the concern to their clients and advise further review by a roofer or insulation expert depending on the nature of the problem.

Final Thoughts

Correct ventilation of attic spaces is a complex process but important to the protection of the home and occupant comfort. If you are unsure of the condition of your attic, consider asking a local home inspector to take a look for you and advise you of any concerns.

By James Bell - Owner Solid State Inspections Inc.