Blower Door Test: How It Works, Results and Costs

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Cold drafts near outlets, rooms that never feel warm in January, an attic full of frost: these are classic signs that a house leaks air. The trouble is that most air leaks are hidden in attics, rim joists and wall cavities, so it is hard to know how leaky a home really is just by walking through it. A blower door test puts a number on it. A calibrated fan mounted in an exterior doorway pulls air out of the house while gauges measure how much air rushes back in through every crack and gap. This guide explains what a blower door test measures, how the test is run, what the results mean, where leaks usually turn up, why combustion safety testing belongs alongside it and what homeowners can expect to pay. Figures below are general ranges rather than quotes, and local code requirements should be confirmed with the building department.

Part of our Crawl Space & Attic Guide guide — start there for the full picture →

What Is a Blower Door Test?

A blower door test is a diagnostic test that measures the airtightness of a building. A technician seals a powerful, calibrated fan into an exterior door frame using an adjustable frame and an airtight fabric panel. The fan blows air out of the house, lowering the indoor air pressure. Outdoor air then pushes in through every leak in the building envelope, the boundary between conditioned living space and the outdoors, attic, crawl space or garage. Pressure gauges measure the difference between indoor and outdoor pressure and how much air the fan must move to hold that difference.

The result tells you how leaky the house is overall, and the test conditions make leaks easier to find. With the house under pressure, drafts that are barely noticeable on a calm day become easy to feel with a hand, see with a smoke pencil or spot on an infrared camera.

Blower door testing is a core part of most professional energy audits, a common requirement for new homes under modern energy codes in many jurisdictions, and a useful before-and-after check when a homeowner invests in air sealing or insulation.

What a Blower Door Test Measures: CFM50 and ACH50

Blower door results are usually reported in two ways. Both are measured at a standard test pressure of 50 pascals, a pressure difference roughly comparable to a 20-mile-per-hour wind blowing on all sides of the house at once.

CFM50

CFM50 is the number of cubic feet of air per minute the fan must move to hold the house at 50 pascals of depressurization. It is a raw measure of leakage. A larger house will naturally have a higher CFM50 than a smaller one with the same quality of construction, which makes CFM50 most useful for comparing the same house before and after air sealing.

ACH50

ACH50 stands for air changes per hour at 50 pascals. It takes CFM50, multiplies it by 60 to get cubic feet per hour, then divides by the volume of the house. The result describes how many times the entire volume of indoor air would be replaced each hour at test pressure. Because it is normalized by volume, ACH50 is the number most often used to compare houses with each other and to check against code limits.

A simple example: a house with 20,000 cubic feet of conditioned volume and a CFM50 reading of 2,000 has an ACH50 of 6 (2,000 x 60 / 20,000). Lower numbers mean a tighter house.

Other numbers you might see

Some reports also include an estimate of natural air changes per hour (sometimes written ACHn), which tries to convert the test result into typical everyday leakage based on climate, height and exposure. Others report effective leakage area, an estimate of the total size of all the holes combined, sometimes expressed in square inches. These are modeled estimates rather than direct measurements and are most useful for ventilation planning.

How the Test Works, Step by Step

1. House setup. Exterior doors and windows are closed. Interior doors are generally opened so the house acts as a single zone. Fireplace dampers are closed, and ash is often covered so it does not get pulled into the room.
2. Combustion appliances turned off. Gas or oil furnaces, boilers and water heaters are switched to off or pilot. This prevents the depressurized house from pulling exhaust gases back down a flue during the test.
3. Exhaust fans and dryers off. Bath fans, range hoods and clothes dryers are turned off so they do not distort the readings.
4. Mounting the door. The technician installs the frame and fabric panel in an exterior doorway and fits the fan into the opening.
5. Baseline reading. Gauges record the existing pressure difference with the fan off, accounting for wind and stack effect.
6. Depressurization. The fan ramps up until the house reaches 50 pascals below outdoor pressure, or the technician takes readings at several pressures and calculates the 50 pascal value.
7. Leak hunting. With the house depressurized, the technician walks through looking for leaks with a hand, a smoke pencil or a thermal camera.
8. Restoring the house. The fan is removed and appliances are returned to normal operation, ideally with a combustion safety check.

