Proper ventilation of high-efficiency dwellings

This is the sort of article that is both enlightening and infuriating.  Of course it is a good idea to keep indoor CO2 levels low.  More evidence that this is true is welcome.  However, there is an assumption in this article that these higher levels of indoor CO2 are an unwelcome side effect of attempts to make a building more energy efficient (see bolded sentence below).

WRONG!  This is true only if you don't do it right.

In 1982, I was fortunate to attend a two-day seminar on Swedish homebuilding methods.  Their industry had been saddled with very ridged energy-efficiency standards following the Arab oil embargo of 1973-74 and now they had newly-developed technology to market.  For me, the most interesting presentation was on why buildings needed to breathe.

Keeping expensively warmed air indoors seemed like a no-brainer.  The idea was that you insulated the structure to minimize radiant heat losses and carefully sealed all joints to minimize infiltration losses.  But the Swedes soon found out that as infiltration losses got near zero, bad things started to happen. The most annoying one was that a dwelling containing bodies exhaling warm moist air created a pressure differential that nature would redress somehow.  So the closer you actually got to creating a weather-tight structure, the harder it was to stop all the leaks.  And when you did get a building down to say, one exchange per hour, the bad air inside would grow nasty molds and other things to make you sick and destroy the house.

The Swedish answer was to create an air exchange system that would equalize the indoor-outdoor air pressure while removing the air that grew molds, etc.  These air exchange systems were fairly complex and really only worked when integrated into the original building design.  But when they worked, the results were magnificent.

In 1996, I spent part of the winter in an exurb of Stockholm in a two year-old condo built by Skanska—one of Sweden's big construction firms.  The air exchange system was remarkable.  There were exhaust pickups in the bathrooms and laundry room, as well as over the stove.  The air-to-air heat exchanger was built into the range hood and was perfectly sized to be washed in the dishwasher.  The warmed outside air was distributed into the living room and bedrooms.  The system ran whenever the kitchen or bathrooms were in use.  The fan was very quiet.  Best of all, since the condo was sited on one of Stockholm's many arms of the sea, the warmed air from outside smelled faintly of the ocean.  It was wonderful—I have never felt so healthy in the winter before or since.

So the answer to the problems caused by CO2 buildup is the same as for all the other nasty things in stale indoor air—design the building to breathe and install first-rate equipment to make it happen.  You cannot make a truly energy-efficient dwelling without actively controlling the location and rates of infiltration anyway, so why not do it right?

OCT 17, 2012

Elevated indoor carbon dioxide impairs decision-making performance

Researchers have found that moderately high indoor concentrations of carbon dioxide (CO2) can significantly impair people’s decision-making performance.

Overturning decades of conventional wisdom, researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have found that moderately high indoor concentrations of carbon dioxide (CO2) can significantly impair people’s decision-making performance. The results were unexpected and may have particular implications for schools and other spaces with high occupant density.

“In our field we have always had a dogma that CO2 itself, at the levels we find in buildings, is just not important and doesn’t have any direct impacts on people,” said Berkeley Lab scientist William Fisk, a co-author of the study, which was published in Environmental Health Perspectives online last month. “So these results, which were quite unambiguous, were surprising.” The study was conducted with researchers from State University of New York (SUNY) Upstate Medical University.

On nine scales of decision-making performance, test subjects showed significant reductions on six of the scales at CO2 levels of 1,000 parts per million (ppm) and large reductions on seven of the scales at 2,500 ppm. The most dramatic declines in performance, in which subjects were rated as “dysfunctional,” were for taking initiative and thinking strategically. “Previous studies have looked at 10,000 ppm, 20,000 ppm; that’s the level at which scientists thought effects started,” said Berkeley Lab scientist Mark Mendell, also a co-author of the study. “That’s why these findings are so startling.”


Berkeley Lab researchers found that even moderately elevated levels of indoor carbon dioxide resulted in lower scores on six of nine scales of human decision-making performance. Credit: Berkeley Lab

While the results need to be replicated in a larger study, they point to possible economic consequences of pursuing energy efficient buildings without regard to occupants. “As there’s a drive for increasing energy efficiency, there’s a push for making buildings tighter and less expensive to run,” said Mendell. “There’s some risk that, in that process, adverse effects on occupants will be ignored. One way to make sure occupants get the attention they deserve is to point out adverse economic impacts of poor indoor air quality. If people can’t think or perform as well, that could obviously have adverse economic impacts.”

The primary source of indoor CO2 is humans. While typical outdoor concentrations are around 380 ppm, indoor concentrations can go up to several thousand ppm. Higher indoor CO2 concentrations relative to outdoors are due to low rates of ventilation, which are often driven by the need to reduce energy consumption. In the real world, CO2 concentrations in office buildings normally don’t exceed 1,000 ppm, except in meeting rooms, when groups of people gather for extended periods of time.

In classrooms, concentrations frequently exceed 1,000 ppm and occasionally exceed 3,000 ppm. CO2 at these levels has been assumed to indicate poor ventilation, with increased exposure to other indoor pollutants of potential concern, but the CO2 itself at these levels has not been a source of concern. Federal guidelines set a maximum occupational exposure limit at 5,000 ppm as a time-weighted average for an eight-hour workday.

Fisk decided to test the conventional wisdom on indoor CO2 after coming across two small Hungarian studies reporting that exposures between 2,000 and 5,000 ppm may have adverse impacts on some human activities.

Fisk, Mendell, and their colleagues, including Usha Satish at SUNY Upstate Medical University, assessed CO2 exposure at three concentrations: 600, 1,000 and 2,500 ppm. They recruited 24 participants, mostly college students, who were studied in groups of four in a small office-like chamber for 2.5 hours for each of the three conditions. Ultrapure CO2 was injected into the air supply and mixing was ensured, while all other factors, such as temperature, humidity, and ventilation rate, were kept constant. The sessions for each person took place on a single day, with one-hour breaks between sessions.

Although the sample size was small, the results were unmistakable. “The stronger the effect you have, the fewer subjects you need to see it,” Fisk said. “Our effect was so big, even with a small number of people, it was a very clear effect.” more