Why Do Attics Need to Be Vented?

Attics need to be vented for many reasons, including to prevent rot on the interior attic side of the roofing plywood and ice dams on your new roof replacement. In your attic, amongst the forgotten boxes, there hides a lot of science. You may not think your attic space has much to do with your roof’s health, but you’d be wrong.

Let’s look at the science behind why attics need to be vented.

The science of air intake and exhaust.

During the winter, your house roof should hold snow equally from top to bottom. If it does, it means it is properly ventilated, insulated, and (relatively) airtight. Cool air is drawn into your attic through your overhang vents located horizontally along the underside of your house eaves where the intake of the cool air exhausts and helps to displace the warmer attic air up and out through the ridge vents.


Attics need to be vented, air-tight and insulated.

If your attic is appropriately air-tight, insulated, and well-vented, it will prevent ice dams, wood rot, and moisture (that eventually would cause shingle blow-off) from building in the attic. Achieving this helps maintain a stable temperature, promoting proper new roof construction and prolonging the roof’s health. 


Air tightness is necessary:

Although common, heat ducting within an attic spells disaster for the air tightness of your attic. Ducts will never be airtight (20% leakage is common) and cause negative pressure, circulating dangerous gases into your home. Your attic should be free of all things except high R-value insulation. All ducting should ideally be within the ceiling.

An airtight ceiling with no penetrations is the best scenario for energy savings, lack of ice damming, and moisture in an attic. Unfortunately, when we renovate (or with new builds), we penetrate our attic-related ceilings relentlessly with all sorts of holes, pot lights and various ventilation channels that are never made airtight. 


High R-Value Insulation:

The sun will heat the house’s walls, and the hot air climbs the walls to heat the airspace under the overhang. The darker the roof, the hotter it is, so on a warm winter day, it creates the perfect conditions for freeze and thaw, which can result in ice dams. So in areas with a lot of snow (mountainous), it is better to insulate the overhang area and locate the air intake at the hidden top point of the long front edge of the facia. 

To a lesser degree, this happens during our Canadian winters. Very high R-Value insulation with ample venting helps.



During winter, moist warm air from the living levels of a home escape into the attic via ceiling openings. It condenses when it hits the cool surface of the roof’s underlayers. The moisture can also be held in fluffy insulation. This can lead to mould and rot.

The warmed interior of the attic melts the external snow layer contacting the roof. It then runs down the roof and refreezes at the coolest lowest level of the roof, at the eave. At this lowest roof point, the ice-melt layer will freeze and thaw. This promotes the degradation of the shingle glue, allowing water to trickle under the shingles. The rot of the plywood sheathing results, and water reenter the attic space. 


Historically, roofs were unvented; why do attics need to be vented now?

In the past, airtight ceilings were the norm. There were no pot lights, ducting holes, or penetrations of ceilings into the attic. Attics were always airtight. Currently, we cut holes into the ceilings all the time, so we must find solutions to exhaust the home’s warm interior air that escapes into the attic. 

Attic airtightness can be improved by carefully sealing all ceiling openings. It requires extra diligence and much more time on the contractor’s behalf and will cost you more money, but in the end, it will save on roof repairs and help your energy efficiency. You’ll have to request that this extra work be done specifically.


Do ridge vents stop working under a heavy layer of snow?

Snow is not an air barrier; it is only an air retardant. Air will still move through the ridge vent covered with snow more slowly.


How can you determine where a roof water leak is coming from?

With so many unviewable points of possible failure, it is always detective work to determine the location and cause of an interior water leak. During the winter, all ice-melt points occur under a layer of snow, making it difficult to see it happening in real-time. It is a good idea to ask Forest City Roofing to come by during a warm day (a thawed day). One of our team will look inside your attic when it may be easier to see water movement.


Since 1993, Forest City Roofing has been a trusted roofing company for London, Ontario and the surrounding areas. Contact our professional team if you have any roof concerns. Call Mon – Fri 9 – 5 at (519) 659 6937.


November 2, 2022

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