The Corsi-Rosenthal box is one of the smartest pieces of public-health engineering of the last decade: a box fan, four or five furnace filters and some tape, assembled into a cube that shifts a huge volume of clean air for very little money. It works. We are genuine admirers of the design. But a cardboard-and-tape cube is bulky, loud and leaks air wherever the seams are imperfect. This site explains exactly how a CR box works and why — and then shows you how to keep the science while skipping the cardboard.
What is a Corsi-Rosenthal box?
A Corsi-Rosenthal box (often shortened to CR box) is a do-it-yourself air purifier invented in 2020 by Richard Corsi and Jim Rosenthal during the early pandemic push for cleaner indoor air. The idea is disarmingly simple: take a standard 20-inch box fan, tape four (or five) MERV-13 furnace filters together into an open cube, seal the fan on top as the lid, and switch it on. The fan pulls room air in through all four sides at once, the filters trap airborne particles, and clean air is pushed out the top.
Because air is drawn through a very large filter area rather than one small panel, a CR box moves far more clean air than most people expect from such humble parts — often rivalling purifiers costing many times more. It was designed to be cheap, open-source and buildable by anyone, and that is exactly why classrooms, offices and homes around the world adopted it.
Why do Corsi-Rosenthal boxes work so well?
The magic is surface area. A single-panel purifier forces all the air through one filter face, which creates resistance (pressure drop) and limits airflow. A CR box spreads that same airflow across four or five filters, so each filter sees only a fraction of the load. Lower resistance means the fan can move more air, and more air moved per hour means more of the room's particles captured per hour.
- MERV-13 filtration captures roughly the same fine-particle range as the filters used in hospitals and good commercial buildings — dust, pollen, smoke and many airborne respiratory particles.
- High clean-air delivery from ordinary parts, because four or five filters in parallel beat one filter in series.
- Low running cost, since a box fan draws only a modest amount of electricity.
In short, the CR box is good physics executed cheaply. The science is not in question. The form factor is where DIY builds run into trouble.
The problems with a DIY Corsi-Rosenthal box
We build these ourselves and we recommend them — but honesty matters. A taped cardboard CR box has real drawbacks, and most people discover them only after it is sitting in the corner of the living room.
- It is bulky and unattractive. A cube of furnace filters with a box fan strapped on top is not something most people want on display at home or in a client-facing office.
- It can be loud. Box fans are built for cooling, not quiet running. On the setting that actually moves useful air, many builds sit at a genuinely intrusive noise level.
- It leaks if you tape it poorly. Every unsealed seam is air that bypasses the filters entirely. A wobbly cardboard shroud, a gap at a corner or tape that peels in the heat all quietly reduce the clean-air output you thought you were getting.
- The fan is inefficient under load. A box fan pushing against filter resistance is working outside its comfort zone, which wastes energy and adds noise.
- It is fiddly to maintain. Swapping filters usually means cutting tape and rebuilding.
None of this means the CR box is a bad idea. It means the idea deserves better hardware.
The engineered alternative: keep the science, skip the cardboard
CleanAirKits took the Corsi-Rosenthal principle — big filter area, low resistance, lots of clean air — and rebuilt it as a proper product. The Luggable is a sealed acrylic CR-style box fitted with genuine hospital-grade MERV-13 filtration and an array of low-pressure fans instead of one straining box fan.
The result is the airflow a CR box is famous for, without the compromises:
- Sealed enclosure — no taped seams, so no bypass leaks. Every cubic metre of air actually goes through the filters.
- Low-pressure fans tuned for filter resistance, running under 35 dB — quieter than a whisper-level library, not a box fan on high.
- Genuine MERV-13 capturing about 93% of airborne particles.
- Around £2 a year to run at roughly 15W, and Energy Star Most Efficient 2026 rated.
- Intertek-tested performance, a free Veya app, and shipping across the UK and EU.
It is the Corsi-Rosenthal box as it would have been designed if it had started life in a lab instead of a garage — same physics, better execution.
The range: pre-built or bring-your-own-fans
Whether you want to open a box and switch it on, or you love the DIY spirit and want to build it yourself, there is a version for you.
| Model | Airflow | Room size | Price |
|---|---|---|---|
| Fanless Kit (bring your own 120mm fans) | Depends on your fans | Flexible | £179 |
| 5-Fan Luggable (pre-built) | 369 m³/hr | up to ~450 sq ft | £229 |
| 7-Fan XL (pre-built) | 681 m³/hr | 600–750 sq ft | £279 |
The Fanless Kit gives DIY builders the sealed acrylic enclosure and genuine MERV-13 filtration, then lets you drop in your own quiet 120mm fans for a truly silent, custom build — the CR-box concept, minus the cardboard, entirely on your terms. The 5-Fan Luggable and 7-Fan XL arrive pre-built and ready to run.
DIY build vs pre-built: which is right for you?
There is no wrong answer here — it depends on what you value.
Build it yourself
If you enjoy the making, want to choose your own fans, or want to spread the cost, the Fanless Kit at £179 gives you the hard part done right (a sealed, leak-free enclosure and real MERV-13) while leaving the fun part to you. You get the satisfaction and control of a DIY CR box without gambling your clean-air output on tape and cardboard.
Buy it pre-built
If you just want clean air today, the 5-Fan Luggable at £229 or 7-Fan XL at £279 arrive tested, sealed and quiet, with the Veya app and warranty-grade parts. No cutting, no taping, no guesswork about whether your seams leak.
Both routes deliver the same core promise: Corsi-Rosenthal airflow, engineered.
Running cost and noise: the numbers that matter
Two things put people off a purifier long-term: the electricity bill and the drone. This is where an engineered CR box pulls decisively ahead of a box-fan build.
- Noise: under 35 dB. That is quiet enough to run in a bedroom overnight or a home office all day. A box fan on a useful setting is typically far louder.
- Running cost: about £2 a year. At roughly 15W, leaving it on continuously costs about the same as a couple of coffees over twelve months.
- Efficiency: Energy Star Most Efficient 2026. Low-pressure fans matched to the filters waste far less energy than a box fan straining against resistance.
Cheap to run, quiet enough to forget about, and honest about its performance because it has been Intertek-tested — that is the difference between a clever weekend project and a device you actually keep switched on.
