Uploaded August 2026 | Updated September 2026, 3 weeks ago
Window screens seem to be rather restrictive to air flow. Many fan guards seem to be restrictive to air flow. I've long wondered if having a window screen in front of a window exhaust fan would reduce the performance of a fan.
I have an 18" Robbins and Meyers window fan downstairs, mounted in a window that typically has a screen on it. Since the fan is blowing out and the rest of the window is sealed off, the screen isn't really necessary as no bugs are going to be able to come in through the fan during operation.
I set up a real-world use case of the fan drawing air through the entire house and used a doorway as a restriction point to measure the CFM, then took measurements both with and without the screen in place.
Surprisingly, the presence of the screen had little effect on the fan whatsoever. There was less than a 100 CFM difference observed during the test.
We'll need to revisit this test with some other fans as I tend to think we will not get the same results with all fans. This particular fan is very good quality and is designed to build good static pressure, allowing it to successfully force the air through the window screen without significantly reducing performance. I suspect something of lesser quality build like a Lasko 3733 would not be able to compete with the window screen so successfully.
On the outside of the building, there was a significant difference noticed. The velocity of the air coming out of the fan was considerably reduced once the screen was in place. The air stream went from a concentrated, forceful tunnel about the size of the fan to a much slower, spread out air flow across the entire window screen.
While the screen's presence didn't have a significant impact on the overall volume of air moved, I tend to think this was largely because the surface area of the window's screen was larger than the fan blades, allowing the air to spread out and still move at the same volume; a lower velocity but over a larger surface area.
Window screens seem to be rather restrictive to air flow. Many fan guards seem to be restrictive to air flow. I've long wondered if having a window screen in front of a window exhaust fan would reduce the performance of a fan.
I have an 18" Robbins and Meyers window fan downstairs, mounted in a window that typically has a screen on it. Since the fan is blowing out and the rest of the window is sealed off, the screen isn't really necessary as no bugs are going to be able to come in through the fan during operation.
I set up a real-world use case of the fan drawing air through the entire house and used a doorway as a restriction point to measure the CFM, then took measurements both with and without the screen in place.
Surprisingly, the presence of the screen had little effect on the fan whatsoever. There was less than a 100 CFM difference observed during the test.
We'll need to revisit this test with some other fans as I tend to think we will not get the same results with all fans. This particular fan is very good quality and is designed to build good static pressure, allowing it to successfully force the air through the window screen without significantly reducing performance. I suspect something of lesser quality build like a Lasko 3733 would not be able to compete with the window screen so successfully.
On the outside of the building, there was a significant difference noticed. The velocity of the air coming out of the fan was considerably reduced once the screen was in place. The air stream went from a concentrated, forceful tunnel about the size of the fan to a much slower, spread out air flow across the entire window screen.
While the screen's presence didn't have a significant impact on the overall volume of air moved, I tend to think this was largely because the surface area of the window's screen was larger than the fan blades, allowing the air to spread out and still move at the same volume; a lower velocity but over a larger surface area.
