Testing the effectiveness of a powered attic fan versus ridge vent and soffit ventilation.
Myth - A power roof ventilator will pull air from your roof ridge vent and short circuit the air flow from the lower eave soffit vents.
Fact - A powered roof fan will cool your hot attic space faster and more efficiently than ridge vent and soffit vents alone without compromising the venting system.
The video below demonstrates the fact. Please pardon the shakiness and lack of professionalism in the video. It was Hot, cramped, dusty and noisy.
The video was shot inside a smaller attic that is connected to a larger attic. This is inside a large home with a hip roof shingled with slate roofing material. There are large rectangular vents located along the soffit around the perimeter of the roof eaves. The roof peak has ridge vent installed along the entire length of the ridge.
The test was performed on a hot day, the outdoor temperature was about 80 degrees fahrenheit. The attic temperature was 92 degrees at the floor and 109 degrees at the peak.
At the start of the video we are looking from the smaller attic space towards the larger attic space. A smoke bomb has just been set off in the larger attic space and the smoke is starting to be seen flowing from the large attic to the small attic. The power vent is already running behind the camera in the hip end of the small roof.
As we progress we pan around the attic looking for signs of air being sucked in from other sources other than the attic spaces. Particularly the ridge/peak of the attic with the ridge vent. Extra time is spent looking between the rafters at the peak to see if the cobwebs are moving, which would indicate air being sucked in from outside at the ridge vent.
As seen there is no indication of air coming from the ridge vent which means there is no "short circuit " or disruption of airflow.
What is seen is the smoke, particularly visible against the beam of the flashlight, from the far attic moving into and through the small attic and being exhausted through the powered attic fan.
Attic Ventilation Video
Conclusion : A powered roof attic fan will help to exhaust hot air from your attic, providing the attic has adequate soffit vent intake, with or without ridge vents.
A simple, and wrong, roof venting analogy often used as an example when trying to support the reasoning for closing off roof vents when installing ridge vents
Think of your roof ventilation system as a drinking straw. The top of the straw being the ridge , or highest, roof vent that exhausts the air from an attic. The bottom is the soffit or eave vent that allows cool replacement air into the attic.
Now, cut a hole between the top and bottom of the straw and let that represent any other vent on the roof. It could be a powered attic ventilator, or a static roof vent, or maybe a gable end louvered vent.
Now place the straw in a glass of water and try to get a drink. Practically impossible. Air is sucked in through the hole in the straw and you go thirsty. The water stays in the glass.
This analogy would be useful if we were trying to suck water into the attic through the soffit vents.
But most of us, maybe all, don't live in a house surrounded by water that we want to suck into our houses. We prefer to keep the water out.
So, instead of trying to suck water into the straw, try putting the end of the straw into "air".
Now,place a finger over the bottom of the straw and inhale. Feel the suction?
No, it won't be quite as strong as if there were no hole higher on the straw, but, if that middle hole were restricted in any way, say a cap or screen or even negative pressure from wind blowing over the outside of the straw, the difference in suction would not be significant.
Attic and roof ventilation in the real world. How it works.
The pretty pictures and diagrams you see on the ventilation manufacturers websites are a far cry from everyday construction reality.
For instance, below is a photo taken in the middle of winter after a winter storm. About 12 inches of snow have have fallen in the past 4 days in this area. The roof is a hip style on a contemporary home near Cleveland, Ohio. The roofing material is Ludowici round butt Lanai flat tile with a weathered texture. The house is about 70 years old and has had annual ice dams in winter caused by poor insulation and inadequate attic / roof ventilation.
The interesting thing in this photo is how it shows what really happens when ridge vents are mixed with static (mushroom, turtle, etc.) vents on a roof for attic ventilation.
According to many roof ventilation manufacturers websites, the static vents should be removed to prevent a "short circuiting" of the ventilation airflow.
As seen below, heat is coming out of the attic from both the lower static vents and the ridge vent. The ridge vent here is a custom installation consisting of ventilation material over a hole cut in the ridge roof boards which was then covered by the roof tile.
There is melting snow all along the sides of the ridge and around the lower roof vents.
If there were any "short circuiting" of the ventilation system, the area around the lower vents would be dry and either covered with snow or there would be a drift of snow entering the vent opening. The additional ventilation lower on this roof is helping to exhaust the attic heat and not reducing it's effectiveness.
Attic Ventilation – Debunking the “Short Circuit” Myth | Save time and money by keeping your existing vents and adding ridge and soffit vents.
If your home has existing roof vents such as gable end or “turtle” vents, a majority of roofing contractors will advise you to remove or close those vents when installing new ridge and soffit venting.
