How to know when you need more ventilation

By Amy Paradysz

IN THE BUILDING SCIENCE WORLD, there’s a lot of talk about energy recovery ventilators (ERVs)—mechanical systems that continuously replace stale indoor air with fresh and conditioned outside air. But, like many of us, Claire Betze wondered if she really needed an ERV.

“I have an older house,” she says. “But we’ve recently weatherized and tightened it up, so we’re using a bathroom fan for exhaust-only ventilation. I know that exhaust-only is simple and works for weatherization projects, but is it enough?” Betze, a professional engineer and home energy consultant with BuildingWorks, knew how to answer that question: with data. She used two handy gadgets that measure carbon dioxide (CO2)—produced through respiration—among other factors:

AN ARANET4 ($180+) measures CO2 levels, temperature, relative humidity and atmospheric pressure in real time and shows a numerical score and a green, yellow or red visual summary.

AN AWAIR ELEMENT ($228+) measures several factors in indoor air quality: CO2, temperature, relative humidity, volatile organic compounds (VOCs) and particulates. The data connects to an app on your phone where you can view the levels independently or total them into an indoor air quality number from 0–100, with a low score indicating poor indoor air quality.

While being interviewed via Zoom in her home office with a window cracked, Betze’s Aranet4 measured the CO2 level at 1,009 parts per million (ppm). Anything above 1,000 ppm is in the yellow (or cautionary) zone. Meanwhile, the Awair Element measured an indoor air quality score of 73 (in the yellow zone); a green score would be 80 or higher.

Using the same measuring devices in her bedroom—where there are two people and a dog breathing in the same room for eight hours or more—Betze observed the CO2 levels spiking for an extended period of time, night after night. Even in October with a window open part of the night, the CO2 levels went from 574 ppm early in the evening to as high as 1,560 ppm. The next day, the CO2 in the empty room dropped back to 491 ppm.

“I wanted to see the data,” she says. “Is exhaust-only ventilation enough? In bedrooms, I think where I’ve landed is that it really isn’t. We’re closing up our houses to keep the heat in, and the CO2 levels spike every night. After measuring this and seeing the need, I’m going to put in an ERV.”

While the Occupational Health and Safety Act (OSHA) has set the permissible exposure limit for CO2 at 5,000 ppm for an eight-hour exposure to avoid oxygen deprivation, much lower levels of CO2 are associated with mild symptoms. For example, sustained exposure at 2,000 to 5,000 ppm has been associated with headaches, sleepiness, poor concentration, loss of attention, slight nausea and even increased heart rate. And sustained exposure at 1,000 ppm has been linked to complaints of “stuffy” air or drowsiness (“brain fog”).

Growing evidence shows potential health risks at CO2 exposures over 1,000 ppm—which is common in crowded spaces and buildings with poor ventilation. If you want to get technical, read the article “Direct human health risks of increased atmospheric carbon dioxide,” in the August 2019 issue of the peer-reviewed journal Nature Sustainability.

If you don’t want to get too technical: The more people— presumably breathing!—there are in a closed space and the longer they are there, the higher the CO2 levels rise, which can make the room feel “stuffy.” If you feel like you need a bit of air, you probably do. Unvented or poorly vented fuel-burning appliances or smoking indoors can quickly spike CO2 levels. An air quality monitor (two aren’t necessary) can be an easy way to determine whether you have adequate ventilation.

“In my own home, the exhaust-only approach to ventilation isn’t distributing the fresh air and providing it to the rooms where I spend most of my time, like my office and bedroom,” Betze says. “The data is a snapshot, but evaluating the trends has led me to a better understanding of the need for better indoor air quality and more effective ventilation in our homes.”

This article appeared in the Winter 2023-24 edition of Green & Healthy Maine. Subscribe today!

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