Blower Door Test: How It Works and What ACH50 Means

What a blower door test is, in plain words

Every house leaks air. Some leaks are obvious, like a gap under a door. Most are hidden: holes around pipes, wires, attic hatches and the joint where walls meet the foundation. A blower door test adds them all up into one number.

ENERGY STAR describes the tool simply. A blower door is a powerful fan that mounts into the frame of an exterior door.

It moves air in or out of the house and changes the pressure inside, which exaggerates leaks through unsealed cracks and openings ².

Technicians use it to find the leakage rate, locate places to seal and measure the result of sealing work ².

Think of it like blowing into a balloon with pinholes. The harder it is to keep the balloon inflated, the more holes it has. A blower door does the same thing to a whole house, then reports how much air the fan had to move to hold a fixed pressure.

The test does not measure insulation, window U-factor or how well your furnace runs. It measures one thing: how much air passes through the shell, which the trade calls the building envelope.

How the test runs, step by step

The steps below follow the setup described by the Vermont Department of Public Service and ENERGY STAR. The formal procedures are set by the test standards the energy code names ³²¹.

1. Set the house up. Exterior doors and windows are closed, interior doors are opened, and fireplace dampers are shut. Any wood fire must be fully out, because the fan will pull air down the chimney.
2. Mount the fan. The technician fits a frame and airtight panel into an exterior doorway and sets the fan in it.
3. Depressurize. The fan pulls air out. Outside air then rushes in through every unsealed crack and opening ³.
4. Read the gauges. A pressure gauge (manometer) reads the difference between inside and outside. A second channel reads the airflow through the fan.
5. Hold or step the pressure. Most tests record airflow at 50 pascals, about 0.2 inch of water column, the pressure the 2021 IECC uses for reporting ¹.
6. Hunt the leaks. While the house is under pressure, the auditor walks it with an infrared camera or smoke pencil. Leaks show up as cold streaks on the camera ³.
7. Calculate the score. The airflow at 50 pascals is turned into ACH50 or into cubic feet per minute per square foot of shell area.

Some tests pressurize the house instead, by blowing air in. The same standards cover both directions, and many technicians run both and average them.

Kinds of blower door tests and results

Example: turning a fan reading into ACH50

This is an illustration of the math with round example numbers, not a measurement of a real home. It shows how the fan reading becomes the ACH50 figure the code limit uses ¹.

So this example house would pass in a hot climate but fail in most of the country. To reach 3.0 ACH50 the crew would need to cut leakage by a third, from 1,200 to 800 CFM50. That is the kind of target an auditor sets before air sealing starts.

Where the result shows up: audit reports, permits and remodel bids

In an energy audit report. A home energy audit usually lists a blower door number near the top, with a list of leaks found under pressure. Vermont notes that a test is often run before and after improvements because it is the clearest way to show a change ⁴.

On a permit or inspection card. Where the 2021 IECC is adopted, new dwelling units must be tested, and the signed report goes to the code official ¹. Additions and major remodels may be covered, depending on how your jurisdiction applies the code to alterations. Our permits and approvals guide explains how to ask.

In a remodeling bid. An air-sealing or insulation bid may promise a target, such as "reduce CFM50 by 25%." That is only meaningful if the same calibrated test is run before and after.

In a HERS rating. A RESNET Home Energy Rating includes field testing by a certified rater, and many state and local codes now accept a HERS Index target as a compliance path ⁷.

What a blower door test does well

Limits and fine print

A blower door number is a test condition, not daily life. Fifty pascals is roughly the push of a strong wind on every surface at once. Your house rarely sees that, so the number is best used for comparison, not to predict your bill.

Results also depend on setup. Whether the tester counts a basement as inside, how volume is measured and whether vents are taped all change the score. That is why the code names specific standards and wants a signed report ¹.

Finally, a test in an occupied home has limits. Furniture, stored items and finished walls hide leaks a camera cannot see. A test during a remodel, when walls are open, can be more useful than one after drywall.

What drives the cost of a blower door test

We do not publish prices for tests or audits; they vary by region and by what is included. What changes the bill is clear, though. A stand-alone code test is narrower than a full audit with an infrared scan and combustion safety checks.

Larger homes, multiple tests (before and after sealing) and travel time add to it. Vermont notes that most contractors charge for assessments to cover their time and equipment, and that utility programs may offer rebates ³.

Federal help has changed. The Energy Efficient Home Improvement Credit (25C), which covered part of a home energy audit, expired for work after December 31, 2025, per the IRS ¹¹.

Check your utility and state energy office for current programs. Our remodeling cost guide covers how to budget testing as part of a larger job.

The test has no lifespan or warranty; it is a snapshot. Plan to retest after any big change, such as new windows, an addition or attic work.

How to get a useful test during a remodel

You can manage this part yourself. The test itself is run by a trained technician with calibrated gear.

1. Decide why you want it: a code test for new work, proof for a rebate or a diagnosis of drafts.
2. Hire a qualified tester. Look for an energy auditor, a BPI-certified professional or a RESNET HERS rater ⁸⁷.
3. Ask for a calibrated test and a written report with CFM50, ACH50 and the volume used ³.
4. If walls or the attic will be opened, test early, so leaks can be sealed while they are reachable.
5. Prepare the house as the tester asks: windows shut, interior doors open, fires out.
6. After sealing, retest with the same method so the numbers compare.
7. If the house gets much tighter, ask for a combustion safety check and a ventilation review ³.

