Why Do Airtight Homes Need Mechanical Ventilation?
I've written a couple of articles recently about the complexities of mechanical ventilation and the battles going on regarding when to install it, how much to ventilate, and whether ASHRAE 62.2 is worth all the resources we're throwing at it. (If you missed the debate between Michael Blasnik and Joe Lstiburek in the comments of the last one, you should go read it. It was the Building Science Super Bowl this weekend.) Those articles were aimed at pros in the building science/green building/home performance field. Today, it's time to step back and remember why we're even talking about ventilation.
Why not infiltration?
In the article I wrote about how blower doors can't tell you how much mechanical ventilation you need, an important question came up in the comments. Remodeler Doug Thompson asked:
"Can someone explain in simple terms just why its so much better to use mechanical ventilation in a tightly built house to provide the same quantity of air as infiltration might give us from a leakier home? This, of course, assumes that we're talking about exterior air of comparable quality and temperature."
As I wrote a couple of years ago, this idea that a house needs to breathe is a myth. In other words, leaky homes don't necessarily have good indoor air quality. Why? Let's start with some of the typical pollutants you might find in a home's air:
- water vapor
- mold spores
- carbon monoxide
If all of those things came from inside the house and nothing bad came from outside, then relying on infiltration might work. They don't and it doesn't. If you try to rely on infiltration for 'fresh' air, you're likely to be disappointed.
The other problem with relying on infiltration is that you don't know how much air you're getting. This was one of the big points of contention in the Blasnik-Lstiburek debate. Blasnik did "a few quick calculations (using all 1,020 TMY3 stations)" and used that analysis to say that in a moderately leaky home, the number of hours per year that a home goes below 50% of the most used ventilation requirement (ASHRAE 62.2) is pretty low.
Still, his simulation assumed a two-story house. One-story homes on slab foundations are shorter and will have less stack effect, so the hours per year of low infiltration will be less. And even for all those hours when the infiltration is above the ventilation requirement, we have to consider the source. If air is leaking in from the attached garage or moldy crawl space, it's probably making the home's air worse, not better.
How tight is tight?
OK, so if you accept that relying on infiltration is a bad idea, how do you know if your home is tight enough to bother with installing mechanical ventilation? I wish I had a good definitive answer that worked for every home, but this is where a lot of the debate about ventilating homes is focused. A lot of homes were air-sealed with ARRA funds over the past four years, and this question came up a lot.
The weatherization programs until recently used something called the Building Tightness Limit (BTL) or Building Airflow Standard (BAS) to determine if a home needs to have a mechanical ventilation system installed. It was based on the older ASHRAE standard, 62-1989. There were two ways to calculate it, but most of the time, it came down to translating blower door results into 'natural' air changes per hour (ACHnat). If the house had more than 0.35 ACHnat based on the blower door test, the weatherization crew did not have to install a ventilation system.
Lstiburek thinks (and I agree) that's a bad way to decide because converting blower door results to natural air changes is not very accurate for ventilation needs. Michael Blasnik disagrees. Lstiburek wrote in a comment this weekend, "the air change rate from hour to hour or day to day does not correlate to a blower door number." He's talking about variations that matter on the scale that ventilation requires - hour to hour.
Back to the main question here, I think most people would say that a home with 3 air changes per hour at 50 Pascals (the pressure used during a blower door test, abbreviated ACH50) is definitely tight enough to require a mechanical ventilation system. If the home leaks at the rate of 20 ACH50, a ventilation system sized to ASHRAE 62.2 levels would be swamped by infiltration much of the time.
For now, we can summarize by saying that leaky homes need air-sealing. When they get tight enough, they need mechanical ventilation. Many homes fall into that grey area with the fuzzy borders somewhere in the middle between leaky and tight.
More important, though, we need a better way of deciding when to install mechanical ventilation. For new homes, it's pretty easy. "Build tight and ventilate right." ASHRAE 62.2 works for this. With existing homes, we need a method that's based on indoor air quality in addition to how much air leakage a home has.
A Blower Door Can't Tell You How Much Mechanical Ventilation You Need
Residential Ventilation Smackdown — The Battle Over Simplicity
Breathe! - Get Fresh Air into Your Home with ASHRAE 62.2
Myth: A House Needs to Breathe