A Corsi-Rosenthal box is the highest clean-air-per-pound you can get: a box fan pulling air through several MERV-13 filters arranged as a cube. It takes about 30 minutes, no tools beyond scissors and tape, and clears a room as well as purifiers costing many times more. These are the practical steps; for the reasoning behind each choice see what a CR box is.
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CleanAirKits Luggable XL Ultra (7-Fan)
681 m³/h CADR, MERV-13, under 35 dB — powerful whole-home clean air — no proprietary lock-in, ships from the UK.
Check price →Use code CORSI10 for 10% off at checkout.Materials
- 1 × 20-inch box fan (a standard square fan).
- 4 or 5 × 20×20-inch MERV-13 filters (five makes a full cube with more surface area; four plus a cardboard base is the classic).
- Cardboard for the base and the fan shroud (the fan's own box is ideal).
- Strong tape — duct or packing tape.
Step by step
- Check the arrows. Each filter has an airflow arrow. It must point inward, toward the centre of the cube — air is pulled in through the filters and out through the fan.
- Form the cube walls. Stand four filters on their edges in a square, arrows facing in, and tape the vertical seams together into an open-topped box.
- Add the base. Tape a cardboard square (or a fifth filter, arrow up) across the bottom, sealing the edges so no unfiltered air leaks in.
- Fit the fan on top. Sit the box fan on the open top, blowing upward and outward. Tape it down all round so air can only leave through the fan.
- Make a shroud. Cut a cardboard ring that covers the fan's corners, leaving only the circular blade area open. This simple step measurably increases airflow by stopping air recirculating at the corners.
- Seal every seam. Run your hand around each joint with the fan on; tape any gaps where you feel air escaping. A well-sealed box is a strong box.
Safety
Use a fan with a modern (post-2012) fused plug, never leave it running unattended for long periods at first until you're happy it runs cool, and keep it clear of curtains and bedding. Do not block the intake filters against a wall — give the whole cube breathing room.