The reason? An erroneous belief that air will circulate between the ridge vent and any other static vent, instead of removing the warm air inside your attic. Thus, allowing hot air to remain in the attic space and in effect, “short circuiting” the ventilation system. This reasoning flies in the face of the laws of physics and common sense.
This video demonstrates what really happens when you mix ridge and gable vents.
On a hot summer day, your attic needs all the ventilation it can get.
First off, the “short circuit” myth can only be applied to a breezy day or mechanically induced air flow. On a hot calm day the hot air in your attic will rise according to the laws of physics. If there are more vents available to exhaust this hot air, the cooler the attic. The air conditioning will run less in the house, and there is less heat damage caused to the roofing shingles.
Basic info.
Gable vents are cut into the end walls of the gable near the peak of the roof. Power vents are large electric fans capable of moving large amounts of air and are usually controlled by a thermostat or a combination of thermostat and humidistat. Ridge vents are cut into the topmost portion of the house roof and usually extend the length of the roof peak. Soffit vents are installed beneath the eave where the wall meets the overhang. (If the house was designed with an overhang) This allows air to move from the lower edge of the roof, through the attic, and out the ridge vent.
The path of least resistance is not what they think.
The basic premise of the “short circuit” myth, is that the path of least resistance or unobstructed air flow, between the ridge vent and any other source of intake air, is between the ridge vent and the upper static/turtle vents or gable end vents.
What the “short circuit” claim disregards is that there is resistance at the gable ends and roof top air vents. The same wind induced low pressure that is pulling air from the ridge vent is also pulling air from the other vents. Both static and idle power vent housings. The wind rolling off the gable edges creates low pressure at the ends of the house which then pulls air from the attic through the gable vents. The actual path of least resistance is the attic full of hot air which is less dense and moving up and out of the attic, and the soffit vent, which in many cases is having air pushed into it by the wind.
Unsupported and unverifiable marketing bunk!
From the early research in the late 30’s that set the standards for attic ventilation ratios. To the attic vent style comparison studies of Hinrichs in the 60’s and the recent research by Tenwolde and Rose at Building Science. In my research I have not been able to find any scientific study or research paper that supports the “short circuit” myth. It seems to come from misconceptions regarding the Bernoulli and Venturi effects, and by marketing associated with manufacturers of ridge vent systems.
About the video
In the above video, I have attempted to replicate a simple gable roof house and attic ventilation system and to illustrate the actual wind flow patterns associated with active wind induced low pressure.
The house; a simple gable roof constructed of corrugated cardboard. The ridge is cut out for venting and a thin filter fabric is inserted to simulate typical ridge vent filters. The cap has been left off, a; to accentuate and make readily visible the air flow. b; I figured it was just too hard to fabricate a tiny metal/plastic ridge vent. An air baffle has been added typical of some ridge vents. This style does seem to work better than those without baffles. There is also a power vent installed that will be discussed in a separate article.
Burning incense sticks placed on a tray and inserted into the attic simulate hot attic air. The wind comes from my forced air furnace. The furnace draws air from the basement staircase through the whole of the basement similar to a wind tunnel. The house is positioned so the wind is blowing perpendicular to the roof eave and ridge. At the soffit, the wind is blowing against the house and pushing to the side, rising up and over the roof eave and blowing into the soffit area. This positive pressure at the soffit helps to move air into the attic as the ridge and gable vents draw air out of the attic. As the wind hits the baffles, it rises up and creates vortices, or low pressure, that pulls the air from the attic space. The wind is also flowing around the house and rolling off the gable edges creating the same low pressure effect which then pulls air from the attic through the gable vents. The “path of least resistance” then becomes the lower soffit vents.
Clearly visible in the video is the stream of smoke that is being drawn out of the attic through the gable vent. There is no disruption of the ventilation system and the hot air is exhausted from the attic space.
When it comes to attic ventilation, the “short circuited venting system” is like an urban myth or sightings of Bigfoot. Anecdotal evidence and reported sightings from friends of friends who read the information on an internet forum.
So when it comes time to install a new roof on your home, don’t be mislead into thinking you need to close up your gable end vents when installing ridge vents and soffit vents. Install all three and if your ridge vent becomes clogged (it can happen) or is installed poorly. You can rest assured that your homes’ attic is still being ventilated by the old gable end vents.
The attic and gable vents. The roof has an 8/12 pitch. Triangular gable vents cut into both gable end walls.
The continuous soffit vent at the roof eave. Both eaves are vented with net free area equal to or larger than the ridge vent.
