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Jan 08, 2024

Clearing the air on ventilation

Kim Shanahan Building Santa Fe The recent guest commentary in The New Mexican by

Kim Shanahan

Building Santa Fe

The recent guest commentary in The New Mexican by Harvard University Associate Professor Joseph G. Allen ("Indoor air guidelines will boost health," May 16) was informative but lacked important caveats.

Allen, director of the university's Healthy Building program at the T.H. Chan School of Public Health, also is chair of The Lancet's COVID-19 Commission — a key figure arguing for increased ventilation rates in schools and other buildings.

The gist of his commentary, first published in The Washington Post, noted two significant developments around increasing ventilation rates came from the Centers for Disease Control and Prevention and the influential standards organization American Society of Heating, Refrigerating and Air-Conditioning Engineers.

The latter's statement was particularly significant, since it reversed a multiyear trend of recommending lower ventilation rates in pursuit of maximizing energy efficiency. That's why Allen's piece needs caveats.

Practitioners of building science for 20 years have chanted the mantra of "Build it tight and ventilate it right." The mantra was necessary to combat historic conventional wisdom that homes should be "allowed to breathe."

Allen noted new recommendations are for at least five whole-house fresh air exchanges per hour, but a typical American home only gets 0.5 exchanges — one-tenth the recommendation. That's a big difference.

Allen doesn't say how air exchanges should happen, so one might think they should keep all windows open all the time. I hear my father's voice in my head yelling to shut the front door; we aren't heating the neighborhood. That's where ventilating properly comes in.

Building tight is crucial for energy efficiency, and Santa Fe's green building code, as measured by the Home Energy Rating System, has driven homes tighter. Tightness is measured by blower door tests that pressurize a house and measure leakage.

Pressurization is measured in pascals, which is too complicated to explain, but blower doors pressurize homes to 50 pascals and then measure air exchanges. Because of pressurization, the value of numbers is different than those Allen mentioned in his piece, although both measure air exchanges. Allen's numbers are for passive exchanges, not pressurized exchanges.

Under New Mexico's current energy conservation codes, which set tightness standards, a pressurized house should have no more than four exchanges per hour. According to current ASHRAE standards, anything less than 3.5 per hour requires mechanical ventilation. Built to passive house standards, a pressurized house can get no more than 0.6 exchanges per hour. That's a very tight house. Tighter houses get better HERS scores.

So how do you build a very tight, energy-efficient house and keep indoor air exchanges at healthy levels? By employing energy recovery ventilators, known as ERVs. They are standard in passive houses and becoming more common. They’re also standard in Santa Fe Habitat for Humanity homes and have been for nearly a decade. They aren't yet code-mandated but someday will be, especially if new air exchange standards become code.

ERVs work by having stale, room-temperature air exhausted outside while passing — but not intermingling with — fresh air coming in. Whether zero degrees or 100 degrees, incoming fresh air is tempered by outgoing 72-degree indoor air. Energy, measured by heat, is "recovered."

Less efficient and less expensive exhaust-only systems are code-allowed, but they create negative pressure in a house that sucks in cold or hot air from wherever there's a leak or crack.

That's not always fresh, especially from poorly closing garage doors or radon-spewing slab cracks.

Opening windows is great, if you’re unbothered by spring dust or clouds of juniper pollen, but the healthiest air is from filtered and temperature-moderated mechanical ventilation, which Allen neglected to say.

Contact Kim Shanahan at

[email protected].

Kim Shanahan

Building Santa Fe

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