Blower Door Tests: Measuring Air Tightness After Spray Foam Insulation

A blower door test is the only objective, quantitative measurement of a building’s air tightness. Everything else — visual inspection, thermal imaging, the installer’s word — is subjective. The blower door gives you a number, and that number does not lie.

The equipment is straightforward: a calibrated fan mounted in an adjustable frame that fits in an exterior doorway. The fan depressurizes (or pressurizes) the building to a standardized pressure differential — typically 50 Pascals, which is roughly equivalent to a 20 mph wind hitting every surface of the building simultaneously. At that pressure, the fan measures how much air it must move to maintain the 50-Pascal differential.

That air flow — measured in CFM50 (cubic feet per minute at 50 Pascals) — represents the total air leakage of the building envelope. Every crack, gap, hole, poorly sealed penetration, and unblocked pathway contributes to that number. The higher the CFM50, the leakier the building.

A blower door test produces two primary metrics. Both come from the same test — they are just different ways of expressing the result.

CFM50 is the raw measurement: cubic feet per minute of air flow through the fan at 50 Pascals. A 2,000-square-foot home might test at 1,500 CFM50. That means the fan moves 1,500 cubic feet of air per minute to maintain 50 Pascals of depressurization. All of that air is entering the building through leaks in the envelope.

ACH50 normalizes the result for building volume: air changes per hour at 50 Pascals. The formula is:

ACH50 = (CFM50 x 60) / Building Volume in cubic feet

For a 2,000-square-foot home with 8-foot ceilings (16,000 cubic feet of volume):

ACH50 = (1,500 x 60) / 16,000 = 5.6 ACH50

This means at 50 Pascals, the entire air volume of the building is replaced 5.6 times per hour through envelope leakage. That is a moderately leaky house.

ACH50 is the more commonly used metric for code compliance because it accounts for building size. A 1,000-square-foot cottage and a 4,000-square-foot home might have similar CFM50 numbers but very different ACH50 results. ACH50 provides an apples-to-apples comparison.

For reference, here is what the numbers mean in practice:

The 2018 IRC establishes a maximum air leakage rate for new residential construction: 5 ACH50 or less, verified by blower door testing. This applies in Oklahoma City and any Oklahoma jurisdiction that has adopted the 2018 IRC.

This is a hard requirement — not advisory, not aspirational, but code. A new home that tests above 5 ACH50 does not pass. The builder must identify and seal leakage pathways and retest until the building meets the threshold.

The 5 ACH50 standard is achievable with standard construction practices and conventional insulation, provided the builder pays attention to air sealing at every penetration, junction, and transition in the envelope. It does not require spray foam. But spray foam makes it significantly easier to achieve and typically results in numbers well below the minimum.

For context, the 2009 IECC — which Oklahoma uses for energy code — included a blower door testing option but did not establish a mandatory maximum ACH50 for all new construction with the same emphasis as the 2018 IRC. The move to mandatory testing and a specific threshold reflects the building science community’s consensus that air sealing is the single most impactful energy measure in residential construction.

Before discussing how spray foam improves blower door results, it is worth understanding where the air is going. In a typical Oklahoma home, the major leakage pathways — ranked roughly by contribution — are:

Ceiling plane / attic interface. This is almost always the biggest single source of air leakage. The junction between the conditioned space and the attic is penetrated by can lights, electrical boxes, plumbing vents, exhaust fan ducts, HVAC boots, attic access hatches, and framing gaps at top plates. In older construction, the gap at the top plate — where the interior partition walls meet the ceiling — can be continuous and substantial.

Rim joist / band board. The junction between the foundation and the first-floor framing is a notorious leakage pathway. The rim joist area is often poorly insulated and poorly sealed, with gaps at the sill plate, the subfloor edge, and the band board itself.

Wall penetrations. Electrical outlets, switches, plumbing penetrations, HVAC registers, and hose bibs all create holes in the wall assembly. Individually small, they add up significantly across the entire building envelope.

Ductwork in unconditioned space. Duct leakage in a vented attic is effectively envelope leakage — conditioned air leaving the building through duct joints and connections. This is a hybrid air leakage and energy loss pathway that blower door testing captures.

Windows and doors. Weatherstripping deterioration, poor installation, and frame gaps contribute to leakage, though modern windows and doors are relatively tight compared to the other pathways listed above.

Foundation penetrations. Gas lines, water lines, electrical conduit, and sewer lines entering through the foundation create holes in the envelope that are often overlooked in air sealing efforts.

Spray foam’s primary advantage is not its R-value — it is the air barrier. When applied correctly, spray foam fills and seals leakage pathways that other insulation types cannot address.

Unvented attic conversion. Moving the insulation from the attic floor to the roof deck eliminates the ceiling plane as a leakage pathway. Every can light, every top plate gap, every plumbing vent penetration through the ceiling — all become interior penetrations within the conditioned envelope rather than leaks to the exterior. This single change can reduce total building air leakage by 30 to 50%.

