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What Is a Corsi-Rosenthal Box, Exactly?

The design comes from a simple engineering trick. A single furnace filter clamped to a box fan restricts airflow so much that the fan struggles to pull air through it. Stack four or five filters into a cube around the fan instead, and you multiply the surface area the air can pass through, which drops the pressure the fan has to fight against. The fan moves far more air, and every bit of that air gets scrubbed through MERV-13 media on the way in.

Engineering professor Richard Corsi and air filtration executive Jim Rosenthal came up with the concept in 2020, aiming to give schools, homes, and community spaces a cheap way to cut airborne virus transmission when N95 masks and commercial purifiers were scarce or expensive. The name stuck because both men get credit: Corsi for the concept, Rosenthal for helping refine and popularize the build. It’s also called a box fan filter, a CR box, or occasionally a “Comparetto cube” after a related design, but Corsi-Rosenthal box is the term that’s taken hold.

This is a legitimate air-purifier category now, not a pandemic-era curiosity. Community groups, teachers, and wildfire-prone households still build them because the math works: high airflow through real HEPA-adjacent filtration, at DIY prices.

What Do You Need to Build One?

Get the parts right and the build takes under an hour. Get them wrong, and you’ll either waste money or end up with a fire hazard.

• Filters: Four or five 20x20x1 or 20x20x2 inch MERV-13 filters (or higher). Two-inch filters trap slightly more and last longer, but they cost more and make a bulkier cube. Never go below MERV-13. Lower ratings won’t catch fine particulate matter or smoke effectively.
• Box fan: A 20-inch box fan with a three-speed setting. Look for a model manufactured in 2012 or later, per safety testing standards. Older box fans have been linked to fire risk and lack modern thermal safeguards.
• Cardboard: Enough for a floor panel and a shroud (explained below). A flattened moving box works fine.
• Duct tape: At least one full roll, sometimes two. This is not a place to skimp.
• Basic tools: Scissors or a utility knife, and a straightedge for clean cardboard cuts.

Skip anything scavenged from a garage sale: a fan with frayed cord insulation, a wobbly motor, or a burning smell on startup is not worth the money you’d save. Nonstandard filter sizes (anything other than 20×20) will leave gaps that no amount of tape fixes cleanly.

How Do You Build a Corsi-Rosenthal Box Step by Step?

Once your materials are in front of you, the build itself is mostly tape and patience. Here’s the sequence UC Davis engineers recommend:

1. Inspect the fan first. Plug it in alone and listen for grinding, smell for burning, and check that all three speeds work before you build anything around it.
2. Check filter arrows. Every filter has a printed arrow showing airflow direction. Arrows must point inward, toward the center of the cube, so the fan pulls air through the media rather than fighting against it.
3. Stand four filters on their edges to form a cube, pleats facing vertically. Tape each seam where two filters meet, running tape top to bottom on both the inside and outside edges.
4. Cut a cardboard floor slightly larger than the cube’s base and tape it to the bottom edges of all four filters, sealing every gap.
5. Set the box fan on top, blowing outward (check the fan’s own airflow arrow, usually molded into the frame). Center it over the open top of the cube.
6. Tape the fan frame to the filter cube around its entire perimeter. Any unsealed gap here is unfiltered air sneaking past your setup.
7. Build a shroud. Cut a cardboard collar that extends a few inches above the fan’s corners and tape it around the fan’s edge. Box fans are square, but their internal blades spin in a circle, so the corners pull in unfiltered air unless a shroud blocks that gap.
8. Run the final seal check. Look for daylight or airflow at any seam, then tape again.

Expect about half an hour to build a box the first time, faster on your second attempt.

Pro Tip: Before you call it done, run a lit incense stick slowly along every taped seam with the fan on medium. If the smoke gets pulled sideways into a seam instead of drawn straight through a filter, that seam is leaking. Add tape or an extra strip of cardboard until the smoke behaves.

Does a Corsi-Rosenthal Box Really Clean the Air?

Yes, and the numbers back it up better than most DIY health hacks. UC Davis testing on a five-filter build found a clean-air delivery rate, or CADR, that increases with fan speed to a high level rivaling many commercial purifiers. CADR measures how much filtered air a device pushes out per minute. A rate in that range competes with, and often beats, purifiers costing several hundred dollars.

The cost math is the real story: that same UC Davis analysis put the cost at roughly $0.08 per unit of CADR, about a tenth of what many commercial HEPA purifiers charge for the same clean-air output.

Four-filter builds test slightly lower than five-filter versions since they have less total filter surface area, but they’re cheaper and easier to move.

