In our last blog we went over why ventilation is so critical and its impact in different regions. In this blog, we’ll discuss the different types of ventilation for sloped roofs, environmental impacts, and ventilation requirements.
Types of Natural Roof Ventilation
Below we list the two types of natural roof ventilation, with our definitions for each.
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Attic Ventilation: Air intake at the eave, into a common attic space area, and exhaust through the ridge.
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Above Sheathing Ventilation (ASV): Vented nailbase is a common type of ASV that consists of sheathing, spacers, and insulation that create an eave-to-ridge air space used for ventilation.
Thermal Effects
Warm air rises and cool air descends. While cool air enters the roof system through lower eave vents, the warmer air will naturally rise and exhaust via the higher ridge vents. Well-balanced ventilation systems utilize this natural thermal movement.
The Imapct of Wind
When wind hits a building, high pressures force air in at the eave. As wind goes over a building low pressures draw air out at the ridge and prevent rain and snow from entering. This happens because wind speed moving against and over a building, is more important than the wind’s velocity. Wind moving against and over a building creates high- and low-pressure zones, also known as Bernoulli’s Principle, which states, “Within a horizontal flow of fluid, points of higher fluid speed will have less pressure than points of slower fluid speed.” More detailed analysis of Bernoulli’s Principle can be found on the NASA website. Information regarding how the speed of a fluid relates to the pressure of the fluid can be found here.
Net Free Area (NFA)
Net Free Area is an important component for all roof ventilation. NFA represents the portion of the opening in the vent that actually ventilates – more specifically – the total unobstructed area through which air can enter or exhaust a non-powered vent.
Building Code Requirements (Commercial vs. Residential)
Commercial and residential building code requirements are actually very similar. However, commercial buildings are usually much larger and require a greater amount of ventilation.
Sources:
Metal-Era Principles of Commercial Ventilation AIA Slides
https://www.khanacademy.org/science/physics/fluids/fluid-dynamics/a/what-is-bernoullis-equation
https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction
https://codes.iccsafe.org/content/IBC2018/chapter-12-interior-environment#IBC2018_Ch12