It should be up to your roofing contractor to choose the right roof ventilation system, as only professionals can accurately estimate what solution will work best for you. However, there are situations in which you, as a customer, will be given two or three alternatives to choose from. In such situations, it is definitely a good idea to have a good understanding of how a roof vent system works and when it makes sense to opt for mechanical ventilation. So let’s ask our experts for their opinion.
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Why Do I Have to Install Roofing Vents?
Every building, whether residential or commercial, is filled with moisture. Falling rain, water vapor exhaled by people and evaporating plants mean that on sunny days, the amount of water in the air inside the building is usually visibly higher than outside.
Another equally important factor from a ventilation standpoint is temperature – roofs are often dark colored. Asphalt shingles from popular brands such as GAF and Owens Corning are most often sold in shades of gray and black, which means that when the sun shines, they heat up much faster than the walls. Adding to this the phenomenon of thermal convection, we can conclude that statistically, for a large part of the year, the temperature and humidity in the attic of your house will be much higher than in the lower parts of the building.
The buildup of heated steam is definitely not good, as it can lead to the growth of mold in the attic. Attic ventilation problems also mean that water vapor is not effectively removed from the rooms, causing it to condense on the windows and walls in winter, while in summer it raises the perceived temperature and drastically reduces the comfort of the building’s occupants.
You must take care to choose good roofing vents, as they are essential to ensure a high level of comfort inside the building. Lack of ventilation, on the other hand, can lead to the growth of mold and other fungi and make it difficult for you to breathe. Although roof ventilation is not the only system responsible for the exchange of air inside the house, the accumulation of heat and water vapor in the attic can drastically reduce the efficiency of the whole house ventilation system.
Does Poor Roof Ventilation reduce the lifespan of the roof?
As a general rule, yes.
During winter, poor attic ventilation causes condensation, resulting in mold outbreaks. Heat buildup in the attic also means that if the insulation layer is poor, snow on the roof will melt and re-freeze at the edges. This results in icicles and ice dams, which can create leaks and pose a serious threat to people and property during snowmelt.
In summer, the phenomena occurring with your roof are different, but still do not positively affect the condition of asphalt shingles. The accumulation of warm air and water vapor limits the roof’s ability to give off heat – when this surface heats up to a high temperature, the shingles begin to degrade quickly. A damaged layer of asphalt or falling granules doesn’t just affect aesthetics – it’s the sections of the roof that are most exposed to the sun that leak, and it’s these that are most often damaged by hail.
When Should I Choose Powered Roof Ventilation and When it’s not Worth it?
As a general rule, powered roof vents are used when gravity ventilation would be too weak.
Probably the most common reason for choosing this technology is high temperatures and humidity. Homeowners living in Florida, Georgia, Texas, Arizona, Nevada and other hot or humid areas have to reckon with the need for a very high-efficiency roof ventilation system because in summer, attic temperatures in these areas can exceed as much as 150°F. In practice, this means that instead of the standard 3 cubic feet of fresh air per hour per square foot of attic floor area, vents should transport as much as 5 to 7 cubic feet.
The shape and design of the roof can also be a problem – airflow in some houses is extremely difficult due to their design. Flat roofs, such as those covered with TPO, are airtight, so water vapor will not penetrate them. The same phenomenon also occurs with asphalt shingles, although this is usually less severe. Also, multi-faceted roofs with many folds and windows may not be efficient when it comes to attic ventilation, as air flow is disrupted.
Few people also know that gravity roof ventilation will not work properly if the attic is air-conditioned. The accumulation of warm air under the roof is perfectly natural, and disrupting this phenomenon can cause steam to not leave the building.
Can Roof Ventilation be too Strong?
Yes, it is possible, and although such a phenomenon is much less common than choosing too inefficient attic ventilation, it is not worth overdoing the amount of vents.
Too much ventilation won’t damage your roof, but it can drastically increase the cost of heating your home or lower the temperature. Energy losses can translate into hundreds of dollars a year that you will have to spend on heating or cooling your home without improving your comfort. As airflow through the roof ventilation increases, so will the cost of using air conditioning.
