You just had a blower door test done. The technician packed up, handed you a printout full of numbers like “CFM50” and “ACH50,” and left you staring at it like it’s written in a foreign language.

You’re not alone. Most people who get this test done are homeowners going through an energy audit, or builders trying to pass a code inspection, not engineers. And yet the report almost never explains what the numbers actually mean in plain terms.

So let’s break it down, one number at a time.

First, What Is the Test Actually Measuring?

A blower door test uses a powerful fan sealed into a doorway to pull air out of a building until the inside pressure drops by 50 pascals compared to outside. While that’s happening, the equipment measures exactly how much air is rushing back in through every crack, gap, and seam in the building.

That’s it. It’s simply a controlled way of measuring how “leaky” a building is.

The First Number You’ll See: CFM50

CFM50 stands for cubic feet per minute at 50 pascals. It’s the raw amount of air the fan had to move to hit that 50-pascal pressure difference.

On its own, this number doesn’t tell you much. A small cabin and a large house could both show very different CFM50 numbers, but that difference might just be because one building is bigger, not because it’s leakier. Think of it like knowing someone’s weight without knowing their height. You need more context.

The Number That Actually Matters: ACH50

This is where things get useful. ACH50 stands for air changes per hour at 50 pascals, and it answers a much more meaningful question: how many times does all the air inside your home get swapped out with outside air in one hour, under that same test pressure?

The formula is simple: take your CFM50, multiply by 60, then divide by the volume of the building in cubic feet. A home that scores 3 ACH50 has its entire volume of air replaced three times every hour at test pressure. Lower is always better. Higher means more leaks.

So What Counts as a “Good” Score?

This depends on what you’re comparing it to.

For new home construction, most versions of the International Energy Conservation Code have required 3 ACH50 or lower to pass. The most recent 2024 edition tightens that requirement to 2.5 ACH50 in many jurisdictions. Older homes, especially anything built before energy codes existed, often test far leakier, sometimes 7, 10, or even higher.

If you’re aiming for Passive House certification, the bar is much higher. That standard requires 0.6 ACH50 or less, which is an extremely tight building envelope.

As a rough field guide:

• Above 5 ACH50 — Leaky. Expect drafts, higher energy bills, and likely code failure on new construction.
• 3 to 5 ACH50 — Moderate. Typical for many existing homes, but usually needs improvement to meet current code.
• 1.5 to 3 ACH50 — Good. Meets most modern code requirements comfortably.
• Below 1.5 ACH50 — Very tight. Approaching high-performance or Passive House territory.

The Part Nobody Warns You About: Going Too Tight

Here’s something that surprises a lot of people. A building can actually be too airtight.

If your home tests very low on ACH50, that’s great news for energy bills, but it also means the building isn’t naturally exchanging stale indoor air for fresh outdoor air anymore. Below roughly 1.5 ACH50, most building professionals recommend adding dedicated mechanical ventilation, like an ERV or HRV, to make sure indoor air quality doesn’t suffer. Airtightness and fresh air need to be planned together, not treated as separate problems.

One More Thing to Watch For: Duct Leakage

If your home’s ductwork runs through unconditioned spaces like an attic or crawlspace, leaky ducts can quietly inflate your blower door results. If a number seems unexpectedly high, it’s worth asking whether duct leakage was tested separately, since that’s a very different repair than sealing window frames or wall penetrations.

What to Actually Do With This Report

Once you understand your ACH50 number, the real value is in what comes next. A high number points toward air sealing work: caulking, weatherstripping, insulation gaps, or sealing penetrations around pipes and wiring. A very low number is a green light to double check that ventilation planning has kept pace with how tight the building has become.

Either way, the report isn’t just paperwork for a code inspector. It’s a diagnostic tool that tells you exactly how your building is performing, and exactly where the next dollar spent on improvements will do the most good.