The difference between Active, Passive and Mechanical roof ventilators
As we’ve previously established in past articles, attic spaces need to be properly vented to avoid several undesirable outcomes. If you haven’t had the chance to read our previous article on attic ventilation, take a few moments to familiarize yourself with the concept.
The roofing industry is currently filled with several different products regarding attic ventilation but how does each product work, what are their pros and cons and ultimately, what is the best long-term investment for your roof?
The first category of available vents and probably the most common in North America are passive vents. What is a passive vent?
Building codes across North America refer to attic ventilation in terms of net free area (NFA) of ventilation to apply to a given roof in regards to the size of its insulated ceiling surface area. The bigger your home, the more openings (NFA) you’ll need to properly ventilate your roof. Applying passive ventilation to a roof implies that the system simply fulfills the national building code requirements in terms of NFA and doesn’t offer any additional venting value. We will see what these extra venting values are later.
Passive type ventilators are vents that offer a specified amount of NFA to allow heat and moisture to naturally escape the attic. The system relies on directional winds (winds coming in cross-sectional directions to the soffits) to enter the soffits and create a positive pressure in the attic, pushing the natural rising heat and moisture upwards to be evacuated. They can be installed on roofs as well as on walls . Here are examples of passive type products:
Shingle or Turtle Vents
The second category of vents are mechanical vents. These types of vents are composed of mechanical moving parts and require electricity to function, either by direct connection or solar energy. The ventilation system functions in the same manner as passive or active type vents except that the homeowner has a fan pulling air out of the attic space at a given volume (CFM). Fresh air comes in from the soffits and is drawn out by the fans, cooling off the attic space. The ventilators can be set to run on a timer, temperature sensor and simply on constant evacuation 24h/7.
As previously mentioned, building codes across the USA and Canada tend to recommend a 1/300 venting ratio for sloped roofs and 1/150 for flat roofs (please refer to your local building codes). But how do you properly determine the number of mechanical vents necessary to ventilate your home’s attic space according to NFA requirements? In Canada, the CSA A93:19 norm offers an easy conversion from mechanical volume flow rate to NFA rating. The conversion is as follows:
This allows us to convert a given volume (CFM) into an NFA rating for mechanical vents and hence apply the same basic calculation to obtain the required NFA.
Mechanical ventilators available on the market vary from direct connection to solar-powered energy and from roof to wall installation options. Different types of fans and control options are also available.
The last category of available ventilators on the market is active type vents. Active type vents are roof ventilators that create a given volume flow rate of evacuation by utilizing average wind speeds. The vents can either be composed of moving or non-moving parts and require no energy to function. Active ventilators are essentially the middle-class division combining both NFA requirements with an additional venting value of volume flow rate. How do they work? Here are the two types of active vents:
- The tower or ‘’Maxi’’ vent: uses the Bernoulli principle to create a pressure differential between the ventilator and the attic space. This pressure differential combined with buoyancy ventilation, otherwise known as the chimney effect, allows this ventilator to exhaust air from inside the attic space at a variable CFM given local average wind speeds. Higher wind speeds flowing through the vent will result in a higher exhaust flow rate (CFM) from the attic.
- The turbine vent: uses wind to make its turbine turn and hence exhausts air from the attic in this matter.
Active vents currently fall under the passive type category according to current Canadian building codes. Hence, the calculation to determine the number of vents installed on a roof is done in the same manner as the passive ventilators. On the other hand, some manufacturers will offer a different, less restrictive, way to calculate the number of ventilators to install on a roof knowing that these types of vents are more efficient than the code requirements. Please note that these recommendations do not follow current building code requirements.
Pros and Cons: Active vs Passive vs Mechanical
Now that we’ve gone through the foundations of each category, how do these different roof ventilators compare to one another? It is important to specify that for the purpose of this exercise we will consider a northern climate (4 seasons) with snow precipitations.
