A Corsi-Rosenthal Cube. This cube was constructed with 4 MERV-13 filters and features a cardboard bottom.

The Corsi-Rosenthal Cube (sometimes called a Comparetto Cube) is an inexpensive, do-it-yourself air cleaner that can be easily constructed out of a box fan and MERV-13 furnace filters. The Corsi-Rosenthal Cube can give whole-room air cleaning performance comparable to commercial HEPA air cleaners that are 10x or more the cost. Total cost is around $100.

Construction guide

What's needed:

A visual guide to constructing a Corsi-Rosenthal cube with a box fan and MERV filters. This design will have the fan pointing to the side rather than up. Photo CC-BY al.hu.
  • 4 or 5 MERV-13 furnace filters. You will want to purchase 20" sized MERV-13 filters. For the 3M Filtrete brand of furnace filters, this is "FPR 1900." If MERV-13 is not available, MERV-11 filters can be used but performance will not be as great. Important note: see the notes on MERV-13 filter tests below for brands to potentially avoid.
  • A 20" box fan
  • Duct tape
  • Some cardboard

How to construct:

  1. Duct tape the filters together, forming an incomplete cube. Try to avoid taping over the filter media part of each filter. 
  2. When taping the filters together, make sure to arrange each filter so that the air intake direction of the filter goes inward. The filters should indicate which direction the air is supposed to flow. So you want each filter's airflow direction to point into the cube, not out.
  3. There should be two empty sides of your incomplete cube. The box fan will go on one of these empty sides and the cardboard will go on the other.
  4. Cut your piece of cardboard to fit over the bottom area of the cube, where the cube will sit against the ground. Duct tape it to the bottom.
  5. Duct tape the box fan to the top of the cube. You want the fan to blow air out (not into) the cube. You can place the fan on the side, rather than the top, of the cube (illustration shows the fan on the side). Having the fan point directly up into the air is a bit better because it's less obtrusive.
  6. Optional: for increased performance you can create a 'fan shroud'. Cut a piece of cardboard so that the circle created by the fan blades is exposed by the edges of the fan are covered (see diagram or these instructions). You can also use duct tape on the fan to create this 'shroud' as well.

Filter brands to potentially avoid

Variations

A 5-filter version of the Corsi-Rosenthal Cube. The cube is raised slightly off of the ground to allow air through the bottom filter.

5-filter version

Rather than placing a piece of cardboard on the bottom side of the cube, another filter can be added in place of the cardboard.  In this case, you would want to raise the cube off the ground (without blocking the entire bottom filter) so that the bottom filter could be utilized.  This would in theory lead to even less resistance on each of the filters, potentially prolonging the life of the cube or increasing the air output.

Two filter "wedge" design. 

A two-filter "wedge" variation of the DIY box fan air cleaner. 

A variation of the air cleaner can be made using two MERV-13 filters (add details). This version is slightly less expensive (2 less filters are needed) and has a slightly smaller footprint.

One filter design, the "classic" DIY box fan air cleaner design.

You can duct tape a single filter to the back of a box fan. In this case there is  more strain on the fan motor, more air resistance and the air cleaning efficacy would not be as great. You would also have to replace the filter on the back of the fan more often. But this design has the advantage of having a low footprint and being the least expensive option.

A Corsi-Rosenthal cube featuring a Lasko 20" high velocity floor fan rather than a box fan. The fan's base was removed and a considerable amount of duct tape was used to seal the fan to the top of the cube. 

Using a non-box fan

With some effort (and potentially a lot of duct tape or custom woodworking), you can make a cube using a higher-CFM floor fan. The advantage of this design is that these floor fans put out a lot more CFM than box fans, so the unit will clean the air faster or be better in larger spaces. The downside is it's harder to construct, considerably louder and the cube is less physically stable.

History

As discussed in his original blog post, the design for the Corsi-Rosenthal Cube was detailed by Jim Rosenthal, CEO of Tex-Air Filters, and was dedicated to Dr. Richard Corsi. Neil Comparetto also detailed a similar design in a popular YouTube video, which he called a "Comparetto Cube."

See also

News / media stories