Are Ridge Vents Useless?

You have all seen the lovely attic diagrams illustrating the convection flow of air moving into and out of a ventilated attic that was built with a ridge vent and soffit vents. The illustrations of the classical view of ventilation, denoted with arrows, depicts this story: Warm air in an attic flows out of the ridge vent, and cooler air flows into the soffit vents to replace the air lost at the ridge vent. Unfortunately, this story is a myth. 

First, let’s look at some of the reasons why attics need ventilation: to reduce moisture in the attic space during cold weather, to cool the attic air in hot weather, to cool the roof surface in cold weather to reduce the likelihood of ice dams, and to cool the sheathing in hot weather to prolong the life of the shingles.

In the summer, the attic temperature can be as high as 130°F; the outdoor temperature could be around 90°F. The difference between the indoor and outdoor temperatures could be 40°F. Under these circumstances, convection (thermal buoyancy) could provide some ventilation for cooling the attic air and sheathing.

In the winter, the attic temperature could be 35°F and the outdoor temperature 20°F, so there is only a 15°F difference in the temperatures between indoors and outdoors. In a home with excellent attic-floor insulation, the difference between the attic temperature indoors and the temperature outdoors could be even smaller; therefore, there are far less pressure-driven forces to induce the “classical view” of attic air convection. Moisture from indoor house air will not be removed by convection alone.

Bill Rose is an authority on attic ventilation and was a researcher with the Building Research Council at the University of Illinois; he is the author of Water in Buildings and has recently retired as senior research architect with Indoor Climate Research and Training. Rose was interviewed for a 1993 article that appeared in Energy Source Builder #30, saying, “Our research shows that the role of thermal buoyancy [i.e. convection] in diluting attic air with outdoor air is negligible.” He went on to say, “If a roof had only one type of vent device, I would choose soffit vents because they work well as inlets and outlets.”

Rose’s research showed that most attic ventilation is wind-driven, and that ventilation air enters the soffit on the high-pressure side of the home and exits the soffit at low-pressure side. Rose also noted that “Soffit vents should always be installed whenever there are high vents. High vents, on ridges or gables, will pull air out of the attic. Without soffit vents, make-up air would be drawn through the ceiling, which increases heat loss and adds moisture to the attic.”

Charlie Headrick is an engineer who years ago designed a ridge vent that worked. Headrick, then with Headrick Building Products, built a roof/attic simulation device to test various ridge vents. The model roof was a short section of gable roof with shingles. At the top ridge was a two-inch slot, over which he could place different brands of ridge vents (you can find the complete company brochure on the library page at mayindoorair.com).

While he heated the shingles with lamps to simulate the sun’s warmth, he made air-temperature measurements in his experimental attic. He compared the air-temperature increases in his model attic to the temperature increase caused by different brands of ridge vents. 

As controls, he compared the air-temperature changes in the “attic” with a two-inch, fully open slot in his model (30.9°F increase) to the temperature increases of a two-inch slot completely blocked with duct tape (65.8°F increase). Some roll-type ridge vents did not perform much better than the duct-taped gap; the more popular brands of ridge vent only reduced the temperature increase by about 20°F. His patented (unfiltered) vent reduced the temperature increase by 31.9°F. Headrick sold his business to GAF and, as far as I can tell, the information he gathered through his experiments and his patent has been ignored.

As an indoor air quality professional and a former home inspector, I don’t think that ridge vents are that effective. First, I’ve seen many properties in which installers had never cut adequate slots for ridge vents. In many other properties, believe it or not, there were no slots at all, and the ridge vent was just installed directly over the shingles.

Second, air doesn’t always flow as expected, even if a ridge vent is properly installed. In one home with soffit vents and a ridge vent, a bathroom fan exhausted into the attic near the soffit. When the fan wasn’t running, there was still passive airflow from the bathroom into the attic (which was warmed by the sun) via the fan. I asked the owner to stand in the bathroom below the exhaust fan with my smoke pencil and to direct the smoke into the fan intake grille while the fan was off. I stood in the attic (with the access stairs closed) and watched as smoke from below entered the attic.

Some of that smoke rose up quickly in a laminar sheet about an inch deep between the rafters and hugged the hot sheathing, which was being heated by the sun. That rapidly moving stream of smoke went over the ridge beam and flowed down toward the floor, and little if any smoke leaked into the open slots for the ridge vent. The bulk of the smoke flowed in a turbulent cloud, rising slowly between the rafters; the cloud of smoke crashed into the ridge beam and then also sank to the attic floor. I could not tell whether any of the smoke exhausted through the ridge vent (which had been properly installed). But there was visible mold growth on the attic sheathing—heaviest on sheathing near the soffit into which the exhaust fan directed moist bathroom air. 

Using a smoke pencil, you could try a smoke test at a ridge vent (with the access hatch closed), and let me know what you find.

One owner called me because his moldy attic had been remediated and the mold growth was beginning to reappear, even though he had installed soffit and ridge vents at the remediation company’s advice. He had also sealed the two gable-end vents with sheet plastic because he’d been told that “those are defeating the soffit and ridge vents.” I was at the property on a hot summer day, and the attic was sweltering. I asked him to remove the polyethylene covering the two gable-end vents and within minutes the attic was more comfortable.

Ventilation contractors do not agree with me. They claim that their installations of ridge vents always improve attic ventilation. In reality, it is the improved ventilation caused by the simultaneous installation of continuous soffit vents that improves the ventilation.

Not all vents are created equal. Net Free Area (NFA) is the total open and unobstructed space in a vent that lets air pass through. For a 3-by-6-inch rectangular metal vent, the NFA is close to the calculated area, 18 square inches. The NFA for continuous metal soffit vents is about 9 inches per lineal foot.

The NFA for one brand of roll ridge vent, three-fourths inch nominal thickness, is 17.5 square inches of vent area per lineal foot. Does this roll vent installation meet the advertised NFA after being crushed to about a half-inch or less?

This article is about ridge vents, but since gable-end vents and soffit vents are so important to proper attic ventilation, I am including some info on these vents. Gable end vents can have excellent NFA ratings if built the original way, with wood louvers (best if also with screening to prevent insect entry and hardware cloth to prevent bat entry).  

Some newer all-plastic vents have a very low NFA that can be reduced to zero by painting.

The same thing can happen with continuous soffit vents. 

Principal Scientist of May Indoor Air Investigations, Jeff May is a former educator, contractor, and home inspector. He is author or co-author of five books on indoor air quality, including two editions of My House is Killing Me. He is also a frequent speaker at conferences and in 2018 was inducted into the Indoor Air Quality Association’s Hall of Fame. His services include site investigations as well as phone consultations; he is happy to speak to anyone on the phone without charge if the person was referred to him by a home inspector. Jeff can be reached at 978-649-1055 or jeff@mayindoorair.com.