The Importance of Roof Vents
The subject of roof vents is not what one would characterize as a sexy topic; but it is certainly an important consideration when installing a new roof, or after increasing insulation levels in a home.
First, it is important to understand the function that roof vents provide. Essentially; roof vents are required to allow hot, moist air to escape from the attic or rafter cavities (vaulted/cathedral ceilings) beneath the roof.
Manufacturers of roofing shingle products require as part of their warranty provisions; that roofs are ventilated properly in order for their product warranties to be valid. Additionally; modern building codes specify that proper roof ventilation is constructed, with minimum standards for compliance being detailed.
Roofs which are under ventilated can exhibit problems both in the short term and the long term. Such examples can include contributing to ice damming in winter months (although not the sole cause); premature deterioration of wood roof decking over time, and poor comfort levels in homes particularly during warmer temperature months. Poorly ventilated attics can serve to “superheat” roof shingles which can result in premature failure; exhibiting curling, granule loss, and early deterioration of the shingles themselves. Typically organic asphalt shingles are even more prone to such failures under poor ventilation circumstances.
In simplified terms; a proper roof venting system requires intake and exhaust; typically in a ratio of 50% intake and 50% exhaust. The concept being that cooler exterior air is introduced through the soffit (underhang) areas of the roof (intake venting); and because heat rises, then pushes the hot attic air out through the vents located near the peak of the roof (exhaust vents).
How to select a vent for your roof?
Different attic configurations may require different types of vents to be used to effectively do the job.
Homes that have typical open configuration attics (characterized as non-living space); will typically require “static” or “turtle” type vents, in a prescribed number; to be distributed evenly near the apex of the roof.
Structures that have vaulted ceilings (often characterized by having finished living space beneath the roofline, lower knee wall spaces, with slanting walls/ceilings), or cathedral ceilings; would properly require some form of ridge venting, located along the entire peak of the roof.
Ridge vents allow air to exhaust between every individual rafter cavity space in concert with the continuous soffit venting required at the roof under hang areas.
Other required forms of attic ventilation include such items as damper vents; which exhaust bathroom or kitchen fans, This allows hot moist air to exit to the exterior of the building, rather than into the attic space.
Plumber’s stacks, exiting through the roof, allow sewer gas to escape from plumbing fixtures such as toilets and sinks.
Specialized ventilation components are often utilized for certain roof configurations, or non-conventional attic designs. Such types include turbine vents which are driven by the wind and work to extract hot air from the attic; power vents, which are hard wired, electrically driven units, which actively draw out hot attic air. Many are thermostatically controlled so that when the attic reaches a particular temperature, the extractor fan is then deployed. Alternative versions of the power vent include units which are solar driven, therefore reducing energy consumption.
Roof vents are rated by a cfm rating (cubic feet per minute). It is therefore necessary to select the right capacity of roof vent, or combination of roof vents, to adequately service the volume of space within the attic.
What are the consequences of selecting the wrong combination of vents?
Selecting the wrong combination of vents can result in failure to achieve the desired result. One example would be installing two turbine vents at opposing ends of the peak of a roof, and having a few static vents located in between. What would result is that exterior air near the peak of the roof would be drawn into the attic, beneath the ridge line, and then would exit out of the opposing turbine vent. The result would be air movement near the ridge, but it would not properly extract the volume of hot air out of the majority of the attic. The phenomenon is referred to as the “Bernuli Effect”; so named after the scientist who illustrated the effect using wind tunnel testing methods.
Equally important when undertaking the installation of correct roof ventilation; is that the work is completed in a competent manner. Such considerations involve items like cutting the correct size of the hole in the roof decking to accommodate the particular vent being utilized. Obviously important is making sure that the roof vents are installed so that they do not result in leaks. While such work can be within the realm of a do-it-your-self; it is advised to hire a professional who is skilled in the correct product selection, and familiar with proper installation techniques, and manufacturer requirements to get the job done right. Of consideration is the access, height of the work area, and steepness of the roof, so enlisting someone who possesses proper fall arrest training, and carries the correct liability insurance, only serves to protect the homeowner and assure a highly satisfactory result.
The subject of proper roof ventilation is most commonly over-looked by many home owners when evaluating their roof replacement proposals. It is a sign of a skilled and knowledgeable roofing contractor, who specifies proper roof venting as part of their proposal.
As many people purchase older homes which have been retrofitted over the years; it is important for them to consider that such changes as the installation of more energy efficient doors, windows, furnaces, and upgraded insulation levels, can also have an impact on various dynamics associated with the building envelope as a whole. Of such considerations, should be the adjustment of ventilation requirements which may be necessary to achieve optimum performance for the heating/cooling system of the home, and the long-term performance of the roofing assembly.