Wall cavity fill. Spray foam in wall cavities seals around every stud, wire, pipe, and electrical box. The foam conforms to irregular framing and fills gaps that batt insulation bridges over. The wall assembly becomes an effective air barrier from plate to plate.

Rim joist application. Spray foam at the rim joist seals the sill plate, the subfloor edge, and the band board in a single application. Closed cell at 2 inches provides both air sealing and R-13, addressing one of the most significant leakage pathways in a single pass.

The cumulative effect of addressing these three areas — attic, walls, and rim joists — typically produces blower door results of 2 to 3 ACH50 in retrofit applications. This is well below the 2018 IRC new construction requirement of 5 ACH50.

In new construction where spray foam is applied from the start, with the builder’s attention to other air sealing details (sealing bottom plates, caulking window flanges, sealing duct boots), blower door results below 2 ACH50 are common.

For retrofit projects, blower door testing before and after spray foam installation provides the most meaningful data. The pre-test establishes the baseline — how leaky the building was before intervention. The post-test measures the improvement.

A typical Oklahoma retrofit scenario:

• Pre-test: 8 to 12 ACH50 (typical for homes built before 2000 with fiberglass batts and no deliberate air sealing)
• Post-test (attic only): 4 to 6 ACH50 (unvented attic conversion addresses the largest single leakage pathway)
• Post-test (attic + walls + rim joists): 2 to 3 ACH50 (comprehensive spray foam application addresses all major pathways)

The delta between pre and post is the measured, verified, indisputable air sealing improvement. It is the strongest evidence that the spray foam delivered the expected performance.

Pre-testing also identifies the leakage pathways before work begins. During a depressurization test, the technician can walk the building with a smoke pencil or thermal camera and pinpoint exactly where air is entering. This information guides the spray foam application — it shows which areas need the most attention and which areas are already reasonably tight.

Post-testing verifies the result and identifies any remaining leakage pathways. If the post-test number is higher than expected, the technician can localize the remaining leaks and address them before the job is considered complete.

Homeowners often ask: “Can a house be too tight?” The short answer is no — but a tight house needs ventilation.

At 2 to 3 ACH50, a house does not get enough fresh air from natural infiltration to maintain indoor air quality. The building is tight enough that deliberate, controlled ventilation becomes necessary. This is not a problem — it is the design intent. The building science principle is: build tight, ventilate right.

A leaky building gets ventilation accidentally — through cracks, gaps, and holes in the envelope. That “ventilation” is uncontrolled, unfiltered, and driven by weather and pressure differentials. It brings in outdoor pollutants, allergens, humidity, and heat. It is not ventilation — it is air leakage dressed up as a feature.

A tight building gets ventilation deliberately — through a mechanical system that controls the volume, filters the air, and (in balanced systems) recovers energy from the exhaust stream. The homeowner controls the ventilation rate. The air is filtered. The energy penalty is a fraction of the air leakage energy loss.

Ventilation options for tight homes include:

Exhaust-only ventilation. A continuously running exhaust fan (bathroom fan or dedicated exhaust) depressurizes the building slightly, and makeup air enters through leakage pathways and dedicated fresh air intakes. Simple and low-cost.

Supply-only ventilation. A duct from the HVAC return plenum to the exterior introduces filtered outdoor air into the system. The HVAC fan distributes the fresh air through the duct system. Effective and integrates with the existing HVAC.

Balanced ventilation (ERV or HRV). An Energy Recovery Ventilator (ERV) or Heat Recovery Ventilator (HRV) simultaneously exhausts stale air and brings in fresh air, with a heat exchanger recovering energy between the two streams. Most efficient but highest first cost.

The appropriate ventilation strategy depends on the home’s tightness level, the occupancy, and the homeowner’s preferences. For spray-foamed homes at 2 to 3 ACH50, some form of controlled ventilation is recommended, and at levels below 2 ACH50, it becomes essential.

A spray foam contractor who does not perform or recommend blower door testing is leaving the most powerful validation tool on the shelf.

Spray foam is sold on air sealing performance. The R-value is part of the equation, but the air barrier is the primary differentiator over fiberglass and cellulose. A blower door test measures that exact performance characteristic. Without testing, the air sealing claim is unverified.

Consider two scenarios:

Scenario A: Contractor sprays foam in the attic, says “you’re all sealed up,” and drives away. The homeowner trusts the claim. There is no measurement, no verification, no number to reference.

Scenario B: Contractor sprays foam in the attic, performs a blower door test, shows the homeowner that the house went from 10 ACH50 to 3.5 ACH50, and documents the result. The homeowner has a measured, verified improvement.

Which installation would you trust at resale? Which installation supports a warranty claim? Which installation demonstrates that the money was well spent?

At Bo’s, we use blower door testing as a verification tool. For new construction where the 2018 IRC requires testing, we coordinate with the builder to ensure the test is performed and documented. For retrofit projects, we recommend pre- and post-testing to quantify the improvement. The test adds modest cost to the project and delivers substantial documentation value.

The blower door does not care about marketing claims, sales pitches, or contractor reputations. It measures air leakage. The number it produces is either good or it is not. That objectivity is exactly why it matters.