Insulation. R-value. Vapor barrier. Those are all terms used to describe products or principles designed to keep our homes comfortable and energy efficient. The concept is simple: keep the heated or cooled air in while keeping the unconditioned air on the outside. So why in the world would we install vents to allow air to pass through on its own? And what is a gable vent anyway?
It’s confusing, especially in today’s world of maximum insulation values and techniques. We’ll discuss gable vents and what they do at length, so you’ll get a better understanding of your home’s ventilation system and why it exists.
Why Do We Need Ventilation?
Trust us, we know that ventilating a well-insulated home seems counterintuitive, but there are a few reasons why it’s so important.
Think back to science class and how warm air rises while cooler air falls. This happens in homes, with hot air rising and becoming trapped in attics and upper floors. But warm air doesn’t just rise; it also takes moisture with it, and warm, moist air collecting in an attic or upper level can cause some serious issues.
When warm air is trapped in an attic space, the moisture in the air collects on surfaces, eventually leading to mold. Also, as the heat intensifies, unvented attics can “cook” roofing systems from underneath, reducing their lifespan and causing expensive repairs. And it’s not just a concern when it’s hot out, as heated air collecting in the attic in the winter can melt roof snow during the day, only to have it refreeze at night and potentially cause ice dams.
But ventilation can prevent these problems. It allows that hot, warm air to escape the building, and gable vents assist in the process.
What are Gable Vents?
Gable vents are louvered, open grates that allow airflow to pass through the highest peaks of a home. That’s the gist of it, at least, but there’s a lot more to the story.
Let’s start with the very basics. A gable, or gable wall, is actually a section of the home. A gable wall is a triangular-shaped exterior wall underneath the point where the two roof planes meet. Most homes with a straight ridgeline have just two gable walls. However, homes with L or T shapes, additions, or other variations have more than one gable wall.
A gable vent is installed at the top of the gable wall, since this is the highest point in the building where the heat and moisture collect. This is often in an attic but might also be in a finished space. They can be triangular, rectangular, circular, octagonal, or a range of other shapes. They may or may not have screens attached inside to prevent pests and debris from passing through. They’re also available in many sizes, allowing contractors, homeowners, DIYers, or other folks to choose the best size for the space.
How do they work?
First, understand that one gable vent does not work on its own. They must work in sets of at least two. This is because they work on the principle of cross ventilation, with hot, moist air exiting the home on one end, and fresh, cooler air being drawn in through the other. This passive ventilation will occur on its own, as long as the space between the gable vents isn’t blocked.
In uninsulated attics and upper floors (these structures are insulated in their highest ceilings, rather than roofs), blockages between the gable vents are rare. However, in finished attics or upper spaces, gable vents require collar ties, which are framing members that stretch from rafter to rafter. The spaces between the collar ties are insulated, and drywall is installed underneath. If installed properly, this creates a clear pathway from one gable vent to the other above the collar ties, facilitating proper airflow.
Do gable vents let conditioned air escape the home?
Gable vents, or vents of any sort, are not supposed to let conditioned air escape the home. However, they must be installed correctly and the home has to be insulated properly. In an uninsulated attic, the floor must be insulated to keep conditioned air below.
In a finished space, some of the roofline, vertical knee walls, and some of the floor must be insulated to capture conditioned air. When this is the case, the gable vents will allow the air in unconditioned spaces to escape while the insulation retains the healthy, conditioned air, allowing vented homes to remain safe and prevent damage.
Soffit and knee wall ventilation
Gable vents in uninsulated or unfinished attics typically vent quite well on their own. However, in spaces with insulated roofs, they aren’t as effective without some help. In these cases, it’s common for gable vents to work in conjunction with soffit vents or knee wall vents.
Soffit vents are installed under the eaves of a roof, or the lowest part of the roof that overhangs the exterior wall. Foam baffles are installed between the roof rafters against the roof deck, preventing the insulation from contacting the deck and leaving air space. Between the soffit vents and baffles, the gable vents are able to draw cool, fresh air from outside up through the soffit vents and into the area above the collar ties, keeping the roof deck cooler to prevent ice dams or overheating the roofing system.
Knee walls present another challenge. To prevent ice dams, the knee walls themselves are insulated, but not the roof within the knee wall space. To promote airflow here, knee wall vents can be installed in the exterior walls. Coupled with baffles, the gable vents can draw air through the knee wall vents, promoting circulation. The floor below the knee wall must be insulated, however, to create the appropriate, energy-efficient building envelope.
Gable Vent vs. Ridge Vent: What’s the Difference?
There is another type of vent commonly used to let hot, moist air escape a house, and it’s referred to as a ridge vent. Ridge vents run along the peak of the roof, known as the ridge. They need to be cut into the roofing deck and installed properly with flashing and ridge cap (a type of shingle for ridges), but they’re extremely effective at moving air.
Ridge vents are long and allow large volumes of air to escape a home. They stretch from one end of a roof to the other, often stopping just 16 inches or so short of the gable. They have baffles that prevent pests, debris, and precipitation from entering, as well. Couple with soffit vents, they’re very effective—much more than standard gable vents.
Ridge vent drawbacks
While gable vents are easy and affordable to install, and can truly be installed at any time, ridge vents are the opposite. Ridge vents cost more in materials, but they also require the roofing crew to cut back the roof sheathing, install the ridge vent, flash it properly, and then cap it with shingles. This costs quite a bit to do and it’s not recommended for existing roofs. When the time comes to replace a roof, however, it’s a wise idea to budget for a ridge vent to be installed.
Also, not all homes are great candidates for ridge vents. Homes with complicated ridge lines don’t have the continuous ridge required to install enough linear feet of ridge vent. In these cases, it’s often better to use multiple gable vents.
You don’t want both
Keep in mind, however, that ridge vents and gable vents don’t work well together, especially when soffit vents and knee wall vents are installed. Many times, this combination creates minimal airflow that defeats the overall purpose of the vents. Homeowners who plan to have ridge vents installed should block their gable vents to get the most from their new vent system.
Are Gable Vents Right for You?
In most cases, gable vents are sufficient if they’re installed correctly and the space they’re venting allows for them. They’re also better than having nothing at all, as that warm air has to leave the home to keep it healthy and prevent damage. Ultimately, ridge vents are typically more effective at moving air, but gable vents are more than capable of doing the trick and keeping those upper spaces cooler and healthier.