How To Make Home Air Cleaner | Build a Corsi-Rosenthal Box

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Wildfire smoke, seasonal pollen, dust, and everyday indoor allergens all recirculate through your home’s air unless you filter them. The most effective DIY air cleaner—backed by Harvard’s Healthy Buildings program—uses widely available furnace filters and a standard box fan to create what’s known as a Corsi-Rosenthal Box.

What You Need To Build It

The Corsi-Rosenthal (CR) Box requires four or five 20″×20″×2″ MERV 13 air filters, one 20″ box fan manufactured after 2012, duct tape, cardboard for the base, and a cutting tool. The thicker the filter (2 inches is the standard), the more particle surface the air passes through. MERV 13 is the minimum rating that captures wildfire smoke particulates, fine dust, and most airborne pathogens.

  • Filters: Four or five 20″×20″×2″ MERV 13 units. (Five-filter cubes add one filter as the top instead of the fan sitting directly on the cube—both methods work.)
  • Fan: A 20″ box fan, post-2012 manufacture (these have safer blade guards). Remove the front grate and control knob if they block the filter surface.
  • Sealing: Duct tape for all seams. Bungee cords or zip ties can help secure the single-filter method.
  • Base: A piece of cardboard cut slightly larger than the filter cube.

How To Assemble The Corsi-Rosenthal Box

The cube’s success depends on one detail: every filter’s airflow arrow must point toward the cube’s center. Reverse any single arrow and the unit barely filters at all.

  1. Check the airflow arrows printed on each filter. All must point inward when the filters are arranged as a square.
  2. Arrange four filters upright to form a box. Tape the vertical edges together on both the inside and outside seams. Do not let tape cover any of the filter mesh—only the cardboard frames.
  3. Cut cardboard to fit the bottom of the cube. Tape it to all four filter frames to create a sealed floor.
  4. Place the box fan on top, oriented so it blows air upward (away from the cube). Tape the fan’s frame to the filter cube on all four sides, sealing every gap.
  5. Optional but effective: create a cardboard shroud—a ring 18–20 inches in diameter—taped to the fan’s exhaust side to prevent the fan from sucking air sideways rather than through the filters.
  6. Set the fan to its highest speed and run it in the room where you spend the most time. Place the unit at breathing height if possible, away from walls and curtains.

Safety And Common Mistakes

Post-2012 box fans include better blade guards, but you still want the fan on top of the filters, never inside the cube. Never tape over the fan’s motor vents or its control knob—the fan needs airflow to cool itself. If the front grate blocks the filter surface, remove it with the screwdriver that came with the fan; the same goes for a protruding control knob.

Unsealed gaps between filters or between the fan and the filters leak air, drastically reducing clean air delivery rate (CADR). Using MERV 8 or lower filters drops particle capture to almost nothing for the smallest particles.

Single-Filter Alternative

A simpler but less powerful version uses one MERV 13 filter taped to the intake side of the box fan (the back). Lay the filter flat, place the fan on top with its intake facing the filter, and tape or bungee-cord the fan to the filter frame. The airflow arrow must point toward the fan. This method works well for a single room and is easier to store when not in use, but it cleans air roughly a quarter as fast as the four-filter cube because it has less filter surface area.

FAQs

FAQs

What size room does a Corsi-Rosenthal Box handle?

For open-plan spaces, position one unit in each zone where people gather.

How often should I change the filters?

The fan will become noisier and move less air as filters load up—that’s the sign to swap.

Is a Corsi-Rosenthal Box safe to run overnight?

Yes, provided the fan was made after 2012 and the unit is placed on a level, heat-safe surface away from bedding or curtains.

References & Sources