The topic of adding mechanical ventilation to old houses comes up on a semi-regular basis, so I wanted to hear from people who have had that conversation with clients. I had read that “there are no field studies to address the question of when a house is tight enough to require supplemental ventilation.” John Straube said that. So, I had a handful of questions in mind when I called folks to talk about this: Does an old house need a ventilation system? Is it worth adding if the envelope hasn’t been tightened up? When is a house tight enough to justify the investment? Should we be relying on air leakage for ventilation? Is that optimal for human health?
The consensus seems to be that it depends on the condition of the house, the way it is used, the number of occupants—including pets—and the homeowners’ priorities and budget. Of course, International Residential Code answers it more concretely: When a house measures 5ACH50 or less, mechanical ventilation is required.
But what’s happening out in the field when architects/designers and builders are re-working old homes? When does the subject of ventilation come up? In what context? Here’s what I heard:
Conditions, concerns, and conversations
Bo Jespersen, owner of The Breathable Home—a Maine-based consultancy focused on deep-energy retrofits and energy audits—talks about the motivation behind installing a ventilation system. Typically, he gets calls from owners of 100-plus-year-old houses because their heating and cooling costs are too high and/or they are uncomfortable in the house. In that case, the need for mechanical ventilation is pretty low, he says. “They have the equivalent of an open window, so they have good air exchange. It’s not filtered, consistent, or measurable but there is air exchange, which makes it hard to justify…