Quick answer: A Corsi-Rosenthal box is a DIY air purifier made from a 20-inch box fan, four or five MERV 13 furnace filters, and tape. It costs a fraction of a HEPA machine, moves more clean air than most of them, and takes about 15 minutes to build. The filters to buy: 20x20x1 or 20x20x2 MERV 13 pleated.
Invented during the pandemic by engineer Richard Corsi and filter maker Jim Rosenthal, the CR box became the go-to for classrooms, wildfire season, and anyone who needed serious air cleaning on a budget. Peer-reviewed testing keeps confirming the same thing: it works.
Why a box of furnace filters beats a cheap purifier
Air cleaning is a simple equation: how much air moves through how much filter. A consumer HEPA purifier pushes a small amount of air through a very fine filter. A CR box pushes a huge amount of air through four large MERV 13 filters. Per dollar, the CR box usually delivers more clean air per hour than anything you can buy in a store — often 2 to 3 times the clean air delivery rate of purifiers costing several times more.
MERV 13 captures the particles that matter: wildfire smoke PM2.5, pollen, dust, mold spores, bacteria, and many virus-carrying droplets. (For the full rating breakdown, see what MERV rating do I need.)
What you need
- One 20-inch box fan (the standard hardware-store kind, ideally 2020 or newer for the safety-tested motor)
- Four 20x20 MERV 13 pleated filters — 1-inch works; 2-inch lasts longer and flows better
- One piece of cardboard for the bottom (the fan's own box works)
- Duct tape or painter's tape
A five-filter build swaps the cardboard bottom for a fifth filter — more filter area, slightly better performance. Both work.
How to build it (15 minutes)
- Check the airflow arrows. Every filter has an arrow on the frame. All four arrows must point INTO the box, because the fan pulls air through the filters and blows it out the top. Arrow direction trips everyone up — same rule as your furnace; see which way does an air filter go.
- Tape the four filters into a square. Stand them on edge, corner to corner, arrows pointing inward.
- Tape the cardboard to the bottom. Seal the edges — leaks are lost performance.
- Set the fan on top, blowing UP. Tape it down and seal the corners.
- Optional but worth it: tape a cardboard "shroud" ring over the fan face, cut to the blade circle. It stops air short-circuiting at the corners and adds a real efficiency bump.
Run it on the highest speed the noise lets you tolerate. Medium on a CR box still outperforms most purifiers on high.
When a CR box earns its keep
- Wildfire smoke events — pair it with your HVAC running a MERV 13; here's how filters handle wildfire smoke
- Allergy season in bedrooms — run it while you sleep
- Rooms without HVAC coverage — garages, workshops, dorms, classrooms
- When someone's sick at home — extra air changes in the sick room
How long do the filters last?
In normal use, replace CR-box filters every 4 to 6 months. In heavy wildfire smoke, check monthly — when the pleats look gray and loaded, swap them. Filters are the whole machine here, so keeping spares makes sense: shipping at Ironside costs the same on 1, 2 or 3 packs, so ordering the rebuild set with your furnace filters is the efficient play.
Every Ironside MERV 13 is made in the USA and tested to ASHRAE 52.2 — grab 20x20x1 MERV 13, browse Maximum Protection, or find any size with the filter size finder. Built here. Breathe better.
Frequently asked questions
Do Corsi-Rosenthal boxes really work?
Yes. Independent testing, including EPA research, shows CR boxes deliver clean-air rates that match or beat commercial purifiers costing far more. They're used in classrooms and clinics for exactly that reason.
What MERV rating do I need for a CR box?
MERV 13 is the standard — it catches smoke, allergens, and virus-carrying particles while still flowing well through four filters. MERV 11 works in a pinch for dust and pollen; going above 13 adds resistance without much benefit in this design.
Should I use 1-inch or 2-inch filters?
Both work. 2-inch filters have more media, so they last longer and flow slightly better; 1-inch filters are cheaper and easier to find in 20x20. If you'll run the box daily, 2-inch is worth it.
How loud is a CR box?
On low or medium, it's a steady fan hum most people sleep through. It's quieter per unit of clean air than most purifiers, because it moves air through a huge filter area instead of forcing it through a small one.
How often do I replace the filters?
Every 4–6 months in normal use, or as soon as the pleats look loaded during smoke events. Dating the frames with a marker when you build makes this easy.