Excessively fast air exchange is a desirable phenomenon in office buildings, but not necessarily in a home environment, as it can impair sleep comfort. Decreasing humidity causes a sore throat, greater susceptibility to colds and dry eyes.
Very efficient roof ventilation can also lead to improper smoke flow through chimneys. The negative pressure created inside the house will suck the products of combustion from the fireplace into the living room, for example, which can lead to a fire or serious poisoning, especially in children and pets staying in the house.
Types of Vents Used in Roofing
Before we get into analyzing the different types of roofing vents, we need to keep in mind some very important principles relating to the design of indoor air circulation:
- Structural limitations – by replacing the roof or roofing vents on an existing house, you are unlikely to be able to significantly affect the air circulation inside the building. Removal of warm water vapor will be simpler for tall houses and complicated if the building is low and spread out. A complicated, multi-slope roof, leaky skylights or inadequately designed roof trusses can drastically restrict the free flow of air, and consequently, stagnant water vapor in the attic.
- The amount of incoming and outgoing air must balance – if you install some very powerful exhaust fans but block all intakes, the whole system will be very inefficient. Obtaining excess pressure inside the building, that is, a situation in which too much air enters the building, is much less likely, but both situations are not beneficial. A roofing ventilation system requires a balance to make it work efficiently and usually, engineers are able to achieve this by installing several smaller vents instead of one that would be very large.
- Sometimes it’s easier to install an air conditioner than to improve ventilation – an air conditioner doesn’t supply air from outside, but it does reduce the humidity of the air, making the roof ventilation system more efficient in practice. This solution consumes much more energy, but there are times when artificially lowering humidity is simply necessary.
- Ventilation must be watertight and non-penetrable to pests – as a roofing contractor with 21 years of experience, we can say that it happens quite regularly that pests such as rodents, birds and insects get into the roof ventilation. A cracked fan housing, on the other hand, can make it so that, although it will exhaust warm air to the outside, on rainy days, water will pour in through the same opening, which will, of course, result in mold growth and rotting of the wooden roof truss components.
Gravity Exhaust Vents
Gravity roof ventilation, also known as passive ventilation, is a system that uses the natural airflow resulting from convection. Water vapor and warm air propagate along a pressure gradient, so they follow from the inside of the building to the outside, where their concentration is lower.
As MyRoofHub mentions, the most popular type of gravity roof ventilation are ridge vents. These are remarkably similar to ridge caps. Installed on top of the roof, just above the ridge beam, they effectively transport water vapor while allowing air to flow between the sheathing and the rooftop. This is an excellent solution used primarily on asphalt shingles, but our company has also used them in conjunction with cedar roofing in Winnetka.
Another solution, also popular, is box vents, which, unlike ridge vents, are installed on the slopes of the roof. Typically, roofing contractors use several box vents of the standard 18“ x 18” size, but if the project requires it, smaller as well as larger models are possible. These roof ventilation parts work well for replacing roofs with complex shapes, when a ridge vent would not effectively dissipate water vapor.
Powered Exhaust Vents
Powered roof ventilation exhaust (attic power vents) visually more resemble box vents, but the mechanism of operation is obviously somewhat different. Typically, powered vents in residential roofing draw in air directly from below the ceiling, thus accelerating airflow. To improve the energy efficiency of the heating system, power vents are often combined with thermostats, ensuring constant temperature and humidity in the interiors.
Intake Vents
Roof ventilation also requires the use of intakes. These can be kynar steel vented soffit panels, gable vents or vents at the attic floor. Air also enters through small leaks in walls, windows and doors. For the purpose of calculating flows for the design of the ventilation system, only dedicated intake vents are considered, such us vented soffit panels and exhausts like boxes and ridge caps.
