Attic Ventilation: The Truth, The Myths, And More

Attic ventilation is one of the most published roofing topics on the internet today. There are countless articles and blogs available for homeowners from various resources. While many of these articles have similar themes, I have noticed some varying opinions on the importance and benefits of ventilation. I have also noticed that ventilation is sometimes a scapegoat for a deeper, more underlying problem. In today’s article, I want to cover what I feel are the truths about attic ventilation regarding its importance and benefits. I also want to mention where I see ventilation (specifically, the lack of) as being unfairly blamed for issues and deficiencies that have deeper rooted problems. I will also discuss some common products used for attic ventilation.
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What Is Attic Ventilation?
In simplest terms, attic ventilation is a process of creating air movement and circulation in the attic space. While there are different product options available, The process is typically done with a source of intake air (fresh air normally drawn from the soffit or near the eaves) and a source of exhaust (typically near or at the roof’s peak)

Why Is Attic Ventilation Important?
Ventilation is important for two (2) main reasons:

Airflow – It is important that flows and circulates through the attic. Without air flow, attic air will become stagnant and be much more susceptible condensing. Condensation in the attic is known to potentially lead to issues with mould. The process of airflow is sometimes colloquially referred to as “letting the roof breathe”
Air Temperature – Having an appropriate venting system will help keep the air temperature in the attic closer to the air temperature outside. A difference between attic and outside temperatures, specifically in the winter, is a major reason for issues such as ice dams and condensation. In the summer, appropriate venting will help prevent the attic space from superheating (a process where hot air in the attic is not exhausting quick enough. Subsequently, hot air from the next day will start an accumulation) Super heated attics can make the upstairs of your home uncomfortable. It can, in some instances, also cause shingles to degrade quicker.

Attic Ventilation

The diagram above illustrates an appropriate attic ventilation arrangement. There is fresh air coming in at the soffits (intake venting) while the warmer air is exhausting closer to the peak (exhaust venting). This arrangement accomplishes both ample air flow to help reduce or prevent condensation while providing a constant flow of fresh intake air, which will keep the attic temperature consistent with the outdoor temperature.

How Do I Know If My Venting Is Correct?

According to OBC (Ontario Building Code) Standards, proper attic ventilation requires two main components:

A balanced venting system
An adequate amount of air flow

A balanced venting system – where an equal amount of air comes in at the soffit region and an equal amount of air exhausts near the roof peak. In theory, balancing the venting system is important in order to maintain appropriate air pressure in the attic space.

Too much intake and lack of exhaust can result in high pressure. In this scenario, the attic is susceptible to summertime superheating, which can make the upstairs space uncomfortable. It can also compromise some warranties with some shingle manufacturers.

Too much exhaust and a lack of intake venting can cause the attic to depressurize. The result is often air from your home will be pulled into the attic. While I have read other resources that claim ice dams and condensation can result from this, my experience has been that an increase in energy costs is likely the worse case scenario (I’ll touch on ice dam and condensation issues a little later)

Of these 2 scenarios, the first is much easier to fix. However, according to most shingle manufacturers, the second scenario is most important for peak performance from shingles. Most homes built in the last 30 years or so have adequate venting, both in a balanced sense and from a volume perspective.

An adequate amount of air flow – In addition to a balanced venting system, having a minimum amount of air flowing in and out of the attic is also important. OBC Standards and most shingle manufacturers define the adequate amount as 1 sq ft of net free venting per 300 sq ft of attic space.

Which Products Are Best For Venting The Attic Space?
There are a number of products available for venting your attic space. When it comes to choosing a venting product, I like to keep it simple and follow a few basic rules:

Try to avoid products with mechanical parts. These types of products often require maintenance or replacement before the lifespan of the roof is complete, which can increase the overall cost of the roof life cycle. Furthermore, many of these products are very powerful and capable of upsetting the balance of the venting system. I prefer to stick with passive or static venting products.

