How to Build a Corsi-Rosenthal Box: DIY HEPA-Class Air Cleaner

Last updated May 20, 2026 · 8 min read

The Corsi-Rosenthal box is the best price-performance air cleaner most households can build. A 20-inch box fan, four MERV-13 furnace filters, a piece of cardboard, and a roll of duct tape — total cost roughly $80 — produces a clean air delivery rate of 300–600 CFM, outperforming most $400 retail purifiers. The design was published by engineering professor Richard Corsi (now dean at UC Davis) and HVAC veteran Jim Rosenthal during the early-COVID indoor-aerosol crisis, and has since been independently validated by EPA, ASHRAE, and university researchers for both viral aerosol and wildfire-smoke filtration.

Why it works

A retail air purifier moves air through one HEPA filter using a small high-pressure fan. A Corsi-Rosenthal box moves air through five times the filter area (four MERV-13 filters in a cube) using a much larger, much quieter box fan. The math is straightforward: filter resistance is approximately inversely proportional to surface area, so quadrupling area drops the pressure drop dramatically — and lets a low-pressure fan move a lot of air through it. The result is more clean air delivered per dollar, per decibel, and per watt than nearly any consumer-grade purifier.

The EPA's published research found that DIY box-fan filters reduced indoor PM2.5 during simulated wildfire-smoke events to a similar degree as professionally manufactured HEPA cleaners costing 5–10× as much. The CDC published similar data during the COVID-19 indoor-aerosol research push.

Materials list

• One 20-inch box fan. The Lasko model is the most-cited because it's cheap, common, and uses a UL-listed motor. Other 20-inch box fans work as long as they're UL-certified. Don't substitute a smaller fan; the math doesn't scale down.
• Four 20"×20"×1" MERV-13 furnace filters. MERV-13 is the sweet spot — it captures fine particulates (including PM2.5) without dropping pressure too much. MERV-11 works in a pinch with slightly less filtration; MERV-14+ adds resistance the box fan struggles with.
• One piece of cardboard, approximately 20"×20". The fan box itself works (cut from the back panel after unboxing). A new piece of corrugated cardboard works equally well.
• One roll of duct tape — any color. You'll use about 30 feet.
• A box cutter or scissors for trimming cardboard.
• Optional: a "fan shroud" — a cardboard ring on the fan exhaust face that boosts airflow about 20% by reducing turbulence. Marginal but real.

Build steps

1. Arrange filters into a cube

Stand the four MERV-13 filters on edge to form the four vertical walls of a cube. Each filter has an arrow printed on the side indicating airflow direction; all arrows must point inward (toward the center of the cube). The fan will pull air through all four filters simultaneously.

2. Tape the vertical edges

Where each filter meets the next, run a strip of duct tape down the seam from top to bottom. Do this on the outside of the cube. The corners must be airtight — gaps here are the most common build error and the most damaging to performance.

3. Cap the bottom with cardboard

Cut the cardboard into a square that matches the footprint of the filter cube. Place it on the bottom (the floor side). Tape it to the bottom edges of all four filters. The bottom is now a sealed plate; air can only enter through the four filter sides.

4. Mount the fan on top

Place the box fan face-down on top of the filter cube, so the fan blows upward (sucking air through the filters from outside the cube and exhausting up). The fan should sit flat on the top edges of the four filters. If it doesn't quite span the full 20"×20" cube edges, that's fine — tape will close any gap.

5. Seal the fan-to-filter gap

This is the second most important step (after the corner seal). Run duct tape around the perimeter where the fan's housing meets the filter tops, completely closing the gap. Any leak here lets unfiltered air shortcut directly into the room and tanks performance.

6. Optional: add the fan shroud

Cut a ring out of cardboard with the inner diameter roughly equal to the fan blade circle, outer diameter equal to the fan housing. Tape it to the exhaust side of the fan. The shroud reduces back-pressure recirculation and boosts measured CADR by ~15–20%. Skip if you don't have time; the box still works fine without.

What it should look like when done

A roughly 20-inch tall cube of four filters with a fan sitting on top, all seams duct-taped. The fan blows up. Air is pulled in through the four sides. Place the assembled unit somewhere with at least a foot of clearance on each side and at least two feet above. If you put it in a corner, performance drops.

Running it

• Highest fan speed during smoke events. The box is quietest at high speed (compared to small high-pressure purifiers at the same airflow), so noise is rarely a reason to run it lower.
• Door closed for a bedroom application. The smaller the volume the box is filtering, the higher the effective ACH.
• Run 24/7 during smoke events. Filters load slowly; the fan uses about as much power as a 50-watt lightbulb.

When to replace the filters

Look at the filters. When they go from white to gray to dark gray, they're loaded and CADR is dropping. Typical lifespan during normal use is 6–12 months. During a smoke event, filters can load fast — sometimes in a single week. The good news: MERV-13 filters cost $15–25 each, and you only replace them when needed. The fan and cardboard last forever.

Safety and code notes

• Use a UL-listed box fan. Older fans that pre-date UL motor-overload protection have been involved in fires when run continuously. The Lasko 3733 and modern equivalents include thermal cutoffs.
• Don't block the fan exhaust. Self-explanatory — a blocked fan can overheat.
• Don't put it where it can be knocked over onto bedding. Common sense; the fan is heavy.
• Test the smoke alarm in the room. A high-airflow purifier can subtly affect ionization smoke alarms; photoelectric alarms are unaffected.

Versions and variations

• "Mini-CR" with smaller fans — exists, but the price-performance falls off quickly. Stick with the 20-inch box fan version.
• 5-filter pyramid (replaces the cardboard bottom with a 5th filter) — adds marginal CADR at a 20% cost increase. Worth it only if you need every CFM.
• Tower-style with two stacked fans — doubles CADR; useful for large open-plan spaces. Twice the noise.

Primary sources: EPA — Research on DIY Air Cleaners · EPA — Wildfires and Indoor Air Quality · CDC — Wildfire Smoke