The Corsi–Rosenthal Box is a proven DIY air purifier that rivals commercial units costing hundreds of dollars. This guide covers everything: what to buy, how to assemble it, and how to get the most out of it.
The science
The design is based on the Corsi–Rosenthal Box, developed by Dr. Richard Corsi and Jim Rosenthal in 2020. It uses negative pressure to force air through HEPA filters on all four sides at once — dramatically outperforming single-filter commercial units at a fraction of the cost.
The fan creates suction that draws dirty room air inward through each filter face simultaneously. Every cubic foot of air passes through HEPA media before reaching the fan.
A cardboard shroud around the fan's base prevents air from bypassing the filters. Without it, unfiltered air would sneak past — the shroud is critical to effectiveness.
Filtered air flows up through the fan grille and back into the room. On medium speed, most rooms get a full air change every 12–20 minutes, depending on room size.
Standard 20"×20"×1" MERV-13 or HEPA furnace filters are sold at any hardware store for $8–15 each. When they look gray and clogged, swap them out and recycle the cardboard.
The secret ingredient
Seriously. A single roll of duct tape — about $6 at any hardware or dollar store — is the only fastener this build requires. No screws, no brackets, no glue gun.
Tape the four filters edge-to-edge into a square cube, seal the seams so no air bypasses the filter media, then tape the cardboard shroud around the fan base. The airtight seal is what makes the design work: any gap you leave is unfiltered air sneaking through.
Foil tape (HVAC tape) is even better if you have it — it holds a cleaner seal and doesn't loosen in humidity. But regular duct tape works perfectly well and is easier to find.