Do not mix and match different venting products. Venting products come in a variety of styles, designs and performance levels. Whichever product you use, it’s important that the exhaust vents all exhaust at the same rate. If you have a passive vent beside a power vent, the power vent will do the vast majority of the exhausting. In fact, the power vent does such a good job, it will take all the air pressure from the passive vents, and turn them into intake vents. This is commonly referred to as “short circuiting” the venting system. The net result is the overall air circulation is less than it could be if the exhaust vents were all the same product, exhausting at the same rate.

Ridge vents should only be used on the right roof. Ridge vents have a couple of attractive features in that they have a low profile (some people would rather not look at vents on their roof) and they are exhausting at the highest possible point. In theory, the higher the exhaust vents, the better. In order for a ridge vent to be effective, the roof must be designed with a ridge (in roofing, the apex where 2 planes converge) that covers the entire roof assembly. Any dormers or jog outs that are below the ridge are not to be vented unless they are within a couple feet of the height of the main ridge. If lower dormers or jog outs are vented, the entire venting system will be short- circuited. I will also add that I recommend a roof pitch of at least 5/12 before a ridge vent is installed. I have scene lower roof pitches with ridge vents become problematic in the winter when there has been a heavy snow load.

Baffles or Rafter Vents are an ideal choice for improving intake venting. Baffles or rafter vents are a styrofoam chute designed to allow air flow into an attic space when the airflow at the soffits have been obstructed. They are a proven and cost effective solution for improving air intake in many homes. It is important to note that some older homes are constructed in a way that does not allow air flow into the attic space. In these instances, baffles alone will not help. An opening through brick or framing plates may be required in order to achieve ample intake air.
The Myths
Referring to some of the theory on roof ventilation as mythical maybe a bit strong and will likely open the door for debate with some. I would, however, like to share my experiences regarding ventilation (particularly the lack of it) in an unbiased and hopefully beneficial way.

Poor ventilation does not cause ice dams. At best, venting can mitigate this issue, but it cannot solve ice dam issues on its own. Ice dams occur when the attic temperature is above freezing while the roof temperature is below freezing. The warmer attic air will melt the ice and snow on the roof by warming the underside of the deck. The melted ice and snow then run to the cooler soffit overhang, where it re-freezes. If the conditions are right (right mix of snow and temperature) the ice will accumulate at the eaves to a point where it creates a level plane to the roof. Water is then directed back towards the shingles, where it can cause leaks. The theory with venting is that warmer air will exhaust and be replenished with cooler air before it can melt the snow. The reality is venting is not the root of the problem. The root of the problem is warm air entering the attic. This is best prevented with attic insulation. If you have issues with ice dams on a regular basis, an insulation top up should be your priority. Ensuring you have both adequate insulation and ventilation is your best defence against ice dams.

Better Ventilation will fix my mouldy attic. For many of the same reasons, improving attic ventilation will not fix mould or condensation in the attic either. The root of the problem is warm/moist air entering the attic space and mixing with cold air. Hot air meeting cold air creates condensation. The first step in fixing this should be to improve insulation levels and fix any air leaks into the attic. Once the source of warm moist air has been stopped, the condensation issues usually go away.

My roof failed before the full warranty term. It must be a ventilation problem. I will tread lightly here, as adequate ventilation is an important part of most shingle warranties. While providing estimates for customers, I often hear the disappointment that their so called 25 year shingle failed in 12-15 years. Homeowners who start to do some research may conclude poor ventilation was the culprit. This is very seldom the case. The reason is usually more to do with the quality of asphalt and stabilizers that were used in the asphalt. Beyond that, UV exposure also plays a big role. That’s why you will notice the West and South sides wearing quicker than the North and East sides (More UV ray exposure in the West and Southern skies.)

In closing, attic ventilation is an important part of the entire roof system. Following best practices as set by shingle manufacturers and OBC Standards should be a priority for all roofing contractors. With this in mind, I find simple, proven venting systems work the best over the long term. I also think it’s important to note that ventilation, while important, is not the answer for larger, more underlying issues such as ice dams or condensation. Fixing these issues usually requires a little more digging.

For more information on venting, roofing or insulation, contact Logik Roofing today.

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