How to Measure Infiltration in Residential Buildings

Air leakage is one of the trickiest inputs in any load calculation. You can measure walls. You can count windows. But how do you quantify the air that sneaks through gaps, cracks, and penetrations you can't even see?

Get infiltration wrong, and your entire load calculation shifts. Overestimate it, and you'll oversize the system. Underestimate it, and the equipment won't keep up on windy winter days.

The Gold Standard: Blower Door Testing

The most accurate way to measure infiltration is with a blower door test. The equipment depressurizes the home to a standardized pressure difference (typically 50 Pascals) and measures how much air flows through the fan to maintain that pressure. That airflow, expressed as CFM50, tells you exactly how leaky the house is.

From CFM50, you can calculate air changes per hour (ACH50) based on the home's volume. Manual J includes charts that convert ACH50 readings into the construction quality categories the calculation uses: tight, semi-tight, average, semi-loose, or loose.

With measured data, there's no guessing. You know exactly where the home falls on the spectrum.

One important threshold to know: if a home tests below 5 air changes per hour at 50 Pascals, building code requires mechanical ventilation. The house is tight enough that it won't get adequate fresh air through natural infiltration alone. That affects both your load calculation and your system recommendations.

The Reality: Most Contractors Don't Have Blower Doors

Here's the problem. Blower door equipment costs money. Running tests takes time. Most HVAC contractors haven't made that investment.

What happens instead? Some contractors make educated guesses based on what they observe. Others—and this is where things go wrong—just pick whatever infiltration value makes the load calculation come out the way they want it to.

That's not a load calculation. That's working backward from a predetermined answer.

If you're going to estimate infiltration without testing, at least base your estimate on something real.

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Research-Based Guidelines

Lawrence Berkeley National Laboratory conducted extensive studies on air tightness in U.S. homes. When cross-referenced with Manual J tables, their findings give you a reasonable starting point based on construction era:

Pre-1950, not weatherized: LooseOlder homes built before modern air sealing practices tend to leak significantly. Balloon framing, minimal insulation, single-pane windows, and decades of settling create plenty of pathways for air movement.

1950-1994: Semi-loosePost-war construction improved somewhat, but air sealing still wasn't a priority. These homes are tighter than their predecessors but still leak more than modern standards allow.

1995 and later: AverageBuilding codes and construction practices tightened up through the 1990s. Homes from this era typically fall in the middle of the infiltration spectrum.

Energy Star certified: Semi-tightHomes built to Energy Star standards undergo blower door testing during construction and must meet specific air tightness requirements. They're meaningfully tighter than code-minimum construction.

Net zero, passive house, spray foam: TightHigh-performance homes designed for minimal energy use prioritize air sealing. If you hear terms like "net zero," "passive house," or learn the home has spray foam insulation throughout, assume tight construction.

These categories aren't perfect. Individual homes vary. But they're far better than guessing randomly or manipulating the number to hit a desired outcome.

What About Weatherized Homes?

Older homes don't always stay loose. Weatherization programs have improved millions of existing homes with air sealing, insulation upgrades, and other improvements.

An 1880 Victorian that leaked like a sieve might now perform like a semi-loose home after professional weatherization work. The construction era alone won't tell you the whole story.

Here's a useful tip: weatherization crews typically perform blower door tests as part of their work, especially when doing combustion appliance zone testing. They often record the results somewhere in the home—frequently on a sticker near the electrical panel.

Ask the homeowner if their home has been weatherized. If it has, ask if they have the test results. That CFM50 or ACH50 number lets you use measured data instead of estimates.

Even without the specific number, knowing that professional weatherization occurred tells you the home is probably tighter than its construction era would suggest. You might bump a pre-1950 home from "loose" to "semi-loose" based on that information.

Visual Clues to Consider

While you can't measure infiltration by looking, you can spot indicators that suggest tighter or looser construction:

Signs of air sealing work:

• Caulk around window and door frames
• Foam sealed penetrations in the attic
• Weatherstripping on exterior doors
• Sealed electrical boxes and plumbing penetrations

Signs of significant leakage:

• Visible gaps around windows and doors
• Daylight visible through exterior walls
• Drafts noticeable near outlets on exterior walls
• Dust patterns indicating air movement paths

Construction quality indicators:

• Original single-pane windows vs. replacement double-pane
• Evidence of insulation upgrades
• Age and condition of siding and exterior envelope

None of these observations replace actual measurement, but they help you make more informed estimates when measurement isn't available.

The Consequences of Getting It Wrong

Infiltration typically accounts for 20-40% of a home's heating load, sometimes more in older, leaky buildings. That's too significant to handle carelessly.

Overestimate infiltration: Your calculated load comes in high. You recommend a larger system than necessary. The equipment short-cycles, controls humidity poorly, and costs the homeowner more than it should—both upfront and in operating costs.

Underestimate infiltration: Your calculated load comes in low. The system you recommend can't keep up on cold, windy days. The homeowner calls back complaining about comfort. You either eat the cost of a fix or damage the relationship.

Neither outcome serves anyone well.

Making Better Estimates

If you're not going to invest in blower door equipment, at least approach infiltration estimation systematically:

1. Start with construction era. Use the Lawrence Berkeley guidelines as your baseline.
2. Adjust for weatherization. Ask about air sealing work and look for evidence of improvements.
3. Check for test results. Weatherization programs, energy audits, and some new construction processes generate blower door data. It might already exist for the home you're evaluating.
4. Consider the whole picture. Window replacements, insulation upgrades, and renovation work all affect air tightness. Factor in what you observe.
5. Don't game the number. Pick the infiltration value that honestly reflects the home's condition, not the one that makes your preferred equipment fit the calculation.

Infiltration estimation isn't perfect without testing. But honest estimates based on real guidelines produce far better results than arbitrary numbers or manipulated inputs.

Conduit Tech incorporates Lawrence Berkeley research into its infiltration defaults, giving you a solid starting point based on home characteristics. Book a demo to see how the platform handles this and other load calculation inputs.