Air Exhaust vs Air intake: finding a balance

These past couple of years, we’ve all been exposed to the consequences of climate change. Natural disasters such as severe hailstorms, record breaking high temperatures and drastic winter cold fronts dropping temperatures in record time have all lead to higher expectations regarding building materials. Moreover, current building code legislations are evolving faster than ever, forcing the building material manufactures to keep innovating. Every year, the ever so temperature oscillations create an increasing amount of roof ventilation issues given the buildings existing system is lacking performance.

Attic venting ratios: What are the building code requirements ?

To properly understand the reason why certain roof ventilation systems fail we must first start to comprehend the basics behind the concept, starting with the building code requirements.

Canadian National building Code (NBC)

Below you will find section 9.19 of the NBC which clearly describes the building code requirements for the ventilation for both sloped and flat roofs. Articles and state that for sloped and flat roofs, the ventilation ratios to be provided to each independently are the following:

Sloped roof ventilation ratio: 1/300

Flat roof ventilation ratio: 1/150

These venting ratios suggest that for every 150ft2 or 300ft2 of insulated ceiling surface area giving into the attic, 1 ft2 of ventilation area shall be applied to the roof. In addition, following the total vented surface area calculation, the building code also requires a minimum of 25% of the total vented area to be provided in the lower and upper portion of the roof. This therefore means that by code requirement, for both sloped and flat roofs, an air intake source shall be provided in the lower portion of the roof as well as an exhaust source in the top portion.

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USA International building and residential codes (IBC & IRC)

Although we will be focusing on Canadian legislations in this article, USA IBC & IRC standards are slightly different than our current Canadian NBC recommendations.

Sections 1202.2 of the IBC and section R806 of the IRC both bring forward very similar recommendations in terms of roof ventilation ratios. As you can see below, from section R806.2 of the IRC, the general rule is to consider a ratio of 1/150 for any given roof (flat or sloped). Following this standard, a less restrictive venting ratio of 1/300 is allowable given the roof satisfies certain conditions as depicted below. To be noted, there are no distinctions between flat and sloped roofs. Please verify your local building codes as this recommendation may change according to local legislations.

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What are the keys to successful attic ventilation?

In both flat and sloped roof applications, the recipe to success is considerably similar; apply proper venting ratios and make sure you provide sufficient space for airflow.

Balancing the system

Introducing proper venting ratios to a roof ventilation system is easy to understand but not always easy to apply. For any type of system (active ventilation, passive ventilation, or mechanical ventilation) a healthy venting ratio is one where the attic space contains a minimum of 50% air intake and 50% exhaust. For more information on the different type of systems, please read: (link to passive vs active vs mechanical article). The ideal ratio, when possible, is 60/40 intake to exhaust. The principle behind this is logical, we want to have balanced flow between the amount of exhaust air and intake air such as to not choke the exhaust unit’s efficiency. An analogy to this concept would be to compare it to someone drinking a glass of water using a straw: the person drinking a glass of water using a straw won’t have any issues drinking the water until someone else decides to pinch the straw midway, cutting off the flow of water. The same can be said about a roof ventilation system having a properly designed exhaust installation but lacking considerably in air intake. The result will lead to the systems exhaust portion being choked off from its intake source and ultimately failing at its task. Moreover, in an active and mechanical roof ventilation system, a lack of fresh air intake will also lead to a depressurization of the attic space. This issue might cause the air inside the building to migrate towards the attic space to compensate the depressurization effect, thus increasing the risks of condensation and mold appearing on the roof deck and structure.

Allowing more space for airflow

The second ingredient to success is making sure the attic space includes a decent amount of open space for a healthy airflow to occur. Without getting too technical with the principles of fluid mechanics, the smaller the allowable venting space is in an attic the harder it is for air to move through it. The opposite can also be said, the larger the allowable venting space is in an attic, the easier it is for air to move through it. A small attic space makes air flow very difficult. The volumetric speed at which the air flows through the attic will be reduced since it will be impossible to move a large volume of air through a small opening. A direct consequence of slower air movement is the possibility that warm, moist air will easily stick to cold surfaces and create condensation. In addition, unless your current installation is composed of rigid insulation, mineral wool or blown insulation will impede air movement due to a non-uniform installation. This will further reduce airflow, thus increasing the risk of condensation and mold growth.

Restrained Attic Space Impeding Airflow

Open Attic Space Allowing Good Airflow

An open attic space makes the movement of air easy and without any restrictions. The air speed is greatly increased, and this also improves the attics hourly air exchange rate allowing the installed exhaust units to operate at their full efficiencies. In addition, by having better ventilation, the risk of condensation and mold creation is greatly reduced.

How to check your sloped roof attic ventilation system?

Now that we’ve gone through the code requirements and the two main keys to success, lets focus on the best ways to apply them to avoid having failing systems.

