The next time you're considering whether to add mechanical ventilation to a house that's much tighter after a retrofit, conside how the house will be used. How many people are likely to occupy one room at a given time? That's key information, because every new person is adding CO2 load to the room with every exhalation. Research published in Environmental Health Perspectives found that once CO2 exceeds the standard 600 ppm used for most ventilation standards, performance goes downhill rapidly. At 1,000 ppm CO2, they found "moderate and statistically significant decrements in six of nine scales of decision-making performance. At 2,500 ppm large and statistically significant reductions occurred in seven scales of decision-making performance." They conclude that "Direct adverse effects of CO2 on human performance may be economically important and may limit energy-saving reductions in outdoor air ventilation per person in buildings."
Hold on. Before we toss out years of building science to freshen the air, the technology already exists to provide a fix that's efficient. Heat recovery ventilators (and ERVs for hot weather) have been around for decades. They can add ventilation to the tightest spaces at minimal energy cost. -Editor