Blower Door Testing in Construction: What Contractors Need to Know | Projul

Blower door testing has gone from a niche energy auditing tool to a standard code requirement in most of the country. If you are building new homes or doing major renovations, you almost certainly need to pass a blower door test. And if your buildings are not passing consistently, it is costing you time and money.

This guide covers everything contractors need to know about blower door testing: what it measures, why it matters, how to prepare your buildings, and how to pass on the first attempt.

What a Blower Door Test Actually Measures

A blower door test quantifies the air leakage through a building’s envelope. The “envelope” is the boundary between conditioned (heated and cooled) space and unconditioned space, including exterior walls, the roof or ceiling, and the floor over unconditioned areas.

The Equipment

A standard blower door setup includes:

• A calibrated fan that mounts in a door frame using an adjustable panel
• A digital pressure gauge (manometer) that measures the pressure difference between inside and outside
• Software or tables for calculating results
• A smoke pencil or theatrical fog machine for finding leak locations (optional but very helpful)

The Process

1. Close all exterior doors and windows
2. Open all interior doors (to equalize pressure throughout the building)
3. Close fireplace dampers and seal any intentional openings as specified by the testing protocol
4. Mount the blower door fan in an exterior door frame
5. Turn on the fan and depressurize the building to 50 Pascals (about 0.2 inches of water column)
6. Record the airflow through the fan at this pressure
7. Calculate the result in air changes per hour (ACH50)

Understanding the Results

The result tells you how many times the entire volume of air in the building leaks out and is replaced per hour at the test pressure of 50 Pascals.

ACH50 = (CFM50 x 60) / Building Volume

Where CFM50 is the cubic feet per minute of airflow at 50 Pascals, and building volume is the total volume of conditioned space in cubic feet.

Lower numbers mean a tighter building. Here are some reference points:

• 7+ ACH50: Typical older home with minimal air sealing. Very leaky.
• 5 ACH50: Moderate air sealing. Would fail most current codes.
• 3 ACH50: Current code requirement in many climate zones. Achievable with standard air sealing practices.
• 2 ACH50: Tighter than code. Required by some programs and jurisdictions.
• 1 ACH50 or less: Very tight construction. Typical of Passive House and high-performance buildings.

Why Air Sealing Matters Beyond Code Compliance

Passing the blower door test is a code requirement, but the benefits of a tight building go well beyond checking a box.

Energy Efficiency

Air leakage is one of the largest sources of energy loss in buildings. In a typical older home, air leakage can account for 25 to 40 percent of heating and cooling energy use. Sealing those leaks directly reduces the homeowner’s energy bills.

Comfort

Leaky buildings have drafts, cold spots, and rooms that are hard to heat or cool evenly. A tight building envelope eliminates most comfort complaints and makes the HVAC system more effective.

Moisture Control

Uncontrolled air leakage carries moisture into wall cavities and attic spaces where it can condense and cause mold, rot, and structural damage. A tight envelope with controlled ventilation keeps moisture where it belongs.

Indoor Air Quality

Counterintuitively, a tight building with mechanical ventilation actually has better indoor air quality than a leaky building. A leaky building pulls air through random pathways, including crawl spaces, attics, garages, and wall cavities, bringing contaminants with it. A tight building with a properly designed ventilation system brings in fresh, filtered air through controlled pathways.

Noise Reduction

A well-sealed building envelope also reduces noise transmission from outside. Homeowners in noisy areas notice a significant difference.

Air Sealing: Where to Focus Your Efforts

Not all air leaks are equal. Some locations consistently account for the majority of air leakage in new construction. Focus your air sealing efforts on these areas for the biggest impact.

Top Plates

The connection between the top of walls and the ceiling or attic space is one of the largest sources of air leakage. Every penetration through the top plate (wiring, plumbing vent pipes, HVAC ducts, low-voltage cables) creates a pathway for air to move between the conditioned space and the attic.

How to seal: Apply fire-rated caulk or foam around all penetrations through top plates. For larger openings, use rigid material cut to fit and sealed at the edges.

