The short answer is yes, you can use just one filter with a box fan. But tests have found that you can get better performance if you use four filters, thicker filters, and shrouds like in a Corsi-Rosenthal box (CR Box). 

How Effective Is a Single-Filter CR Box?

In one study, researchers tested 13 CR Box configurations. They burned pine needles to create PM2.5 smoke inside a chamber the size of a small room, about 29.3 m³. Then they measured the Clean Air Delivery Rate (CADR), which shows how good the purifier is at removing small particles from the air. 

One setup they tested was using a single MERV-13 filter attached to a box fan. The CADR they got was 111 cfm (189 m³/h). That’s about the same as Sqair on medium.

That means even a simple one-filter design can meaningfully reduce particulate matter in a room, especially compared to having no air purifier at all.

What Happens When You Add More Filters?

When researchers tested multi-filter designs, effectiveness increased a lot. Designs with two filters reached 263 cfm (446 m³/h). With four filters, the effectiveness reached 400 cfm (681 m³/h).

That means a four-filter design often delivers 2-3 times more clean air per minute than a single-filter setup.

Why Adding More Filters Improves Performance

The improvement comes down to airflow and surface area. When you add more filters, you increase the total surface area that air can pass through. With more surface area, there is less resistance, making it easier to move air through the filters.

More airflow through the filter means a higher CADR, which directly translates to faster particle removal from the room. This is why multi-filter designs are especially useful in larger rooms or in situations with lots of pollution, such as during wildfires.

Is There a Filter on the Bottom?

In a standard four-filter CR box design, the bottom is usually covered with cardboard, not a filter.

Some versions use a fifth filter on the bottom instead of cardboard. Both designs can work. The original CR box instructions show cardboard as a simple and low-cost way to seal the bottom so air is pulled only through the side filters.

How About Using a Thicker Filter?

Using more filters is one way to increase the surface area. Another way is to use thicker filters. 

Researchers tested this by comparing a 1-inch MERV 13 filter and a 4-inch filter. In other words, these are the same filters, but one is thicker. 

With the 1-inch filter, effectiveness was 111 cfm (189 m³/h). But with a 4-inch filter, effectiveness was more than double–248 cfm (421 m³/h). That means you can increase performance just by switching to a thicker filter.

The key is airflow resistance. Thicker filters have more surface area, which allows air to pass through more easily. With lower resistance, the fan can move more air through the filter each minute, increasing the CADR. That means faster particle removal and more effective air cleaning in the same room.

What’s the Shroud for?

Another way to improve effectiveness is to add a “shroud.” Shrouds are the circular cardboard (or plastic) cutout that covers the corners of the box fan on top of a CR box. 

Its job is simply to prevent air from coming back inside the front of the fan. Here’s why: Box fans are square, but the spinning fan blade inside is round. Without a shroud, air can come back in the unused corner spaces. That means the fan is pushing some air around the front, rather than in through the filter. If we cover up the corners, that forces the fan to push air through the filters. 

In the study, researchers tested the same DIY purifier setup with and without a shroud. They found that adding a shroud increased CADR by around 40%, depending on the configuration tested. That’s a big boost from a simple design change!

Bottom line: Yes, a single-filter box fan purifier is effective. But if you want more clean air, adding more filters, using a thicker filter, and using a shroud will increase performance.