Step-by-Step: How to Build a Corsi-Rosenthal Box

An Easy, Effective DIY Air Purifier

Building a Corsi-Rosenthal Box is a simple process that requires no special tools or technical expertise. With just a few affordable materials, you can assemble a powerful air filtration system that significantly improves indoor air quality.

Follow this step-by-step guide to build your own Corsi-Rosenthal Box and start removing airborne mold spores, dust, and fine particulates from your home.

🛠 Materials Needed

Before you begin, gather the following materials:

✔ 4 MERV 13 or MERV 14 HVAC filters (20”x25”x1” recommended)

✔ 1 box fan (20-inch standard box fan)

✔ Duct tape (or weatherproof tape for better durability)

✔ Cardboard or foam board (optional for top cover)

✔ Scissors or a utility knife (for cutting tape and cardboard)

🔧 Assembly Instructions

1️⃣ Arrange the Filters

• Stand the four MERV 14 filters upright to form a square box shape.

• Make sure the airflow arrows on each filter are pointing inward.

• This setup ensures that air is pulled through the filters before being exhausted by the fan.

2️⃣ Tape the Filters Together

• Using duct tape, securely seal the filters together along the edges.

• Make sure there are no gaps where unfiltered air could pass through.

• Reinforce each corner with extra layers of tape for a sturdy structure.

3️⃣ Create a Bottom Cover

• Cut a 20-inch square piece of cardboard to fit as the bottom cover.

• This prevents unfiltered air from entering the bottom of the box.

• Tape the bottom cover securely to the filters, sealing all edges.

• (Tip: The side panel from the fan’s original box often works perfectly for this step!)

4️⃣ Attach the Box Fan on Top

• Place the box fan on top of the filter box with the blowing side facing up.

• The fan should sit evenly on top of the four filters.

• Use duct tape to secure the fan to the top edges of the filters, making sure it sits snugly with minimal gaps.

5️⃣ Create a Top Cover for Efficiency

• For better performance, cut a cardboard or foam board top cover with a fan-sized hole.

• This cover helps prevent air from recirculating around the fan instead of being pushed out.

• If using a cover, tape it securely around the edges.

6️⃣ Plug It In & Test the Airflow

• Turn on the fan at medium or high speed.

• Hold a tissue or piece of paper near the filters to ensure airflow is being pulled through them.

• If airflow seems weak, check for gaps or loose tape and seal them as needed.

💡 Tips for Best Performance

✔ Place the CR Box in a central area where air circulation is needed most.

✔ Run the fan on high for maximum airflow, but medium can be used for noise reduction.

✔ Monitor filter condition regularly—if they appear dark or clogged, it’s time to replace them.

A Simple Build with Big Results

With just $50–$100 and 15 minutes, you can build a Corsi-Rosenthal Box that effectively reduces mold spores, dust, and pollutants in your home.