Rim Joists and Band Boards

Where floor framing meets exterior walls, the rim joist area is a major leakage point. The junction between the sill plate, rim joist, and subfloor creates multiple gaps that allow air and moisture movement.

How to seal: Spray foam applied to the rim joist area is the most effective method. It seals air leaks and provides insulation in a single step. For builders who prefer not to use spray foam, rigid foam board cut to fit and sealed at the edges with caulk or tape works well.

Window and Door Rough Openings

The gap between the window or door frame and the rough framing is a common leakage point. This gap must be sealed to prevent air movement.

How to seal: Low-expansion spray foam is the standard approach. Do not use high-expansion foam, which can bow window frames and interfere with operation. Alternatively, use backer rod and caulk. Some builders use specially designed air sealing tapes around rough openings.

Exterior Wall Penetrations

Every pipe, wire, duct, and vent that passes through an exterior wall creates a potential air leak.

How to seal: Use caulk, foam, or pre-manufactured sealing boots around all penetrations. Pay special attention to plumbing drain and vent penetrations, electrical cable and conduit entries, gas piping, hose bibs, and HVAC line sets.

Recessed Can Lights

Standard recessed can lights are essentially holes in the ceiling connected to the attic. Even “IC-rated” cans allow significant air leakage unless they are also rated as airtight.

How to seal: Specify airtight (AT) rated recessed fixtures. If standard fixtures are already installed, cover them from the attic side with an airtight enclosure and seal around the edges.

Electrical Boxes on Exterior Walls

Outlet and switch boxes on exterior walls allow air to leak around and through the boxes.

How to seal: Use airtight electrical boxes or apply foam gaskets behind the cover plates. For boxes that are already installed, seal around the cables entering the box with caulk or putty.

Plumbing Penetrations

Plumbing drain and vent pipes often pass through multiple floors and the roof, creating vertical air pathways through the building.

How to seal: Seal around all plumbing penetrations at every floor level and at the roof. Use fire-rated sealant where penetrations pass through fire-rated assemblies.

HVAC System

Ductwork, especially in unconditioned spaces like attics and crawl spaces, is a significant source of air leakage. Leaky return ducts in attics can pull hot, humid air into the building, while leaky supply ducts push conditioned air into unconditioned spaces.

How to seal: Seal all duct joints and connections with mastic or approved tape. Consider placing ductwork within the conditioned envelope (inside insulated and air-sealed spaces) rather than in attics or crawl spaces.

Attic Access

Attic hatches, pull-down stairs, and whole-house fan openings are large potential air leaks.

How to seal: Weather-strip attic hatches and pull-down stairs. Install insulated covers over pull-down stairs from the attic side. Whole-house fan openings need insulated, sealed covers for the heating season.

Testing Strategy: Rough-In vs. Final

You have a choice about when to conduct the blower door test, and the timing matters.

Testing at Rough-In

Testing before drywall installation has significant advantages:

• You can see everything. All air sealing work (or the lack of it) is visible and accessible.
• Leaks are easy to fix. Applying caulk or foam to a top plate penetration takes seconds when it is exposed. After drywall, it requires cutting and patching.
• You catch problems early. If a subcontractor missed air sealing on their penetrations, you can address it immediately.
• Cost savings. Fixing leaks at rough-in is dramatically less expensive than fixing them after finish work.

The downside is that some leakage points (like window installation, exterior cladding, and attic access) may not be in their final condition at rough-in. This means the rough-in test may show slightly higher leakage than the final result, so you need to account for that margin.

Testing at Final

Testing after the building is complete gives you the most accurate measurement of the finished building’s air tightness. All components are installed, and the result reflects the actual performance.

The downside is that leaks discovered at final are harder and more expensive to fix. You may need to remove finishes, make repairs, and reinstall finishes, all while trying to maintain the schedule and the client’s patience.

Recommended Approach

Test at rough-in whenever possible. Use the rough-in test to identify and fix problems while everything is accessible. Then test again at final to verify the finished result. This two-test approach costs a bit more for testing but saves significantly in rework costs.

Preparing for the Test

Preparation before the test day makes the difference between passing and failing.