When it comes to intake vents, by far the most popular solution is soffit ventilation. Due to its low price, aesthetics and high level of reliability resulting from the dispersion of the vents over a large area, soffit panels located in the roof overhang are chosen by contractors regardless of the size of their projects. With a simple manufacturing process and a wide range of available materials such as steel, aluminum, vinyl and wood, vented soffit panels are finding their place in almost every roof ventilation system in the United States.
There are times when the roof overhang is too small to achieve adequate airflow. In such situations, it is possible to use fascia vents or drip edge vents. Although these are two separate designs in theory, the principles of their use remain almost identical. Typically, fascia and drip edge vents are installed just above the gutters, around the entire house. Such solutions require consideration as early as the design stage of the roof structure – the roofing contractor must install the shingles so that air can circulate under the roof. An additional disadvantage of fascia and drip edge vents is their low efficiency. The small size of the openings means that powered vents may be necessary to improve airflow.
What Kind of Roof Ventilation Will Be the Best for My House?
To help you choose roof ventilation, let’s use a few examples. In each case, our suggestions will be geared toward homeowners living in mild climates such as Illinois. We base our calculations on standards guided by 2021 International Building Code, section 1202.2. In many states, local building laws are enacted based on this document.
As a general rule, the area of the vents must be no less than 1/150 of the ventilated area. The regulations allow the use of twice as small an area of vents if at least 40% of the vent area is at the top of the roof, with the remainder at the bottom. This is, of course, the natural solution used in most pitched roofs, so we can assume that the vents should be about 1/300 of the floor area of your home.
Small house of 1,500 to 2,500 sqft with a gable roof
Roof ventilation in a 2,000 sqft single-story house can be gravity as well as mechanical – in the vast majority of cases, the difference between the pressure at the attic floor and just below the ridge will be large enough for gravity air circulation to be efficient enough.
How much should the area of the vents be? About 14 sqft for roof ventilation using only ridge vents, or less than 7 sqft when both soffit and ridge are vented.
How much air should the vents transport? It is assumed that during each hour, attic air should be replaced at least 10 times. Assuming a standard attic volume in a 2,000 sqft house, we should expect about 1,670 CFM (cubic foot per minute) of airflow. With standard sized ridge vent caps and a standard pressure differential, this effect is achievable with just about 1 square foot of ventilation area for every 220 feet of floor space in the house single-story house!
How many linear feet of ridge vents will I need? Standard ridge vents provide about 1 square foot of surface area for every linear foot of their length. So if your home is 2,000 sqft, you will need to install about 14 ft of ridge vents to meet the regulatory requirements. However, we recommend that it be no less than 10 ft for every 1,000 sqft of floor area.
For two-story homes, the situation is not fundamentally changed. Provided you maintain at least acceptable airflow between floors, even a small number of vents will allow you to get a good roof ventilation system performance.
Medium house of 2500 sqft to 4000 sqft with a gable roof
Creating an efficient roof ventilation system for homes around 3,000 sqft becomes a bit more difficult. If you’re dealing with a single-story house, the relatively large area as well as the volume of the building means that airflow is often disrupted by many objects along the way. Water vapor does not penetrate very quickly through the ceiling – in most cases, it reaches the upper part of the house through slots or individual ventilation ducts, which are usually located on the outer walls of the building.
For relatively large homes, many designers also recommend that the attic ventilation system be distributed. Distributing box exhaust vents throughout the roof allows for smoother airflow. Large powered vents are also often used to remove air from, for example, the living room and kitchen, where a slightly more frequent air exchange is required compared to the bedroom, study or garage.
How much should the area of the vents be? As with smaller homes, the recommended area of the vents is 1:150 of attic floor area, so if your roof ventilation system uses only ridge caps, they should be about 20 ft long. If, on the other hand, you use vented sofft and ridge, about 10-12 ft ridge caps are sufficient.
How many box vents should be installed? A standard 18“ x 18” box vent exhaust is equivalent to an opening of 144 – 160 square inches. So if you wanted to use them as your only vents, you would need to use as many as 20 box vents. In practice, for homes of about 3,000-4,000 sqft, box exhausts are used to improve the ventilation of individual rooms in the attic, such as a bathroom or guest room.
