Joe Lstiburek’s Airtightness Goals
The renowned building engineer explains why the Passivhaus limit of 0.6 ach50 is just fine — except when it isn’t
In commentary recently posted to the Building Science Corporation website, building scientist Joe Lstiburek takes a stroll down memory lane and reflects on his attempts in the early 1980s to help develop an airtightness standard for residential construction in Canada. Later, after signing on with production builders in the U.S., he launched a successful campaign to plug “big holes” in the envelopes of the homes these builders were constructing so that the finished products would be more comfortable to live in.
“I picked one of the big builders’ worst divisions in terms of comfort complaints — it turned out to be Chicago — and focused on making the comfort complaints go away. Notice, energy had nothing to do with any of this,” Lstiburek writes. “I figured that if they got rid of the big holes, the comfort problems would go away. The list of big holes was pretty easy to compile. Bathtubs on exterior walls, fireplaces and chimneys on exterior walls, garages attached to houses with bedrooms over them, cantilevered second floors, interior soffits dead-ending into exterior walls, and dropped ceilings under attic insulation. Done. Make these go away with draft-stopping and, presto, the comfort problems would go away.”
Blower-door tests in the Chicago-area houses consistently came in at 3 air changes per hour at 50 Pascal pressure difference – an ACH value that, coincidentally, had already been written into building code in Sweden and had originally inspired the Canadians to come up with their own airtightness standard.
However, the 3 ach50 goal was rejected as too leaky when Lstiburek was working with the National Research Council of Canada, whose contingent in Saskatoon, he says, favored 1 ach50 as the standard. Eventually, the NRC folks compromised on 1.5 ach50 as the standard for a new residential program dubbed R-2000. As Lstiburek expected, however, the standard was not widely adopted.
As airtight as you like
Lstilburek salts his discourse on building-envelope airtightness with a few key points:
(1) Blower-door tests are terrific for measuring equivalent air-leakage area, but they do not precisely measure the leakage rate of a building in service or the locations and severity of leakage paths in the envelope;
(2) Blower-door test results alone shouldn’t be used to declare that the tested building doesn’t need mechanical ventilation;
(3) Draft-stop the air leaks;
(4) Install a controlled ventilation system and sealed-combustion or power-vented appliances;
(5) Airtightness below 3 ach50 is terrific, 1.5 ach50 is difficult to achieve, and the Passivhaus requirement of 0.6 ach50 is really difficult to achieve – but if that’s your thing, fine. Just don’t try to impose 0.6 ach50 – a “number doesn’t seem to be based on anything that makes any sense” – on everyone else, including and most especially Joe Lstiburek.
He elaborates on this point somewhat more crisply near the end of his essay: “From my own personal perspective, you should be able to pick any number you want. And hats off to the Passivhaus folks for their own personal program number. It is their program, and they get to do what they want as far as I am concerned. I only get irritated when they criticize everyone else … arbitrary and capricious is OK too — in your own personal program and your own personal life — but not in mine. Public policy and national standards should not be arbitrary and capricious.”
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