A basic test often takes an hour or two; a full energy audit with combustion testing and an infrared scan usually takes longer.

Pairing the test with infrared

Blower door tests are often paired with infrared thermography. When the indoor-outdoor temperature difference is large enough, such as on a cold Front Range morning, air pulled in through leaks shows up on a thermal camera as streaks of cold along baseboards, around outlets, at top plates and around attic hatches. Missing or settled insulation shows up too. Homeowners curious about the equipment can browse thermal cameras used for home inspections, though interpreting images under blower door conditions takes practice.

What the Results Mean

There is no single “good” number for every house, but some general ranges are widely used as reference points. Treat them as rough guides rather than strict cutoffs:

• Very leaky. Older homes with no air sealing work often test well above 10 ACH50, and some much higher.
• Average existing homes. Many existing homes fall somewhere in the mid to high single digits.
• Code-built new homes. Newer energy codes generally target low single digits, often in the range of 3 to 5 ACH50 depending on the code edition and climate zone.
• High-performance homes. Builders aiming for very tight construction may reach 1 to 2 ACH50 or lower, and some voluntary programs set much stricter targets.

New-construction testing requirements

Many jurisdictions now require blower door testing of new homes as part of energy code compliance. Recent editions of the International Energy Conservation Code (IECC), published by the International Code Council, include air leakage testing requirements and maximum leakage rates that vary by climate zone. Much of the Colorado Front Range falls in a cold climate zone where these limits are relatively strict. Adoption varies by city and county, and local amendments are common, so builders and buyers should confirm which edition and requirements apply with the local building department.

Too tight?

A very tight house needs planned mechanical ventilation, such as an energy recovery ventilator, a heat recovery ventilator or continuous exhaust fans, to keep indoor air fresh. ENERGY STAR notes that sealing a home can trap indoor air pollutants and may call for added mechanical ventilation. Its attic air sealing guidance also recommends testing for radon after reducing air leaks. The blower door number is a starting point for those decisions, not a goal to push as low as possible without a ventilation plan.

Where Air Leaks Usually Hide

Blower door tests in existing homes tend to find the same culprits again and again. The biggest leaks are often the hardest to see.

Attic bypasses

Openings between the living space and the attic are usually the largest leakage paths. Common examples include gaps around plumbing vent stacks, wiring holes in top plates, chases for ducts and chimneys, dropped soffits over kitchen cabinets, and unsealed or uninsulated attic hatches. ENERGY STAR’s attic air sealing project walks through finding and sealing these before adding insulation, and the guide to attic insulation types and R-values covers what comes after.

Rim joists and the foundation

Where the floor framing sits on the foundation, the rim joist area often has gaps at the sill plate and around penetrations. It is a common source of cold-floor complaints in basements and crawl spaces. The guide to rim joist insulation and air sealing explains materials and approaches.

Ducts

Leaky supply and return ducts running through attics, crawl spaces and garages can pull in unconditioned air or push conditioned air out. Duct leakage is measured with a separate duct blaster test, covered in the companion guide to duct sealing.

Recessed can lights

Older recessed lights in insulated ceilings are often not airtight and can act like small chimneys into the attic. Replacement airtight fixtures or approved covers can reduce leakage; older non-IC-rated fixtures should not simply be buried in insulation.

Other common spots

• Fireplace dampers that do not close tightly.
• Gaps around window and door frames behind the trim.
• Electrical outlets and switch boxes on exterior walls.
• Plumbing and gas penetrations under sinks and behind appliances.
• The door between an attached garage and the house.

Combustion Safety: Why It Belongs With Every Blower Door Test

Tightening a house changes how air moves through it, and that can affect fuel-burning appliances. Naturally drafting gas water heaters and older furnaces rely on warm exhaust rising up a flue. In a tight house, a large kitchen range hood, bath fans, a clothes dryer or even a fireplace can depressurize the space around the appliance enough to pull exhaust back into the house. This is called backdrafting.