Let’s start by considering the recommended venting ratio given in the Canadian National Building Code. Very often, when homeowners or contractors are having condensation or mold issues with an active type system, the first impression they have regarding their system will be to assume the roof lacks exhaust. Very rarely do they have the reflex to address the existing fresh air intake condition before thinking about the exhaust. The verification can be done quickly and easily:

  1. First step is the calculate the ventilation surface area using the NBC requirements. Hence, we will want to apply the 1/300 ratio to the total surface area of insulated ceiling area giving into the attic. For example, let’s say we’re looking at a 1200ft2 home with roof condensation issues and we want to verify our current attic ventilation system. When applying the ratio, the result will give us 4ft2 (1200ft2 ÷ 300 = 4ft2) of ventilation area.
  2. Second is to consider a neutral air intake to exhaust system ratio. As mentioned above, a minimum balanced system is one where 50% of the total venting surface area is applied to both intake and exhaust. In the case of our example, this would mean we would have to apply 2ft2 of net free area (NFA) to both air intake and exhaust. Additionally, having a higher intake to exhaust ratio such as 60/40 is even better.
  3. Third step is to verify (or conceive in the case of a new construction) our system. The exhaust portion of the system is easy to check, simply determine the NFA rating of the given product installed on top of the roof and multiply it by the number of units. In the case of our example, having 2 units of model 301-12 from Ventilation Maximum (both having 1ft2 NFA each) will indicate sufficient exhaust in the system. The exhaust being verified, let us move our attention to the intake. Verifying the intake requires the following steps:
    1. First, roughly estimate the total surface area of open vented soffits on the home. Coming back to our example, the given home contains open vented soffits at the eaves of a 2-sloped roof. The soffit length on the home is equivalent to 35 feet and the depth is equivalent to 1 foot. Hence, the total soffit surface area is 35ft x 1ft = 35ft2 for one side, so total surface area is of 70ft2 for both sides.
    2. Second step is to calculate the total NFA rating of the soffit. Usually, this step requires us to have the specific efficiency of the installed soffit but if this is not available to you, simply use the standard market efficiency of 3% NFA. Back to our example, we will apply 3% efficiency to the total 70ft2 surface area, giving us 2.1ft2 NFA (70ft2 x 3% = 2.1ft2). Hence, this would confirm that we have the minimum amount of air intake NFA to supply a balanced system as a total of 2ft2 was calculated earlier.
  4. Once we’ve verified the NFA ratings to our roof ventilation system and that we’ve concluded the system is up to standards, the next step is to verify if the given roof has sufficient space to allow proper air movement. This step is often a little harder to verify as we will have to visually inspect the allowable space for air to move from the soffits into the roof. Following this, we will also verify the allowable open space in the roof structure allowing for proper ventilation. The NBC current standards speak of a minimum gap of 2in but we at Ventilation Maximum have concluded that 2in is often not enough given obstructions in the insulation installation or roof structure can impede airflow. Ventilation Maximum recommends having a minimum gap of 4in to allow proper ventilation in the roof. Hence, verifying that the vented soffits are not blocked by insulation or roof structure and that the air is able to move into a minimum air gap of 4in to the roof attic space will confirm that the flow is properly moving through the attic space.


Once the system verification is done and the issue still hasn’t been depicted, the last step is to verify that both the exhaust units and intake sources are installed in a uniform matter on the roof. Often, roofs containing possible dead zones lacking air movement (intake and/or exhaust) inside an attic having a properly functioning system, may cause or be subject to condensation issues. If this is the case, local modifications to that specific dead zone section will be necessary to improve airflow.
Finally, in most sloped roof applications the issue of having sufficient space for airflow is generally not of concern because the average sloped roof attic space contains enough height for an average person to stand up. To every general condition there are exceptions, here are the roof types to watch out for to which a possible lack of airflow space may occur: low sloped roofs (slopes ranging from 4/12 to 2/12), cathedral roofs and dormer sections on standard sloped roofs.

How to check your flat roof attic ventilation system?

Flat roof ventilation systems are slightly different than sloped roof systems and often more difficult to properly conceive. First, a flat roof doesn’t have the given contributing effect of having hot and humid air naturally rise along the roof deck as in a sloped roof application. This requires the system to be designed in such a way as to move air in a desired orientation, thus covering the entire roof surface area. More so, not all flat roof buildings contain or supply sufficient air intake to the given system.
To properly verify the system, the same steps as depicted in the sloped roof section above is to be used but this time by applying a total vented ratio of 1/150. Additionally, a minimum height clearance of 4in is once again recommended by Ventilation Maximum to allow proper airflow. Depending on the size of the building and attic to ventilate, 6in or more can further reduce the risks of mold and condensation creation by allowing the airflow to move with more ease.
Once done, if the verifications do not depict any problematic features, the impact of other existing conditions such as the lack of a vapour barrier or building imperfections in the ceiling composition may contribute to the given issue. For more tips on what to look out for, please refer to: link to ‘’avoiding condensation problems in your attic’’.

To conclude, every homeowner or contractor witnessing a failing attic ventilation system should verify the buildings attic ventilation ratio, including both intake and exhaust balanced NFA ratings, as well as the given allowable space for airflow to occur before looking further into identifying the problematic source. Feel free to contact us for more information on attic ventilation.

Author : Jonathan Ramsay Massicotte, P.eng
Technical and development Director at Ventilation Maximum