House Airtightness Testing Using a Duct Blaster

Sealing Vents

Matt Bower seals up the air intake vent for the building's energy recovery ventilator (ERV) as he gets ready for a house airtightness test.

Air-Sealing a Plumbing Drain

This pipe is part of an unfinished drain system that communicates with the outdoors. It has to be air-sealed with tape so it won't affect the airflow measurements during the building airtightness test. "We had to tape off some of the plumbing drains," explains Matt Bowers, "otherwise we would be pressurizing or depressurizing the leach field."

Creating a Custom Duct-Blaster Frame

Using a scrap piece of ZIP System sheathing, Bowers made a frame to fit one of his building's windows. Here, he applies a Pro Clima gasket to a hole in the frame that will be used for manometer tubes.

Gaskets for the Duct Blaster Frame

Bowers applies weatherstripping to the custom Duct Blaster frame. The peel-and-stick weatherstripping sticks to the ZIP panel, and will help seal the assembly to the window frame when the frame is placed in a window opening. The air pressure created by the fan during the test will force the weatherstripping against the window frame.

Building the Frame

Bowers adds another piece of weatherstripping to the test frame. In this view, we can see the Duct Blaster housing taped in place, a handle for pulling the frame tight to the window opening, and two strips of Velcro for attaching the manometer to the frame during the test.

Tape-Sealing the Frame

Bowers seals the Duct Blaster fan to the ZIP sheathing with foil duct-sealing tape.

Air Tube Penetration

The "reference pressure hose" for the manometer has to penetrate through the test assembly panel, so that it can sense ambient outdoor air pressure during the test for comparison with indoor pressure. The hose is open to the outdoors, and plugs into the "reference port" on the manometer. This closeup shows the Tescon VANA gasket used to air-seal that penetration.

Test Assembly Ready to Use

The completed "blower window" test assembly sits next to the window, ready for use.

Removing the Window (1)

Bowers pulls a hinge pin from the tilt-and-turn window.

Removing the Window (2)

Bowers pulls the active light out of the Zola tilt-and-turn window. The window is designed to make this process routine: you just pull the top hinge pin out, tilt the window inward, and slide the active light off the lower hinge.

Setting Up

Bowers feeds the manometer air tube through the gasket in the test assembly.

Placing the Apparatus

Sealing a Window Air Leak

Dogging Down

Bowers snugs the test assembly panel tight to the window by laying an aluminum pole across the window frame and clamping it to the pull handle in the center of the panel. The gasket around the perimeter of the panel forms the seal with the outside of the window frame. During the test, the air pressure will act to hold the panel tightly to the window, compressing the gaskets.

Inserting the Duct Blaster Fan

Test Apparatus in Place

Setting a Baseline

Before starting the depressurization test, Bowers establishes a baseline for the test, noting the difference in air pressure between indoor and outdoor air with the Duct Blaster fan not running. For a valid test, the difference must be within three pascals (as it was in this case). For the test, the manometer will automatically compensate for this baseline pressure differential.

The Gasket Seal

This closeup view shows that the seal between the gasket and the window appears to be good.

"Flow at Fifty"

A view of the manometer readout during the test. With actual pressure at -51.8 pascals, the airflow through the Duct Blaster displays as 52 CFM50. Because achieving a pressure difference of precisely -50 pascals for any length of time is practically impossible given the effect of outdoor wind, the device needs to calculate the standardized result.

Homing In

Bowers fine-tunes the fan speed, trying to get the test pressure as close as possible to -50 pascals. Note the small ring on the Duct Blaster fan: this is the smallest ring available for the device off the shelf. The airflow during this test, about 52 CFM, is much lower than a HERS rater would typically observe when testing ductwork with the device (its ordinary purpose): in his experience, Bowers says, duct systems test at 300 to 400 CFM25.

Setting Up for Pressurization

With the depressurization test complete, Bowers flips the test apparatus around and installs it on the inside of the window, clamping the panel into place from the outside.

Removing the Shroud

Bowers removes the protective shroud from the Duct Blaster fan. The green hose to Bowers' left is the "reference hose" that allows the manometer to sense outdoor pressure during the test.

Set Up for Pressurization Testing

A view from inside the building shows the test apparatus in place and the equipment connected for the test. The white accessory with honeycombed holes in the Duct Blaster ring is a "flow conditioner," which helps prevent outdoor air turbulence from affecting the measurements. Sitting on the windowsill is the manometer, with hoses connected for sensing indoor pressure, outdoor pressure, and fan pressure.

Test Apparatus in Place

Another view of the Duct Blaster fan in its ZIP System panel frame, clamped into the window for the pressurization test.

The Duct Blaster Fan

A closeup view of the Duct Blaster fan in place, with hoses connected and penetrations in the panel sealed with tape.

Ready to Test

Another view of the test apparatus in place.

Pressurization Test Results

With the house under 50 pascals of pressure, the manometer registers an airflow of 65 CFM. The pressurization test and depressurization tests are slightly different because materials in the building envelope may respond differently to pressure from one side or the other (for instance, housewrap that is sucked against a wall may leak less than housewrap that is blown away from a wall). But in each case, the numbers are very low for this house.

Join the Discussion

Please read our Content Guidelines before posting

Close X