Keeping a tight result tight

A house does not stay as tight as its best test forever. These habits protect the number you paid to reach.

• Reseal after new holes. Every new pipe, wire or vent through an outside wall or the attic floor is a new leak.
• Check attic hatches and pull-down stairs. Weatherstrip and latch them.
• Replace worn door weatherstripping. It wears where people walk.
• Keep caulk joints whole around windows and trim; recaulk when they crack.
• Retest after major work, such as an addition, new windows or a kitchen exhaust upgrade.

Risks to watch, and when to call a professional

• Backdrafting appliances. A gas water heater, furnace or fireplace may rely on air leaks for combustion air. Vermont notes safety tests are run before and after air sealing so flue gases do not spill indoors ³.
• Stale or humid indoor air. A much tighter home may need mechanical ventilation ³. The 2021 IECC requires mechanical ventilation in new dwelling units ¹.
• Ash and soot. Depressurizing pulls air down chimneys. Fires must be out and dampers shut.
• Old dust and fibers. Strong airflow can stir dust in attics and walls; see the warning below.

Call a qualified auditor or HVAC contractor if a combustion appliance shows spillage, if you smell exhaust, or if a carbon monoxide alarm sounds after sealing work.

Code rules: 2021 IECC testing and leakage limits

Testing is required. 2021 IECC Section R402.4.1.2 says the building or dwelling unit shall be tested for air leakage. Under any compliance path, leakage may not exceed 5.0 air changes per hour or 0.28 CFM per square foot of enclosure area.

Testing follows ANSI/RESNET/ICC 380, ASTM E779 or ASTM E1827, reported at 50 pascals (0.2 inch water gauge). Where the code official asks, an approved third party must test, and a signed written report goes to the code official ¹.

The prescriptive limit. Section R402.4.1.3 sets the leakage rate at no more than 5.0 ACH50 in climate zones 0, 1 and 2, and 3.0 ACH50 in zones 3 through 8, when complying with Section R401.2.1 ¹.

Air sealing details. Table R402.4.1.1 requires a continuous air barrier with sealed breaks and joints, and says air-permeable insulation shall not be used as a sealing material ¹.

Adoption varies. States and cities adopt code editions on their own schedule and may amend the testing rules. Existing homes being remodeled fall under the existing building provisions, which treat alterations differently. Ask your building department which edition applies, or check it with our permit checker.

Blower door test vs. other home diagnostics

Common myths about blower door tests

Myth Any fan in the door gives you a score.

Reality Only a calibrated blower door measures leakage; an uncalibrated one only locates leaks ³.

Myth ACH50 is how much air your house leaks every day.

Reality It is leakage at a fixed 50 pascal test pressure ¹; daily leakage is much lower and changes with wind and weather.

Myth Insulation stops air leaks.

Reality The 2021 IECC says air-permeable insulation shall not be used as a sealing material ¹. Seal first, then insulate.

Myth A tighter house is always healthier.

Reality A tight house needs controlled ventilation, and fuel-burning appliances need safety checks after sealing ³.

Myth Old houses cannot be tested.

Reality Any house can be tested. Older homes often show the biggest drop after sealing.

How your climate zone changes the target

When a blower door test is worth ordering

Order one when you plan air sealing or insulation and want proof, when a permit for new work requires it, or when drafts and high bills have no obvious cause. It is most valuable early in a remodel, before walls close, because that is when leaks are cheapest to fix.

Skip a stand-alone test if you only need a single obvious repair. In that case, a full energy audit later, or a test bundled with a whole-home renovation, gives more for the visit.

Your next step: ask any insulation or air-sealing bidder whether they will run a calibrated test before and after, and put that promise in the scope of work.

Related terms

Air Sealing

Air sealing is the work of closing the cracks, holes and gaps in a home’s outer shell so that heated or cooled air cannot leak out and outdoor or attic air cannot leak in, using caulk, foam, gaskets, weatherstripping and rigid blocking.

Whole-House Ventilation

Whole-house ventilation is a fan-driven system that swaps stale indoor air for outdoor air on a steady schedule across the whole home, instead of relying on leaks and open windows.

Blown-In Insulation

Blown-in insulation is loose, fluffy insulation, usually cellulose or fiberglass, that a machine blows through a hose into attics, wall cavities and other hard-to-reach spaces, where it settles into a blanket that slows heat flow.

Weather-Resistant Barrier (WRB)

A weather-resistant barrier (WRB) is a continuous sheet or coating installed over the studs or sheathing of an exterior wall, behind the siding, that catches water getting past the cladding and drains it back outside before it can soak the wall.

Thermal Bridging

Thermal bridging is heat flowing around your insulation through solid parts of a wall, roof or floor, such as wood studs, steel framing, headers and slab edges, that run from the warm inside surface to the cold outside surface.

Window U-Factor

Window U-factor is a rating of how fast heat passes through a whole window, frame and glass together, for each square foot and each degree of temperature difference between inside and outside.

Also referenced from: Blown-In Insulation, Batt Insulation, Thermal Bridging, Ductless Mini-Split System, Whole-House Ventilation, Smart Thermostat Compatibility, Building Envelope, Air Sealing