DIY Corsi-Rosenthal Box: Cost, Construction, and Performance Analysis

Volume I  ·  May 2026  ·  1,098 words

The Corsi-Rosenthal box — a cube of four or five MERV 13 furnace filters taped to a 20-inch box fan — emerged from the COVID-19 pandemic as a low-cost air cleaner design validated by university and EPA testing. At $50–80 in materials, it delivers a smoke CADR of 300–450 CFM, comparable to commercial HEPA purifiers costing $200–400. This article analyzes the performance data, material selection, construction considerations, and the circumstances in which a Corsi-Rosenthal box is — and is not — the appropriate choice.

Design and Construction

The standard design uses four 20×20×1-inch MERV 13 furnace filters forming the vertical walls of a cube and a 20-inch box fan as the top face, pulling air through the filters and exhausting upward. A fifth filter can replace the cardboard bottom panel to increase filter area at the cost of structural rigidity. Filters are joined with duct tape or painter's tape at the edges, and the fan is sealed to the filter edges to prevent bypass. The shroud — the cardboard or plastic panel between the fan blades and the filter edges — is the critical construction detail: without a shroud, the fan recirculates a significant fraction of air that has already passed through the blades, reducing clean air delivery by 30–40%.

CADR Measurements

Independent testing at UC Davis, Brown University, and the EPA has measured Corsi-Rosenthal box CADR values ranging from 250 to 450 CFM for smoke, depending on the fan model, filter count, and shroud design. Key findings:

• A four-filter design with a Lasko 20-inch box fan and an optimized shroud achieved 307 CFM smoke CADR in UC Davis chamber testing — equivalent to a $250–300 commercial HEPA purifier.
• A five-filter design (adding a bottom filter) increased CADR by 15–20% but required more floor space and tape joints. The marginal CADR gain has diminishing returns.
• Fan speed setting has a nonlinear effect: high speed delivers approximately 3× the CADR of medium speed, and low speed delivers approximately one-third of medium. The energy cost of high-speed operation is approximately 50–60 watts.

Filtration Efficiency: MERV 13 vs HEPA

MERV 13 filters are rated to capture ≥ 90% of particles 1.0–3.0 µm and ≥ 50% of particles 0.3–1.0 µm (per ASHRAE 52.2). HEPA H13 filters capture ≥ 99.95% of particles across all sizes. The single-pass efficiency difference is significant — but single-pass efficiency is not the relevant metric for a recirculating air cleaner. A Corsi-Rosenthal box processes room air at 300–450 CFM; the fraction of particles removed per pass is lower than HEPA, but the higher airflow partially compensates. The EPA compared a four-filter Corsi-Rosenthal box to a commercial HEPA purifier in a COVID-19 aerosol challenge test and found the two reduced particle concentration at comparable rates, with the Corsi-Rosenthal box achieving 85–95% of the HEPA unit's reduction within 30 minutes.

The filters used in testing — typically 3M Filtrete 1900 MERV 13 — are electret-treated, relying on electrostatic charge for a portion of their efficiency. As the charge dissipates over months, efficiency at sub-micron particle sizes declines from approximately 70% to 35–50%. This decline is gradual and does not produce a sudden failure, but it means a Corsi-Rosenthal box's effective CADR decreases over its filter life more rapidly than a mechanical HEPA filter's.

Cost Comparison

At $90–120 initial cost and $60–80/year in filters, the Corsi-Rosenthal box is cost-competitive with the Coway Airmega 1512 ($200 purchase, $45/year filters) over a five-year horizon: Corsi-Rosenthal TCO of $390–520 versus Coway TCO of $425. The cost advantage narrows over time because the higher annual filter cost of the Corsi-Rosenthal box compounds.

When to Use — and When Not to Use

The Corsi-Rosenthal box is an evidence-based option for: temporary high-pollution events (wildfire smoke season, prescribed burns), budget-constrained households that otherwise would have no filtration, large shared spaces (classrooms, community centers) where multiple units are needed at low cost, and rapid deployment during air quality emergencies.

It is not the appropriate choice when: noise is a primary concern (box fans at high speed produce 55–60 dBA — 15–20 dB louder than a quality HEPA purifier on equivalent CADR setting); aesthetics or floor space are constrained; automated operation is required (no air quality sensor integration, no timer, no auto mode); or the user has severe respiratory disease where every percentage point of filtration efficiency matters. In these cases, a commercial HEPA purifier with verified CADR, quiet operation, and sensor-driven automation justifies its higher purchase price.