Pre-Test Walkthrough

Walk the entire building the day before the test and check every air sealing location on your list. Look for:

• Missed penetrations that were not sealed
• Incomplete foam or caulk applications
• Gaps at rim joists
• Unsealed rough openings
• Open electrical boxes without gaskets
• Any other pathways between conditioned and unconditioned space

Test Day Preparation

On the day of the test:

• Close all exterior doors and windows
• Open all interior doors
• Close fireplace dampers
• Seal combustion air intake openings on gas appliances (per testing protocol)
• Turn off exhaust fans and HVAC equipment
• Close dryer vent dampers
• Follow the tester’s instructions for any additional preparation

What to Do During the Test

If you are present during the test (which is recommended), use the depressurized building as a diagnostic opportunity. Walk the building with a smoke pencil or incense stick and check for air movement at suspected leak locations. The 50-Pascal pressure difference will make even small leaks visible with smoke.

Mark the locations of any leaks you find so you can address them if the test fails.

When You Do Not Pass

If the test result exceeds the code threshold, do not panic. Most failures are fixable.

Common Failure Scenarios and Fixes

Slightly over the threshold (within 0.5 ACH50): Focus on the easy wins. Seal any penetrations you missed during the pre-test walkthrough. Check electrical boxes, plumbing penetrations, and the rim joist area. A few tubes of caulk can often close the gap.

Significantly over the threshold (1+ ACH50 over): There is likely a larger issue. Common culprits include:

• A section of air barrier that is missing or discontinuous
• A large penetration that was not sealed (like a bathtub drain or tub surround opening)
• Ductwork in an unconditioned space that is not properly sealed
• Recessed can lights that are not airtight rated
• Attic access that is not weather-stripped

Use smoke testing to identify the major leak locations and focus your sealing efforts there.

Way over the threshold (2+ ACH50 over): There may be a fundamental issue with the air barrier strategy. Step back and assess whether the air barrier is continuous and complete. Common systemic issues include:

• No air barrier at the rim joist area
• Balloon-framed walls that allow air to travel from the basement to the attic
• Attic knee walls without an air barrier on the attic side
• Cantilevers or bump-outs that were not sealed to the main building envelope

Building Air Tightness into Your Process

Rather than treating air sealing as a last-minute task, build it into your construction process.

Design Phase

Identify the air barrier location and strategy during design. Know where the air barrier will be on every surface of the building envelope: walls, ceiling/roof, and floor.

Framing Phase

Seal the bottom plate to the subfloor with gasket or caulk during framing. This is much easier before walls are standing.

Before Rough-In Trades Arrive

Seal all penetrations through the top plate and exterior walls after framing but before insulation. Some builders designate a specific air sealing day in the schedule.

During Rough-In

Require each trade to seal their own penetrations through the building envelope, or assign a dedicated person to seal penetrations as they are made.

Before Drywall

Conduct the rough-in blower door test and address any failures before drywall installation.

At Final

Verify the final blower door result and address any remaining leaks.

Tracking and Documentation

Document your air sealing work and test results in your project management system.

Photo Documentation

Photograph air sealing work at key locations before it is covered by insulation or drywall. These photos serve as evidence of your work quality and can be valuable if questions arise later.

Test Reports

Keep copies of all blower door test reports in the project file. Note the result, the tester’s name and certification, and any diagnostic findings.

Lessons Learned

Track your blower door test results over time. If you notice patterns (certain house plans or construction details that consistently produce tighter or leakier results), adjust your approach accordingly.

Project management tools like Projul make it easy to store test reports, photos, and notes in a central location that is accessible to your entire team. Over time, this data helps you refine your air sealing process and pass tests consistently.

Wrapping Up

Blower door testing does not have to be a source of anxiety. With a systematic approach to air sealing, a focus on the major leakage points, and testing at rough-in to catch problems early, you can pass consistently and deliver tight, comfortable, energy-efficient buildings to your clients.

Start by identifying the air barrier location in every project, build air sealing into your construction schedule, and use each blower door test as a learning opportunity. Over time, your buildings will get tighter, your pass rate will improve, and your reputation for quality construction will grow.