Everything You Need To know About Roof Ventilation

Blog, roofers of Winnipeg

The life expectancy of your roof system is not limited to the quality of shingles you install.
The life expectancy of your roof system is not limited to the quality of shingles you install. Inadequate roof ventilation alone can throw your investment out the window, leaving you confused as to why your promised 30-year roof system only lasted about half the period.
How Ventilation Works

Simply said, ventilation is the flow of air through an intake and exhaust system. In your home, attic ventilation is one of the things that gets a say on whether your roof system will last for the intended period or not.
Photo credits to CertainTeed 

Attic Ventilation
Attic ventilation systems work by allowing air to constantly flow in and out to maintain the temperature difference between the inside and outside of your attic relatively low.
This prevents the hot summer air or the moist winter air that gets in the attic from being trapped and causing all sorts of problems.

The unwelcomed air can end up in your attic in one of two ways:
During the hot summer:
The hot sun's heat is radiated onto the roof deck and part of it is transferred to the attic.

The way proper ventilation serves you then is by expelling that heat through the exhaust vents before it can travel further into the living quarters of your home and cause a strain on your air-conditioning systems.
During the Cold Winter:
The Warm indoor air--produced by your furnace and other activities such as cooking, moving, breathing, and bathing, to name a few--travels up into the attic by going through the vulnerable parts of your ceiling (i.e recessed lights, attic access hatch, and other ceiling penetrations.) in the form of warm water vapor
This process of moisture transfer occurs due to a pretty cool phenomenon, called Stack Effect (also, thermal effect)
Stack effect is the warm air's natural tendency to rise. And speaking of cool, as the warm air rises, cooler air tends to fall as well.
You want to take advantage of this effect by having a well-designed ventilation system where you place intake vents at the lowest point of the attic (i.e Soffit) and exhaust vents at the highest point of the attic.
Vapor diffusion is another reason moist air travels upwards during winter.
That is because water vapor naturally travels from high-humidity conditions to low-humidity conditions.

During summer, the power of wind is a key element in this equation, and without it, the mentioned natural forces will not create the air movement you need. 
The wind creates pressure outside the building which forces air inside the attic at the eave level. The Wind above the roof creates pressure that pushes air outside of the building.

Photo credits to CertainTeed

When things get chilly, however, the stack effect is a far greater factor in facilitating air movement inside your home. This is due to the temperature difference between the outside and inside of your house. Colder air enters your home at the ground level and turned-warm indoor air rises to the attic.
When the absence of wind poses an issue. Air movement can be created mechanically using a powered ventilation system.
Mechanical ventilation relies on a power source in order to function.


Type of vents
The type of ventilation systems can be divided in two categories: Intake and exhaust vents.

Each type provides a certain NFVA (Net Free Ventilation Area). And you'll understand what that means further below.
Intake vents can come in the form of soffit vents which go along the underside of your roof's overhang and they can be continuous or come in the form of individually spaced vents, called Undereave vents.
As far as exhaust vents, the most widely used are roof louvers and those are the square box-shaped vents you see on roofs. Although, recently one type that has been gaining popularity is the ridge vents; mainly due to their increased efficacy in ventilating the attic.
Many will agree that the most effective type for exhaust ventilation is the ridge vent. Because it sits at the highest point of the attic and it's a lot less likely to cause a leak in your roof (as opposed to the roof louver). However, ultimately the right type of ventilation you may need will depend on the shape of your roof and other factors. This will be determined by someone like ourselves (lien à la page d'estimation) after a thorough inspection of your roof.
When An Attic Is Pooly Ventilated
By now, you probably have a good idea on how attic ventilation works in an ideal environment. But what happens when your roof is not adequately ventilated?