Pros and Cons Passive Ventilation
Passive ventilators, in a general manner and excluding the gooseneck, are aesthetically appealing to the average consumer. Homeowners tend to like their low profile and discrete appearance, generally blending in very well with the roof or walls. On another note, these units usually offer very small NFA ratings and require large amounts of vents to be installed on a roof to satisfy code requirements. Additionally, their aesthetically appealing concept is also the main reason why these vents have issues through winter, especially when installed on low/medium sloped roofs. Snow rapidly covers them and heat from the attic space melts the surrounding snow to create a block of ice, condemning the exhaust outlet. At this point, any attic space equipped with roof installed passive type vents become unvented and condensation issues, as well as water infiltrations originating from the unit, are bound to occur.
Additionally, it is important to keep in mind that these types of ventilators require directional winds to function properly . If your local winds are not able to push enough fresh air into the soffits, air movement in the attic will be slow and non-uniform, possibly creating dead zone pockets resulting in condensation issues. Also, plastic manufactured passive vents have been known to have issues with rodent intrusions.
Pros and Cons Mechanical Ventilation
Mechanical ventilators on the other hand are known to be less aesthetically pleasing than passive vents but offer higher efficiency ratings as they steadily draw out a given amount of volume (CFM) from the attic. Having a continuous rate of evacuation allows the attic to stay cool and dry during winter while also offering possible energy bill savings during the summer. The downside to these ventilators is that they are mechanical and moving parts are always bound to fail. Unfortunately, having the fan installed on the roof often means a homeowner won’t realize possible malfunctions until it is too late, as it only takes a few hours for condensation to build-up in an attic space.
An important point to consider when installing these ventilators is to make sure your home has sufficient open air intake to compensate for the volume being evacuated. Not having the right amount of fresh air intake will lead to the creation of a negative pressure in the attic, consequently drawing hot and humid air from inside the home into the attic which may lead to condensation issues. Also, a disadvantage about solar-powered fans is that they do not work at night. The nocturnal period is the most important in attic ventilation as it allows the roof to cool down within a few degrees of the exterior temperature. Lastly, the high efficiency of these ventilators may allow possible energy bills savings during summertime but on the other hand will also cost you energy to operate on a 24h/7 basis.
Pros and Cons Active Ventilation
Active ventilators seem to have a mixed opinion on an aesthetic basis as they stand up above the ridgeline, utilizing the wind to operate. The ‘’maxi’’ vent does not have any moving parts and offers continuous venting year-round as it stands high above the possible snow accumulations. This vent also has anti-gust deflectors eliminating the risk of snow or water infiltrations.
The turbine vent, on the other hand, relies on its moving turbine to draw air from the attic and may freeze during freezing rain or heavy snowfall winter outbreaks. Once blocked, its overall efficiency is greatly reduced and the unit also loses its means to eliminate weather infiltrations, as the spinning turbine drains water out from the vent. Additionally, these units can sometimes be noisy as the spinning turbine can give out a squeaking sound.
Active ventilators also offer possible energy bill savings during the summertime as they can cool down the attic efficiently. Keep in mind that these units do not need any electricity to operate whereas the mechanical ventilators do. In general, active vents are usually built from galvanized steel or aluminum, making them very reliable against rodents.
What is the best long-term investment for your roof?
I think it is important to say that all types of vents have been proven to properly ventilate an attic space under given conditions. If your home is located in a warm climate where cold weather and snow are not variables to consider, I’d say any type of vent will work given the right concept and installation. Mechanical and active vents will have an advantage over passive vents as they offer efficient venting and possible energy bill savings.
On an opposite note, if your home is located in a northern climate where cold weather and snow are major factors to consider, the ‘’Maxi’’ (or Maximum) ventilator would be the best choice. It is built from galvanized steel, stands high above the ridge line, offers year-round venting, offers continuous and variable exhaust flow rate according to local wind speeds to cool down the attic and lastly, will also save you money on energy bills during warm seasons.
For more information on our Maximum ventilators, visit our website.
Technical and development Director at Ventilation Maximum