What you give up is footprint and volume. A finished cube is roughly two feet on each side, visually blunt compared to a sleek tower purifier, and a box fan on high speed can be noticeably louder than a purpose-built HEPA unit. If you need something quiet for a bedroom or a compact unit for a small apartment, comparing commercial air purifier brands may serve you better. If you need to filter a large room fast and cheap, a Corsi-Rosenthal box usually wins.

How Many Boxes Do You Need for Your Space?

Sizing comes down to matching CADR to room volume and how many air changes per hour (ACH) you want. Health researchers generally aim for four to six ACH in occupied spaces during a pollution event.

The math: multiply your room’s length, width, and height in feet to get cubic footage, then figure out how many minutes it takes your CADR to move that volume once. A 400 cubic foot bedroom filtered by a 600 CADR box gets multiple full air changes per hour easily.

• Small bedrooms typically can be served by one four-filter box.
• Larger living rooms may need one five-filter box on high or two four-filter boxes on medium.
• Classrooms generally require two to three boxes, positioned on opposite sides of the room rather than clustered together.

Place boxes away from walls and corners so air can circulate freely on all sides, and avoid pointing multiple units directly at each other, which just recirculates the same air. For more on matching output to square footage, see this guide to air purifier coverage.

How Do You Keep a Corsi-Rosenthal Box Safe to Run?

Safety mostly comes down to the fan, not the filters. Puget Sound Clean Air Agency recommends fans manufactured in or after 2012, since older models have a documented history of overheating. Inspect your fan periodically for unusual motor noise, a hot-to-touch housing, or any burning smell, and unplug it immediately if you notice any of those.

• Never run a box fan on a frayed cord or through a damaged extension cord.
• Check filters visually every few weeks; darkening or visible debris buildup means it’s time to swap them.
• Replace filters after any heavy smoke event even if they look fine, because fine particulate can load a filter faster than dust does.
• Store spare filters flat and dry, away from moisture that can warp the cardboard frame.
• Keep the running unit away from unsupervised toddlers and pets, since the fan grille, while safer than an open blade, still poses a pinch risk.

Pro Tip: Write the build date on the cardboard floor in marker. It’s an easy way to track filter age without relying on memory during a smoky week when you’re not thinking clearly about maintenance schedules.

What Are the Most Common Build Mistakes?

Most performance complaints trace back to one of a handful of assembly errors, all fixable in minutes.

• Reversed filter arrows force the fan to pull air against the filter’s intended pleat direction, cutting efficiency and straining the motor.
• Skipped shroud leaves the fan’s round blade sweep uncovered at the square corners, pulling in unfiltered air that never touches the filter media.
• Under-taped seams are the single biggest CADR killer; even a gap the width of a pencil measurably drops airflow through the filters instead of around them.
• Running on the lowest speed by default feels quieter but moves noticeably less air; two units on medium often beat one unit on high, for less noise per box.
• Skipping the incense or smoke test means leaks go unnoticed for weeks.

Pro Tip: If noise is the main complaint, add thin rubber furniture pads under the fan’s base to cut vibration transfer to hard floors, and try running two boxes on medium speed rather than one screaming on high.

When Does a Corsi-Rosenthal Box Make the Most Sense?

The best use cases share one trait: a need for a lot of clean air, fast, without a long-term budget for commercial equipment.

• Wildfire smoke events, when outdoor particulate spikes and you need coverage across multiple rooms at once, are the classic case. See our guide to wildfire smoke cleanup for pairing strategies.
• Classrooms and shelters benefit from quick, cheap deployment across many rooms, per the Corsi-Rosenthal Foundation’s build instructions.
• Temporary pollution spikes, like a smoky evening or a renovation kicking up dust, are ideal short-term jobs for a CR box.

What it isn’t: a replacement for whole-house HVAC filtration or a permanent living-room fixture if you care about noise and appearance. Treat it as a powerful, disposable-feeling tool for acute problems, not a forever purifier.

Where to Verify These Build Instructions and Numbers

For the official build steps, go straight to UC Davis’s engineering guide and the Corsi-Rosenthal Foundation’s instructions. For performance data and cost-per-CADR figures, the Wikipedia summary collects the UC Davis study results in one place. For safety rules on fan age and wiring, check Puget Sound Clean Air Agency’s DIY filter guidance. Peer-reviewed contaminant data is available through this non-targeted analysis study.

Sources

• Science in Action: How to Build a Corsi-Rosenthal Box
• Corsi–Rosenthal Box
• Effects of Corsi-Rosenthal boxes on indoor air contaminants: non-targeted analysis
• DIY Air Filter | Puget Sound Clean Air Agency

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