During the hot summer, your goal should be to maintain a "cold roof" so that the radiated sun's heat does not sit in your attic for long.
By having adequate intake and exhaust ventilation you are ensuring that there is a constant flow of air entering your attic through the area near the eaves edge and escaping through the exhaust vents before the built up attic heat becomes a concern.
When this balance of air flow is not established, the solar-heated hot air will build up in the attic.
an overheated attic will cause the hot air to sneak in to your home's interior and cause a strain on your air conditioning system.
Over time, the excessive attic heat will eventually cause your roof deck to deteriorate and your shingles to fail prematurely.

During the cold winter, however, we know that the purpose of having adequate ventilation in place is to facilitate for the rising moist air to escape through the exaust vents in order to maintain a uniform temperature between the surface and the underside of the roof deck.
When you have a poor ventilation system in place, the rising warm air will tend to stick around for longer and create the perfect environment for a condensation effect.
In the context of your attic, condensation happens when the ascending warm water vapor comes into contact with the cold rafters of your roof and loses its gaseous form to a liquid state.
You could probably guess what happens from this point on. The condensation effect causes the underside of the roof deck to soften and warp over time. And it causes the moisture to drip in the form of droplets onto the insulation.
Usually (or hopefully) it won't get to the point where the droplets reach and go through the drywall ceiling from condensation alone but it will definitely cause the insulation on the attic floor to loose it's R-value (its ability to keep heat from leaving or entering your home) over time.
A wet insulation also means trapped moisture, which promotes corrosion of all the existing metal inside the attic (i.e wall ties, roofing fasteners) and can also form a breeding ground for mold and bacteria to grow.
Another known consequence of inadequate ventilation is the formation of ice dams.

Ice damming is one of the most common roof issues homeowners face in colder climates. And since this is roofing in Winnipeg, we can't go without talking about ice damming.
In a poorly ventilated attic, built up warm air accompanied by a sunny day outside causes the snow above the roof deck to melt and run down only to then refreeze at the eave level, where the temperature tends to be colder (remember? warm air rises to the highest level? Good.).
The formed ice acts as a wall, stopping the melting snow from running off the roof.Bear in mind, this is all happening under a thick layer of snow that's acting as an insulation blanket.
The formed pool of water is then forced back up under the roof shingles causing damage to the sheathing and the interior and exterior walls of your home.
Now that is Ice damming for you!

But to be fair, we can't blame it on poor ventilation alone. An insufficient amount of insulation is also a factor because it allows a bigger volume of indoor warm air to escape into the attic.
How to properly ventilate your roof
So how do you make sure your roof is properly ventilated?
I am going to demonstrate a quick formula to help you determine if your home's roof has sufficient ventilation.
The purpose of this of course is not to try to turn you into a certified home inspector but to at least put you in a position of knowing whether your roofing contractor knows what they're doing or simply whether your roof is safe the way it's sitting today.

The amount of ventilation required depends on the size of your roof but both the National Building Code and manufacturers require that a minimum ratio of 1:300 be obtained. That is 1sq.ft of ventilated space (called Net Free Ventilation Area) for every 300 sq.ft of attic space; given you have both an intake and exhaust system in place, as well as a vapor barrier on your attic floor.

The required ventilated space must be distributed between the intake and exhaust systems and while a 50/50 balance is a good start, more ventilation at the eave level will create more pressure at the higher points of the attic and therefore provide more efficiency.
That said, a 60/40 split would be the sweet spot.
Here is how we figure out the math using an example of 1200 sq.ft of attic space: 
1.Determine how much NFVA you need:  1200/300 = 4 sq.ft of Net Free Area
2. Divide the amount between the intake and exhaust: 
              60% of intake = 2.4 sq.ft
              40% of exhaust = 1.6 sq.ft

3. Convert the amount to sq.in: Since NFVA for ventilation product is listed in Sq.in.
              2.4 x 144 = 345.6 sq.in of NFVA needed for Intake
              1.6 x 144 = 230.4 sq.in of NFVA needed for exhaust
4. Apply those numbers to the particular type of vent you have and the NFVA each one provides.

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