Air Exhaust vs Air intake: finding a balance

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.