What a blower door number means for a Central Ohio house

A blower door test turns a house full of drafts into a single number. That number decides more than people expect: which leaks to fix first, whether a sealed house needs a ventilation fan, and whether a new Central Ohio home passes code. Here is how to read it.

How the test works

A calibrated fan is fitted into an exterior doorway inside an airtight panel. With windows shut, interior doors open, and exhaust fans off, the fan pulls air out of the house until the inside sits 50 pascals below outdoor pressure, a steady difference comparable to a stiff wind pressing on every side of the house at once. The airflow needed to hold that pressure is recorded in cubic feet per minute at 50 pascals, written CFM50.

To compare houses of different sizes, that airflow is converted to air changes per hour. Multiply CFM50 by 60 and divide by the house volume in cubic feet, and the result is ACH50. Say a two-story bungalow in Grandview Heights has 1,400 square feet of living space under 8-foot ceilings, or 11,200 cubic feet. If the fan needs 2,000 CFM50 to reach the test pressure, the house measures about 10.7 ACH50. Every leak you seal lowers the airflow and the number with it.

The one number the Ohio code sets

For new homes, section 1102.4.1.2 of the Residential Code of Ohio requires a tested result of no more than 5 ACH50, using RESNET/ICC 380, ASTM E779, or ASTM E1827, with a written report signed by the tester and given to the building official. The test is run after every penetration of the envelope has been made, with exterior windows and doors closed but not taped, dampers closed, interior doors open, heating and cooling off, and supply registers open.

So a 2020s house in Powell or Lewis Center had to meet that number. Houses built before the testing requirement never faced one, and many older houses around Columbus would land well above it.

Reading your own result

The code offers no grading scale for existing homes, so treat these as rough bearings rather than official categories.

• Far above 5 ACH50: a house that has never been sealed. The basement rim joist and the attic floor usually hold the cheapest large wins.
• Somewhere near 5: a house that was built with some care or has had upgrades. Targeted sealing still pays, but the remaining leaks take more hunting.
• Below 5: tighter than the Ohio new-home limit. The main envelope work is done, and a planned fresh air supply matters more than more sealing.

The number matters less than the trend: a before and after pair shows what the work actually changed.

Finding leaks while the fan runs

The test earns its fee as a leak-finding session. With the house held at negative pressure, a smoke pencil or a thermal camera shows where outside air rushes in: along the basement sill and rim joist, around attic hatches and pull-down stairs, at plumbing and wiring holes through top plates, at recessed lights, behind kitchen soffits, at fireplace dampers, and around window and door trim. In older Grandview Heights and Clintonville houses, the gaps where radiator or duct risers pass between floors are frequent offenders. Write the list down, because it becomes the scope of work.

Things that move the number besides insulation

A few details can shift a result more than people expect, so they should be recorded with it.

• Which spaces count. Whether a basement or a conditioned crawl space is inside the test boundary changes both the leakage and the volume in the math.
• The garage door into the house. An attached garage behaves like outdoors; a leaky door between the garage and the kitchen shows up in the number, and it is worth sealing for air quality reasons too.
• Ducts outside the envelope. Supply and return ducts in a vented attic or crawl leak to outdoors, so their leakage appears in the whole-house number.
• Fireplaces. An old masonry fireplace with a loose damper can account for a surprising share of the total.
• Wind and temperature. Testers correct for outdoor conditions, but a very windy day adds noise, so before and after tests are best done in similar weather.

What the number says about fresh air

ASHRAE 62.2 sets a whole-house fresh air rate from floor area and bedroom count, then lets part of it be credited to natural infiltration based on blower door results. A leaky house earns a large credit, and a tight one earns little, so after a sealing job the share that a fan must supply goes up. The retest after the work answers that question with data instead of guesswork; ASHRAE 62.2 in plain English walks through the formula.

What the number says about safety

Pulling a house into negative pressure can draw exhaust gases back down the flue of a natural-draft water heater or furnace. A careful assessment includes a combustion safety check, often with exhaust fans and the dryer running to recreate a worst case. Building Performance Institute analysts are trained in exactly this procedure. After foam makes a house tighter, the check should be repeated, because the house now behaves differently.

Put it on paper

Ask for three things in the closeout package: the CFM50 and ACH50 before the work, the same numbers after, and the house volume used in the calculation. With those, anyone can check the math later, including a future buyer. If the numbers point at the basement band, the rim joist service is usually the place to start.