ENERGY STAR advises homeowners with oil or gas appliances to consider having a qualified contractor test them before and after sealing, because sealing can in some cases cause naturally vented appliances to backdraft. Its guidance on identifying home problems also notes that inspection and testing by a home energy professional can help identify combustion-related health and safety issues. EPA lists back-drafting from furnaces, gas water heaters, wood stoves and fireplaces among indoor sources of carbon monoxide.

Professionals often perform a combustion appliance zone (CAZ) test. With exhaust fans running and interior doors set to create a worst-case condition, they measure the pressure in the room containing the appliances, check whether each appliance drafts properly, and measure carbon monoxide in flue gases and ambient air. Problems are typically solved by adding makeup air, adjusting or servicing equipment, or replacing atmospheric appliances with sealed-combustion or power-vented models.

Every home with fuel-burning appliances or an attached garage should have working CO alarms regardless of test results. The guide on where to put a carbon monoxide detector covers placement.

How to Prepare for a Blower Door Test

A little preparation makes the test faster and the results more useful. Before the technician arrives:

• Close and latch all exterior windows, including basement and garage-side windows.
• Clear a path to the attic hatch, the basement, the crawl space access and the furnace and water heater.
• Let the auditor know about any fireplace or wood stove in use. Ashes should be cold, and some auditors will ask that a fire not be burned that day.
• Plan to keep pets contained, since an exterior door will be occupied by the fan for a while.
• Make a list of comfort complaints, such as cold rooms, drafty spots, ice dams or high bills, so the auditor knows where to look first.

Test-in and test-out

The most informative way to use a blower door is twice: once before any work (the “test-in”) and again after air sealing is complete (the “test-out”). Comparing the two CFM50 readings shows how much leakage was actually eliminated, which is far more reliable than judging by feel. Many contractors use a blower door during the work itself, sealing a set of leaks and then checking the reading again to decide whether the next round of sealing is worth the effort. When an incentive program is involved, a test-out is often required to verify the result.

ENERGY STAR estimates that homeowners can save an average of 15% on heating and cooling costs by air sealing and adding insulation in attics, floors over crawl spaces and basements. A test-out is the simplest way to see whether a particular project delivered a meaningful share of that potential.

How Much a Blower Door Test Costs

Costs vary by region, house size and what is included. As a general guide:

• Stand-alone test. A basic blower door test with a written result often costs a few hundred dollars.
• Full energy audit. A comprehensive audit that includes blower door testing, an infrared scan, combustion safety testing and a prioritized report commonly runs several hundred dollars, sometimes more for larger homes.
• New-construction compliance testing. Testing for code compliance is usually arranged by the builder and priced per house or per project.

Some utilities and local programs offer discounted or subsidized audits, and some contractors include testing as part of an air sealing job. Ask whether a test-out after work is included, so you can confirm the improvement. ENERGY STAR estimates that sealing and insulating can save up to 10% on annual energy bills, though results in any given house depend on its starting condition and climate.

Blower Door Testing as Part of an Energy Audit

Most homeowners encounter a blower door during a home energy audit. A good audit uses the test to locate and rank leaks, checks insulation levels, looks at duct leakage, tests combustion safety and produces a prioritized list of improvements. The value lies less in the number itself than in how it guides the work: sealing the biggest attic bypasses first, then the rim joists, then smaller leaks, and finally confirming the gains with a second test.

For houses with crawl spaces, air sealing and insulating the foundation area is often part of the plan. The overview of crawl space insulation and sealing explains how that piece fits with whole-house air sealing.

A standard home inspection does not usually include a blower door test. Buyers who want one before closing typically schedule it separately with an energy auditor or a home performance contractor, often alongside the general inspection.

References

• Seal and Insulate with ENERGY STAR — ENERGY STAR
• Attic Air Sealing Project — ENERGY STAR
• Identify the Problems You Want to Fix — ENERGY STAR
• Why Seal and Insulate? — ENERGY STAR
• Carbon Monoxide’s Impact on Indoor Air Quality — U.S. Environmental Protection Agency

Want to know how a home on the Front Range really performs before you buy it? Get in touch through our contact page and we can help connect you with a qualified local inspector or energy professional.