Venting About Ventilation

There is much debate within the building science community about how much ventilation is enough.

Building science as a discipline has to deal with a lot of messy, noisy data, entrenched mythologies, and strong tendencies towards confirmation bias.

On the other hand, public health as a discipline has to deal with a lot of messy, noisy data, entrenched mythologies, and strong tendencies towards confirmation bias.

And the ventilation debate is, of course, occurring right where building science research meets public health research. A musical analogy might be Charles Ives’s “Three Places in New England,” the middle movement of which was inspired by an incident where Ives heard two marching bands playing completely different music meet at a street corner. It yielded a fascinating cacophony that he did a great job of reproducing in his composition. But as a piece of music, it’s not everybody’s cup of tea.

Which is why we have building codes—to tell us practitioners what we should do. Following the code gets us off the hook. Sort of, anyway: the problem with following the code is that it doesn’t always yield good ventilation. It’s a prescriptive standard, not a performance standard. In other words, the code says “install this sort of system” rather than “install a system that achieves these results.” So we can install systems that don’t achieve very good results and still be code-compliant.

It’s also not everybody’s cup of tea to try to plow through all the literature on air quality and health and ventilation rates and reach some supportable conclusion about what sort of ventilation system is the right one for a particular project.

Which is why we have building codes—to tell us practitioners what we should do. Following the code gets us off the hook. Sort of, anyway: the problem with following the code is that it doesn’t always yield good ventilation. It’s a prescriptive standard, not a performance standard. In other words, the code says “install this sort of system” rather than “install a system that achieves these results.” So we can install systems that don’t achieve very good results and still be code-compliant.

This article was written using data from HeatSpring expert instructor (and DEAP Energy Group founder) Mike Duclos‘ projects. Want to continue learning about Passive House? Enroll in Passive House Design or Passive House in the Real World and learn more!

Below are two graphs. They’re both from homes whose ventilation systems meet code. The first one shows carbon dioxide levels in a master bedroom exceeding 2100 parts per million (ppm) by morning. Carbon dioxide is a decent proxy for indoor pollutants. It’s also relatively inexpensive to measure CO2 concentrations accurately. For these reasons, we do a fair amount of CO2 measurement in our work. It’s not perfect, but it’s a lot better than guessing—or than making no attempt to monitor indoor air quality. We’ve measured CO2 levels in bedrooms that exceeded 3000 ppm; 2100 ppm is by no means the worst we’ve seen.

Because of the particular messiness of trying to find correlations (let alone causality) between indoor air characteristics and human health, we don’t know for sure what too high a CO2 level is. There’s some evidence, however, that 2100 ppm is way too high.

The second graph shows a well engineered, balanced, commissioned ventilation system. It reliably keeps the CO2 concentrations within a range of about 500 ppm to 1250 ppm. Most researchers within the building science and public health communities would say this ventilation system is doing a good job, in particular (though you can’t tell this from the graph) because it’s an extremely energy-efficient system that transfers heat from the stale warm exhaust air to the incoming stream of fresh cold air without also transferring indoor pollutants.

Why doesn’t the code just require practitioners to install the second system and avoid the first system? Well, it probably will—eventually. For now, however, the building industry feels (with some justification) that the second sort of ventilation system adds more costs than benefits—that the hard-to-quantify added health benefits cannot possibly be worth the all-too-easy-to-quantify added costs.

But that gets us into the world of cost-benefit analysis and economics, yet another discipline that has to deal with a lot of messy, noisy data, entrenched mythologies, and strong tendencies towards confirmation bias. So we’ll just leave that there.

In the meantime, all we can offer is that we’ll continue to do our best to measure and monitor the effectiveness of the ventilation systems we install and stay on top of the latest building science research. We’ll also do our best to stay on top of the latest public health research.

Stay tuned.

–Paul Eldrenkamp, DEAP Energy Group

Speaking of ventilation… did you know that the air tightness of a small Passive House can present issues for exhaust ventilation strategies that rely on the building enclose to leak make up air? In this spreadsheet, an example of a 400 CFM exhaust fan is used to illustrate this issue. Download it for free: