Attic Ventilation Explained:
Homestead Roofing invited Slate Baker from Lomanco Vents to demonstrate — live on camera with a convection display — exactly how different vent filters and screens affect attic airflow, and why the products that look good on a supply house shelf do not always perform the way the packaging suggests.
Most conversations about attic ventilation stay theoretical. You hear about intake and exhaust, the 1:300 rule, and why balance matters. Tracy Bookman of Homestead Roofing and Slate Baker from Lomanco Vents took a different approach. They built a working convection display, called the SmokeHouse, and tested actual vent products live, measuring airflow in real time.
The demonstration setup is simple. A light bulb inside a sealed box provides heat, mimicking the warmth that builds up in any attic. That heat rises and escapes through an exhaust vent at the top, spinning a small fan driven purely by the natural convection current. An anemometer measures the airflow and feeds the data to a laptop. At the bottom, an intake opening mimics the soffit vents that draw cool outside air into the attic. With no obstruction, the system runs at 100 percent airflow capacity. Slate Baker then places real-world vent products over the exhaust opening to measure exactly how much airflow each one costs.
The Screen: A Necessary Tradeoff at 4 to 5 Percent Airflow Loss
The first product tested is a standard aluminum screen, required by fire code in many areas including El Paso County, Colorado where Homestead Roofing operates. The purpose is to prevent burning embers from being sucked into the attic during a wildfire. During the Black Forest fire in Colorado Springs, fire investigators discovered that most homes that burned did not ignite from embers landing on top of the roof. They ignited because embers were pulled in through attic vents by the convection current.
The screen reduces airflow by about four to five percent. That is a small, acceptable tradeoff for a meaningful safety benefit. Slate Baker points out that the screen is the baseline. Everything else they test performs worse, in some cases dramatically worse.
Branded Filters: Losing Half Your Airflow on Day One
Slate Baker tests two different branded vent filters, brand new and straight out of the package. When he places the first filter over the exhaust opening, the anemometer drops and the fan slows visibly. Airflow falls by roughly half. Remove the filter and the fan immediately spins back up to full speed. The same thing happens with the second filter, a different brand with a similar design. Once again, airflow drops by about 50 percent.
Slate Baker makes a point that is easy to miss but critical: this is the filter's performance on day one. Over months and years, dust, pollen, and debris get pulled through the attic and trapped in these filters. The airflow restriction that starts at 50 percent on installation day gradually worsens until the vent is doing almost nothing. Building code requires enough ventilation for ten complete air exchanges per hour, or fresh attic air once every six minutes. A filter that cuts airflow in half means five exchanges per hour instead of ten. And that is before any dust accumulation.
The Mesh Ridge Vent: Two-Thirds Airflow Loss
The next product is a mesh-type ridge vent that used to be popular and is still promoted by some shingle manufacturers. Slate Baker places it over the exhaust opening and roughly two-thirds of the airflow is blocked. The fan barely moves. He explains the fundamental tension: the manufacturer is trying to keep weather out, which is a legitimate goal. But the more weather protection they build into the vent, the more airflow they sacrifice. At a certain point, the product stops functioning as a vent because you have cut a hole in the roof and then plugged it.
The Corrugated Plastic Filter: Nearly Zero Airflow
The final filter is from a widely-installed corrugated plastic ridge vent product. Slate Baker places it over the exhaust and the fan stops almost completely. The anemometer plummets. He tells Tracy to put his hand over the exhaust to feel the heat. Nothing. No heat, no airflow. Then Tracy puts his hand over the unobstructed opening next to it and describes it as a little sauna of hot air pouring out. Slate Baker notes that the material is woven so tightly that if you poured water on it, the water would not even drip through. If air cannot get through, moisture cannot escape. And if moisture cannot escape, it is going to condense inside the attic.
The Intake Side: When Insulation Blocks Everything
Slate Baker shifts the demonstration to the intake side. He places a barrier over the intake opening to simulate what happens when insulation covers soffit vents, one of the most common attic ventilation problems in real homes. In thousands of homes, blown-in insulation was added years after original construction and the insulation contractors covered the soffit vents under a foot of fiberglass or cellulose. From the outside, the soffit vents look fine. From inside the attic, they are completely blocked. Air cannot enter. When Slate Baker blocks the intake on the convection display, the fan on the exhaust side slows to a stop. Without intake, there is no exhaust.
The solution for blocked soffits is to install baffles, thin plastic or foam channels that create an air pathway from the soffit vent up into the attic. This is a relatively inexpensive fix compared to the cost of replacing a mold-damaged roof deck or dealing with widespread condensation damage. But it has to be done before or during a roofing project. The signs of poor roof ventilation guide covers what to look for in your attic. For a detailed walkthrough of ventilation mistakes, the roof ventilation types guide covers each one in depth. Homeowners in the Colorado Springs area can reach Homestead Roofing at homesteadroofingcolorado.com or 719-433-6991. In all other areas click here for Roofing Repairs and Replacement or Call: (702) 620-6514
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