For as many types of varying roof designs, there are a variety of different types of roof vents designed to service particular circumstances of roof design.
What Is The Purpose of Roof Vents?
Roof vents are designed to allow the attic or rafter cavities of a structure to breath. A roof that breathes allows attic moisture, humidity and retained heat within an attic to exhaust. The result being an attic that is dry, resists mold and mildew, and which aids the heating and cooling system of the home to work more efficiently.
Having the correct types of roof vents also can contribute to making upper floors of the home more comfortable in hot weather conditions.
Where roofing ventilation is concerned, it is important to have what is termed a balanced ventilation system. Ideally that requires that fifty percent of ventilation is intake ventilation, which is most typically achieved through the soffits (under-hang of the roof).
The second component of a balanced ventilation system involves exhaust ventilation; typically achieved by having roof vents located near the peak or at the peak of the roof.
Ideally a balanced roof ventilation system would be in equal proportion at the soffit and the top of the roof. It is possible to have too much exhaust ventilation, but having an excess of intake ventilation does not pose any particular problems.
We observe many homes that do not have any intake ventilation, due to the original design characteristics of the home. Examples include houses that have roof rafters that are pocketed into the exterior brick walls. Such homes often demonstrate open tongue and groove wooden soffits. Many older homes built in the last century were not designed with high levels of interior wall or attic insulation (If any in some instances). Such homes would literally leak air and were not thermally efficient, but they could “breathe”.
Over the decades, many such homes were renovated and had high levels of insulation added in attics and walls, high efficiency furnaces were introduced, thermally efficient windows were retro-fitted to replace old wood windows.
The result was that such homes of the era had increased efficiency, but retained heat and resulting moisture would be created in the environment and migrate into the attic (where all hot air rises to the highest point in a structure).
A consequence being that older retro-fitted homes, and newer high efficiency builds required an increased level of ventilation.
Most assuredly today, a homeowner should be alert to receiving a roof replacement estimate that incorporates a specification for increased or improved ventilation; particularly if no such work had been performed previously.
Manufacturers of roofing products will void their warranties if it is discovered that proper intake and exhaust ventilation does not form part of a replacement roof assembly.
Some home designs are very easy to calculate. Traditional open attics can have the volume calculated based on square footage and pitch, and a prescribed number of exhaust roof vents can be determined.
But what about an interrupted attic space (often characterized by functioning dormers through the roof, or homes that have vaulted or cathedral types of ceiling/attic designs)?
Those are circumstances which have to be factored in terms of specifying the correct type of roof vent.
Another circumstance involves homes built with gable end vents. If one adds exhaust roof vents in combination with existing gable end vents; it is possible to actually draw snow or moisture into the attic.
Related Content: How a Properly Insulated and Ventilated Attic Preserves Roof Health
What Are The Different Types of Roof Vents?
In fact using different styles of vents in combination near the peak of the roof, can have a similar result, and at the very least render the exhaust system ineffective.
As an example; having a turbine vent in combination with fixed (static) vents located across the peak of a roof will result in the following: air being sucked by the turbine vent from all of the other vents and no effective exiting of hot, moist air from the attic.
The use of turbine vents or Maxi™ large capacity vents on a roof should be a sole product option used (in concert with an open attic configuration and proper intake ventilation at the under-hang of the roof).
In the case of a vaulted or cathedral ceiling, continuous ridge venting is the correct approach. Ridge venting can also be employed for conventional open attics, provided that there is sufficient ridge line which translates into the correct number of square inches opened up at the ridge to meet the exhaust venting requirement, based on attic volume.
Gable end vents should always be closed off if introducing a different type of roof exhaust ventilation system.
On the subject of intake roof ventilation; if one does not have a traditional soffit that can permit air into the attic; there are some options and specific types of vents that can be used to achieve the result.
Most typically roof eaves vent strips or vent boxes can be installed at the junction of the fascia and roof eaves area. The design is reminiscent of a drip edge flashing detail which incorporates a continuously vented box as part of the design profile. These are preferred to be used in warm climate areas, as regions which experience high snow volumes can have problems where ice formation at roof eaves areas can prevent intake, or accumulated snow in the gutter can be “sucked” in through the soffit box.
Slot type vents are another alternative, which are located a few feet up from the roof eaves area. The design permits water to shed over the top surface of the vent, and has a baffled grill design to allow air to be introduced into the attic cavity. Installing such products involves cutting a continuous slot in the roof boards or plywood sheeting, and then applying ice and water barrier membrane over the area and surrounding the slot vent component.
Mostly commonly observed as a method to achieve such ventilation, is to employ a series of slant back vents (surrounded by Ice and water membrane), at the lower roof eaves areas. This also can result in snow ingress under severe winter conditions.
Related Content: Attic Ventilation and Its Importance
The most ideal approach would be to remove bricks between eaves rafter spaces and install individual grill vents, to permit air intake at the exterior wall/soffit junction.
It is important to note that installing any type of intake ventilation lower down on the roof surface can result in the increased velocity of water shedding from the roof.
Roof designs which may require unconventional intake vent solutions include homes with interrupted attics (functioning dormers), or attics which are converted to living spaces below the gable roof line.
In both instances, such designs result in a tiny attic space beneath the peak of the roof, and lower attic spaces (termed knee walls), located above the roof eaves areas. Such spaces are connected by the common roof rafters spanning from ridge to eaves. The sloping “wall” portions of the interior space can be likened to having a vaulted ceiling design. In other words, the space between each pair of rafters in effect becomes it’s own attic space.
As discussed earlier; ridge venting becomes the correct exhaust approach, and continuous soffit ventilation is required to introduce air into each and every individual rafter cavity.
Damper vents are a type of ventilation product which are designed and required to exhaust kitchen, laundry or bathroom fans that carry moist humid air from such locations, and which must exhaust through the roof. Typically there is an insulated, flexible hose which connects from an electrically powered exhaust fan located in the interior space. The end of the hose that exits the roof is connected to the damper vent which has a mechanical flap which lets exhausted humidity out, but does not permit water in.
Motorized fans which act to draw air from an interior or attic space are typically referred to as “power fans”. These purpose driven units are hard wired by an electrician and can be controlled thermostatically or by the occupant controlling the operation of the fan and related fan speed.
Power operated fans in a residential application context for roofing, are often used to actively draw air out of a fixed attic space. An example would be a shed roof design which abuts to a brick wall exterior structure.
In the context of a commercial flat roof application; such exhaust vent fans are used to draw smoke from range hoods in restaurants, ventilation for auto body shops or fume mitigation in factory environments as examples.
Variations of a power vent include those which are solar powered and those that are thermostatically controlled so that when an attic reaches a particular temperature; the vent is automated to turn on and shut of respectively.
When considering flat roofing; there are vents that are specific in design to the nature of the work, but achieve the same purpose as those used on steep slope applications. One unique product is termed an insul-vent. Specifically the product is designed to exhaust moisture and vapours which can be trapped beneath an insulation layer or between roof assemblies in overlay circumstances.
Related Content: Which roof vent is best for your home?
The ventilation of wooden substrate flat roofs is typically best achieved by installing cross strapping as part of the structural design. Where flat roofs are concerned; design characteristics like the use of internal or external roof insulation, location of vapour barriers or vapour retarders, and in relationship to building use, are integral design requirements which can be unique to each building.
As an example; a building which has freezers within the building may require very specific specifications and ventilation design, relative to a more conventional warehouse structure.
Complicated flat roof designs and their ventilation requirements are often best determined by qualified engineers for such projects.
To learn more about roof ventilation, commercial, industrial, or residential roofing; visit: www.avenueroadroofing.com
