What does this air leakage report actually mean?
I had an energy audit performed on our home a while ago and am just now getting around to doing something with it.
Here’s what the report states for air leakage:
Envelope Leakage at CFM50: 2,965
Natural ACH: 0.41
ASHRAE Recommended Level: 0.35
The house is a 1965 ranch. 3,000 sf split between ground floor and partially finished basement. 2,500 finished sf.
The infrared camera showed clear air leakage all over the house, and we have 40 unsealed can lights upstairs, but from reading numerous reports and blog posts it seems like these are already remarkable baseline numbers for an old house with all these can lights.
But maybe I’m reading it wrong or the numbers are reported in a non-standard way?
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CFM50 is the standard. They will often include the ACH/50 in the report (which is mere arithmetic on the volume of the house in cubic feet and the CFM50 number) not just the ACH/Natural, which is derived from the ACH/50 number based on long standing models for different locations.
The "Natural" number is a very squishy number, because CFM/50 doesn't tell you WHERE the leaks are, only the aggregate size of the hole. The location matters, since holes in the bottom and top of the house have a lot more natural infiltration drive from stack effects compared to same-sized holes somewhere in between. The ACH/Natural number could be twice the real infiltration, or it could be half, depending on the locations of the holes.
A sea of can lights penetrating into the attic is a collection of holes in a bad location from a real-world air leakage point of view. The band joist & foundation sill is usually another large, consequential leak in a bad place.
Thanks, Dana. I've read more now about ACH/Natural and understand why it's not always a reliable metric.
And I read much of what Peter suggested below. Seems like, depending on which standard I compare myself against, I either have a relatively tight home, or a terribly leaky one!
As usual, the more you learn, the more you realize how much more there is to learn.
My goal is simply to make this house as tight as is reasonable without undergoing a major retrofit.
To that end, I've already sealed the attic and I've sealed the rim joists and sill plates where they are accessible in the basement (about 1/3 of the perimeter).
Seems like I have essentially tackled the big things, and can now focus on little things like improving windows/doors, sealing wall penetrations, etc. And if I renovate in the basement or replace siding at some point, I can access the remaining rim joists for additional sealing.
Guess I'll perform another blower door test and see how I did.
>"Guess I'll perform another blower door test and see how I did."
When you run the next blower door test have a couple cans of temporary marker paint and an IR camera (a relatively expensive FLIR that works with a phone or tablet computer is good enough) then go around and mark as many trouble spots as you can find while the house is actively pressurized/depressurized. When starting out with big leakage numbers it's easy to miss a couple of easy yet substantial leaks on the first pass, and they become more obvious after you've fixed most of it on the first pass.
Check out the numerous blogs on GBA about blower doors, but particularly this one:
https://www.greenbuildingadvisor.com/article/blower-door-basics and this one:
https://www.greenbuildingadvisor.com/article/time-move-away-ach-50-pascals
Peter