When it comes to two-story or three-story houses of 3,000 to 4,000 sqft, be aware that the layout of rooms can make it much easier or harder to plan adequate roof ventilation. We unequivocally recommend that rooms with special air circulation requirements, such as bathrooms, have a separate ventilation duct. Even if your attic is not in use, but there is a room just below it where the vapor concentration and temperature are higher than in the rest of the house, it is definitely worth improving the roof ventilation in this area.
Large house of more than 4000 sqft with a gable roof
Even if the roof structure is simple and the attic airflow is not disturbed by additional obstructions, roof ventilation for large houses over 4,000 sqft virtually always requires the use of power exhaust vents. This is due to the large volume of the attic, which, especially when the roof is also high, requires a lot of energy to make the removal of warm air effective. In addition, if the attic is more than about 14 feet high, air currents that normally move along the sheathing from the soffit to the roof ridge may lose their direction, depleting momentum.
The more floors, of course, the smaller the attic area, but probably also the higher the temperatures – as we have already mentioned, heated air rises by convection, ultimately going all the way to the attic of the house. At the stage of designing the roof ventilation for a large house, your roof replacement contractor from Glenview should pay attention to the layout of the rooms – this greatly affects the later temperatures in the attic.
Houses with multi-gabled roofs
For houses with multi-gabled roofs, designing a roof ventilation system becomes a much more difficult task. Keep in mind that in the case of dormer, dutch and mansard roofs, the flow of air currents can be very disrupted – instead of going straight to the ridge and thus going to the outside, it stops in some spaces.
If your roof consists of multiple valleys, pitches and dormers, several smaller exhaust vents should be efficiently deployed. While the use of mechanical attic ventilation allows you to use only one fan, in our opinion, this is not exactly the optimal solution, and on top of that, it is relatively energy intensive.
We strongly recommend that each dormer have its own separate box or ridge vent. If, on the other hand, the ridge is very long and it would be pointless to completely replace the existing ridge caps with vents, it is worthwhile to distribute the exhausts at equal intervals along the length of the house – this way, the airflow will be more even.
Why Should You Have Your Roof Ventilation System Installed by the Top-Notch Roofing Contractor?
At first glance, constructing roof ventilation is not difficult – just install exhausts and intakes. In practice, however, the situation is much more complicated.
Design process
Without access to specialized software or experience, you may not be able to accurately determine the amount of air that should pass through your roof ventilation. To do such a design, you need to be well versed in the specifics of the region as well as the other materials used, and understand how air moves through your attic. Neither building standards from the IBC nor tutorials from the Internet can replace experience and professional equipment for tens of thousands of dollars.
Selection of ventilation systems
Failure to properly construct a roof ventilation system can result in the installed equipment not operating at full capacity or even worsening attic conditions. Your roofing contractor takes responsibility for selecting intakes and exhausts in such a way that they do not interfere with each other. For example, a powered attic vent installed too close to a ridge vent will cause air to go inside the building through the passive vent, rather than outside as it should.
Reasonable search for savings
It’s perfectly normal that when contracting your roofing contractor for a service, you don’t want to spend too much.We understand this, and when preparing a proposal for roof replacement, we take into account the customer’s economic preferences. Saving money on attic ventilation is possible, but it must be reasonable. As in the case of choosing the best asphalt shingles, you can choose from many available options. Although the most expensive does not always mean the best, buying the most budget ridge vents or box vents usually ends badly.
Properly secure all holes
The theory is that properly constructed attic ventilation does not pull air from inside the building. If this is not the case, warm air and water vapor can raise the humidity level of the wooden structural beams in the ceiling. Leaks at the junction between ceilings and chimneys, poorly sealed lights and even a lack of seals to the attic hatch can greatly degrade the performance of the ventilation system, which of course can result in mold growth.
