Sub-1.0 ACH50 Without Stuffy Interiors
What the ACH50 number means for a coastal house, why our target is under 1.0, and how a sealed house still gets filtered fresh air.
A blower-door test measures how much air leaks into a house through gaps nobody planned. The lower the number, the tighter the house. A tight house still gets plenty of fresh air, brought in on purpose through a filter.
What the blower-door test measures
A blower door is a strong fan set into an outside door frame. It pulls air out of the house, or pushes air in, until the inside pressure is 50 pascals away from the outside.
A gauge then shows how much air the fan has to move to hold that pressure. That air is passing through gaps somewhere in the house. Fifty pascals is about the push of a stiff wind on every side of the house at once.
Every test uses the same pressure, so one house can be compared with another.
What the number means for you
The result is called ACH50, short for air changes per hour at 50 pascals. It counts how many times the air inside the house is replaced by outside air in one hour at that pressure. A result of 5 means the whole house fills with new air five times an hour.
North Carolina's residential code accepts up to 5 when a house is tested. Our design target is under 1.0.
On this coast, a low number matters for moisture as much as energy. Summer air here holds a lot of water. Humid air can slip through a gap and meet a cool surface, like the back of the drywall or a cold duct. There it turns to water, inside the wall.
The same leaks make rooms drafty and uneven. That is why comfort problems and moisture problems start in the same place, at the same time. Airtightness decides where a house breathes.
A tight house breathes where you intend it to.
How a house gets that tight
Tightness comes from details drawn before anyone frames a wall. The key is one continuous air barrier, a sealed layer that wraps the walls, roof, slab and openings without a break.
Air finds the one gap left open, so every joint counts. A few places decide the number:
- Where the walls meet the roof and the foundation
- Around every window and door
- Every hole cut for a pipe, a wire or a duct
- Rim joists, chases and dropped ceilings, which are easy to miss once they are covered
- The attic floor, where the ceiling meets the top of the walls
Each of these is drawn before construction starts. The air barrier is checked while every seam can still be seen, and the blower door runs at the end.
How fresh air gets in
A tight house still needs fresh air, so a fan brings it in on purpose. It arrives through a filter you chose, at a rate someone calculated for the house.
Salt, pollen and humidity arrive either way. In a tight house, they come through that filter. Because coastal air is often damp, the fresh air system is planned with dehumidification.
It is also planned with the heating and cooling. A tight house has smaller loads, so its equipment can be smaller. Equipment sized to the house runs long enough to pull water out of the air. Oversize it, and the house reads 72 on the thermostat and still feels clammy.
Why we test before you move in
Under 1.0 ACH50 is a design target. It is written into the drawings, carried into construction and confirmed by a blower-door test before handover. On a new house, it is part of the performance work planned from the first drawings.
Testing before you move in matters. If the test finds a leak, there is still time to track it down and seal it.
If your house is already built
An older house can be tightened too, and the same test can measure a house that already stands. A performance retrofit can include air sealing, moisture and drainage fixes, dehumidification and right-sized equipment.
It starts with an assessment of the house, done before any work is priced. How far the work goes is decided with you, against what the house needs.
Planning a new house on the coast? Send us the lot address from our new construction page, and we'll run it through our lot checklist.