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 the cost of adding a mechanical system.” Here, the budget would go toward tightening up the envelope.
OK, so say the budget is shot on beefing up the envelope. Now the house is tighter with possibly inadequate air exchange and/or inferior air quality. Are you shooting yourself in the foot to invest in the envelope if you don’t have money to put into proper ventilation? “We have to get pretty far down the road of efficiency before we start talking about whole-house ventilation,” Bo answers, noting that he always checks bath fans and range hoods to ensure good spot ventilation.
“When I started out 15 years ago, I was far more worried about that,” he continues. “I would bang that drum loudly. If we were going to tighten the house, there had to be a ventilation plan—knowing we would get to a building tightness limit (BTL) based on the blower-door test results, the volume of the house, and the number of occupants/bedrooms. We always wanted to have a ventilation plan in place, in case we reached the BTL, but we have found it’s hard to do that in older homes.”
Bo says when it comes to deep-energy retrofits that include the installation of Zip System, spray foam, Intello membranes and other insulation and air-sealing products that make the house more airtight, it’s critical to have the mechanical ventilation discussion up front. The house will perform much differently once those upgrades have been made, and fresh-air supply becomes a must. At that point, for Bo, the conversation is about choosing between an exhaust-only system, which is a bath fan on a timer, or balanced ventilation with an HRV or ERV. Of course, cost is usually top of mind for clients.
“It’s not a matter of ‘every dollar you spend will save $4,’” Bo tells clients. “Ventilation is not going to save money—it’s going to cost money. And if people haven’t been researching it, they are going to be suspect if I show up and start talking about a $6000 system. You have to discuss the cons of exhaust-only: depressurization issues; unknown sources of fresh air; low efficacy; admittance of warm, moist summer air and cold, dry winter air. If their goal is to save money on their bills but I won’t do the work until we agree on a ventilation plan, most people will pick exhaust-only.” Exhaust-only systems meet code, and they are less expensive but they are inferior to whole-house ventilation.
Bo says this conversation needs to happen early on, and if contractors aren’t bringing it up, homeowners must. He recommends that people ask about air exchanges and indoor air quality. The contractor will either know the answers or will be able to provide a contact who does. “A house can hide a lot of sins,” Bo notes. “It takes someone to come in and point out the ways it is begging for attention. Unless you are looking, you aren’t going to find the problems.”
Bo is currently working with a client whose old home is begging for mechanical ventilation. There’s high moisture content—especially in attic, which has mildew—that needs addressing first and foremost. “In this case, we don’t discuss efficiency upgrades until we talk about mechanical ventilation, moisture mitigation, and air exchanges. That is all this client should be worried about right now.”
Sometimes occupant sensitivities force the issue—the off-gassing of VOCs from new building materials can be bothersome. Air-quality testing makes sense in this situation—regardless of what the blower-door test numbers read. Bo’s services include air quality monitoring and measuring. If that is not in the budget, he says using a low-cost air-quality monitor is a good starting point; it can help determine the rate of exchange between indoor and outdoor air.
It’s important to know that blower-door test results do not provide the full picture, and that air exchange is not going to solve every problem. Sometimes filtration is the real problem solver. “You can have proper air exchange and still have comfort issues that require a more robust filtration system,” Bo explains. “You need to be mindful of the habits at play in the home.”
As the way people use their homes changes—the pandemic, of course, has resulted in near-constant occupancy in many cases—ventilation for CO2 levels, which when high can affect brain function, are of more concern. Bo asks: “How much should we be blaming a home’s condition versus how much time we spend in it?” Now that we are using our homes as offices and schools, he wonders if we need to redefine what our houses should provide—beyond shelter and warmth.
In his own old house, Bo has been experimenting with taping a HEPA filter to the face of a box fan—inspired by Allison Bailes’s DIY Comparetto Cube. He put one in every room and, after three months, the filters have collected a significant amount of particulate matter. “With leaky old houses, it’s more about filtering the air that is coming in—if it’s an indoor air quality issue but there is enough air exchange,” he concludes.
A diagnostics enthusiast weighs in
“The BlueAware Indoor Air Quality Monitor changed a lot of my thinking around ventilation,” says Doug Horgan, vice president of BOWA. “It measures CO2, temperature, and humidity. It also has two low-accuracy instruments for particulates and one for VOCs, which I haven’t gotten a lot of information from, but the CO2 reading is great because you can tell if you are getting enough air exchange in a room for the number of occupants.”
In his house, Doug learned that closing the children’s doors at night is a bad idea. After a few hours, the CO2 levels were well into the “not-recommended” range. “I would have thought with a somewhat leaky house that we would be fine—and we are, in the overall space, but not in individual rooms. That really changed how I think about whether ventilation is needed or not.” Like Bo, he recommends testing and advises homeowners buy a $200 air monitor to get a general sense of the air quality and where/what the issues are.
Doug’s house was built in the early 1950s; it is not super tight, and is on the one-third natural ACH50 level, where it would make sense to add mechanical ventilation. Like many homeowners, he relies on a range hood and bath fans, saying they are good practice, even if not a whole-home ventilation strategy. “It is easy enough to add a bath fan with a programmable timer—it’s simple and better than nothing. The flaw is that you don’t know where the air is coming in and whether or not it is enough.”
He points out that code requires full-house ventilation on the majority of new projects and remodels but it allows exhaust-only or balanced systems without making much of a distinction between the two. “I’m pretty sure exhaust-only is going to fail the way my house did, which was by exchanging an adequate amount of air but it wasn’t distributed optimally,” he says. “There were spaces that weren’t getting much, if any, clean air.”
Doug mentions a third system: intake-only ventilation, which is ducted to the outdoors with a mechanical damper and controller. It has the advantage of distributing air wherever the ductwork goes. However, he notes, in cold climates, it is not a good idea to pressurize your house by only taking air in and letting it work its way out through the building envelope.
The health of the house and its occupants
Residential designer Michael Maines shares a couple thoughts: “I would recommend ventilation whenever a house is measuring below-code-minimum air exchange levels.” He, too, brings up CO2, noting that it makes people sleepy and unproductive—not ideal when working from home. And, of course, indoor air moisture can be bad for the building envelope but people are more comfortable with humidity levels that are not optimal for the house.
“My rule of thumb is that more ventilation is always good but it is expensive,” Mike says, adding that for a budget-driven project, it can be hard to justify a multi-thousand-dollar Zehnder system—or even a $2000 Broan Nu-Tone system. It’s a good investment in occupant health but it can be a hard sell because it is not standard practice yet.
“Sometimes I push ventilation pretty hard, sometimes I don’t even touch it,” Mike says, noting that he is currently working on a 200-year-old Cape house. The budget is tight, and the house is getting tighter but it belongs to a young couple, who are not concerned about ventilation or indoor air quality. So, as with many clients, it’s a nonstarter.
For my part, I’m left with the nagging question: Are our homes built for full-time occupancy? And if not, how do we make them work best for us—within our budget? In an ideal world, all houses would be tight enough to require mechanical ventilation, and we would all breathe fresh, measured, and well-distributed air. But that’s not the case. So what’s the next best thing?
Kiley Jacques is senior editor at Green Building Advisor. She can be reached at [email protected] Illustration courtesy of Fine Homebuilding magazine.
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