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DIY Solar Book, home

Part II: Conservation FirstChapter 2. Find the Leaks

Build a Low-Cost Blower Door from a Box Fan

A fan sealed in a doorway panel that makes house leaks visible. It is not a certified airflow test.

Difficulty
Intermediate
Cost
$25–70
published

A blower door is a fan sealed into an exterior doorway so you can put the house under a steady pressure and watch where air sneaks in. A professional rig reports airflow at a standard pressure, often 50 pascals, and that number can be compared with a code target. A box fan in a plywood or foam panel does not. It gives you a repeatable breeze and, if you add a gauge, a pressure you can try to match next time. Use it to find leaks. Do not use the number as a certified air-change rate.

The leaks you are hunting are the ones in the heat-loss guide and the air sealing weekend. The fan makes them obvious. Smoke, in the next guide, makes them visible.

Difficulty

Level
Intermediate
Time
A day to build the panel, an hour to test
Skills
Cutting a panel square and taping a seal

Cost estimate

$25–70

People who already own a box fan spend this on foam board or a scrap of plywood, tape, and weatherstrip. A cheap digital manometer adds money and is worth it if you want before-and-after pressure. It still does not calibrate the fan's airflow.

Materials

ItemQtyNotes
Box fan1A fan you can seal around. More flow reaches a higher pressure in a leaky house, and may still not reach 50 pascals.
Rigid panelCut to the doorwayFoil-faced foam or thin plywood. It has to resist the fan without tearing.
Weatherstrip or closed-cell tapeThe perimeterThe panel-to-jamb leak will dominate if you skip this.
Digital manometerOptionalA tube indoors and a tube outdoors. Without it you can still hunt leaks, you just cannot quote a pressure.
Painter's tape and a helper1 setTo hold the panel while you check the seal.

How to set it up

Pick an exterior door you can block for an hour. Cut the panel to the stop, a little snug, and cut a hole the fan fills. Tape or gasket the fan to the panel so air cannot sneak around the housing. The fan blows out of the house. That puts the house under negative pressure, so replacement air enters through the leaks and a smoke trail indoors bends toward those leaks.

Open the interior doors so the whole house is one volume. Close windows. Turn off bath fans and range hoods so they are not a second, unknown hole. Tape the obvious panel gaps until the only intentional hole is the fan.

If you have a manometer, one port senses the indoors and the other the outdoors, with the outdoor tube sheltered from wind. Wind makes the reading jump. Test on a calm day if you want the number to mean anything. Write down the fan speed and the pressure. After you seal a list of holes, repeat at the same fan speed. A higher pressure means the house got tighter. That comparison is the result this tool can honestly give.

A professional test converts fan airflow at 50 pascals into leakage. Your box fan's cubic feet per minute is not on a nameplate you can trust once it is taped into a panel, and the pressure may not be 50 pascals. Do not divide a guess by the house volume and call it ACH50 for a permit or a sale.

A box fan in a doorwayA panel fills an exterior door. A fan blows out. A tube runs to a pressure gauge. A smoke trail bends toward a leak in the wall.Fan blows outGaugeSmoke finds the leakThis finds leaks. It is not a certified CFM50 test.
The fan blows outward. Smoke inside bends toward the leaks. The gauge tells you the pressure, not the certified leakage.

What you should see

At the attic hatch, at can lights, at the rim joist, and at plumbing stacks, smoke or a tissue will move when the fan is on and sit still when it is off. Those are the holes to seal. A gentle drift at a closed window is a different, often smaller, path. Seal the dramatic ones first and retest. If you chase every outlet gasket before the hatch, the weekend is gone before the big holes are sealed.

If the panel bows hard or the pressure will not stabilize, stop. A foam panel can crack. A house can be too leaky for the fan to build pressure, which is itself a finding: look for a disconnected duct, an open flue, or a window you missed.

  1. Make the house safe to depressurize

    See the safety note. Combustion appliances must not be firing. CO alarms stay active.

  2. Seal the fan into the door

    Panel to the jamb, fan to the panel, fan blowing out.

  3. Walk the house with smoke or tissue

    Hatch, top plates, rim, doors, and any hole you already suspected. Mark what moves.

  4. Record fan speed and pressure

    Then seal the marked holes and repeat. Keep the comparison. Do not present it as a code test.

  5. Open the house back up

    Remove the panel, restore fans, and do not leave a combustion appliance in a state you are not sure how to relight.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.