GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Green Advocate

Adding Mechanical Ventilation to Old Houses

When should we get serious about supplemental air?

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, the International Residential Code (IRC) 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.


  1. charlie_sullivan | | #1

    Great article on an important topic that is too often swept under the rug!

    Some of the comments are potentially misleading:

    "off-gassing of new building materials can be bothersome. CO2 testing makes sense in this situation—regardless of what the blower-door test numbers are, CO2 build-up can be a problem."

    It's true the off-gassing of new building materials can be a problem, it's true the CO2 build-up can be a problem, and it's true the CO2 testing/monitoring is a good idea whenever there are questions about adequate fresh air. But new building materials don't off-gas CO2 (unless they are on fire). They off-gas VOCs, and that a VOC monitor would be the way to spot that. CO2 comes from people breathing in the space or from gas stoves.

    "ventilation for oxygen levels in the brain and blood stream are of more concern. "

    If you have too little ventilation for people breathing in a space, it's the build-up of CO2 in the blood and the effect of that on the brain that's a concern. CO2 would build up to highly toxic levels in a unventilated space long before the oxygen levels would be a concern.

    1. GBA Editor
      Kiley Jacques | | #2

      Thanks so much for the feedback, Charlie. I have made some changes to reflect your comments.

  2. Expert Member
    ARMANDO COBO | | #3

    And what are the socio-economic costs of bad indoor air quality? In millions houses across the country, regardless of the price point of houses, we have VOCs, CO, carpets, cleaning supplies, water damage and mold, formaldehyde, dander, radon, gas cooking, wood burning, tobacco smoke. What are the medical and insurance costs to everyone?
    How about the health costs disparity in lower-income housing, with higher percentage of illnesses, higher pollution-related mortality, higher unemployment due to health issues, etc., etc.
    Many years ago, I remember a presentation at a green conference that show a statistic that blew my mind; according to the ALA, the American family spends around $100/family/month, on health related issues, because bad IAQ. I wish I had the time to search for that figure now.
    One can spend less than $500 installing a spot ERV/HRV for a small house. So, it’s my impression, that bad IAQ is more of a matter of weak choices, than economics.

  3. user-723121 | | #4

    IAQ is really overlooked and what could be more important today. Filtration of the supply air needs improvement, filters inside HRV's never get checked and cleaned. These filters need to be external like a furnace filter and need to have a cleaning or replacement schedule. Also the HRV/ERV fresh air intakes ( we need improved intakes) easily get covered over with nature's seed dispersal mechanisms, cotton from trees and the like. A smart house takes just a bit of effort to perform as designed.

    1. mattbrennan4 | | #16

      Doug, I agree but struggle on how to find the parts to incorporate a large (MERV13+) filter into my duct line (6") how would you do it?

  4. rdanomly | | #5

    I've been out of Weatherization for quite a while, but as of at least six years ago in Indiana, each home had to have an ASHRAE 62.2 calc performed on the home based on anticipated final blower door results. If the client wasn't willing to have mechanical ventilation installed (when there was a ventilation deficit), they would not be able to participate. Granted, most Wx homes are smaller, so a stout, quiet, continuous bath fan usually was more than enough to address most ventilation deficits. And most clients were OK getting a new, quiet bath fan that actually worked & was vented outdoors.

  5. nshirai | | #6

    Doug Horgan, if you read this, do you have an updated IAQ monitor that you like these days? Seems the Blueair Aware is discontinued, according to the link in the article.

    I'd love to find an affordable unit with decent CO2 monitoring. We have a Foobot, but if I understand correctly it's CO2 readout is inferred from VOCs, not directly measured with a dedicated sensor.

    1. pioneerbuilders | | #10

      I like the IQAir VisualPro.

    2. AussieInSeattle | | #13

      I'm looking into sensors at the moment. My requirements are that I'd like to measure temp/humidity in multiple rooms as well as outdoors and also IAQ in a single location.

      My favorite so far is the ecowitt IAQ sensor (does not do VOC):
      You buy the above and then the gateway if you just want IAQ monitoring:

      You can also add 7 other sensors for a total of 8 (water leak, indoor or outdoor temp/humidity, rainfall, etc) to the system and then access all the data on your phone - available sensors:

      The ecowitt system seems to fit my needs on paper but I'm yet to pull the trigger. Anyone else have this system?

      1. mattbrennan4 | | #17

        I have MYSA thermostats which provide temp and humidity at each location they are installed. Doesn't give IAQ stats though.

  6. Yeldog | | #7

    Seems everybody has a different solution -- opinions. How about houses where the ERV/HRV was used as the bathroom vent w/ less than satisfactory results. Not enough flow when you need it (even with boost) -- too much other times (especially in the winter) Likewise when make up air is introduced for the stove hood in a way not factoring winter conditions. One I saw dropped cold air down from the kitchen ceiling ?

    With the new variable speed HVAC equipment it seems the easiest way is to simply introduce outside air into that system. This way it gets distributed throughout the house. The new systems have low fan setting that do a great job of moving air 24/7 ... even in systems with zones that I always do. I'm in a radon area -- the idea of the constant exhaust seems ill advised -- negative pressure in the house will bring it in. I'm not too worried about the bath exhaust fans -- the Fantech units are powerful and quiet. On a timer they only need 10 min to clear a bathroom of anything. That's not going to be a huge radon problem.

    Have used dedicated dehumidifier units w/ outside intake and dampers ..... I'm wondering about the need here as well. My last house had no humidity issues once the VS HVAC equipment was installed -- the unit never ran.

    I'm not sure what to do with my new build -- it's a deep rebuild and addition to an old stone building (mid-atlantic). Have an intake vent set up -- controls from the stove hood. I'm not sure I need the dehumidifier here either .... With the dehumidifier you duct the outside vent to the dehumidifier and use it's control to introduce a timed refresh.

    With the need for fresh air and make up air from a stove hood -- you would think more options would be available.

  7. lyoung_veic | | #8

    I have been deeply bothered by many of these issues for almost exactly a year now. When it became evident that cleaning surfaces wasn't going to stop the pandemic, but that ventilation was the most important measure we could undertake to make indoor spaces safer, I started looking critically at all the existing buildings I go into. Many of them both have no effective ventilation, and no easy way to add it to the building. Since then I have not let a single customer get away from me without The Ventilation Talk. It boils down to this: seize every opportunity to add effective ventilation to your home (I work residential mostly, but same for commercial buildings).
    The cost of a ventilation system is a necessary investment all building owners need to be prepared to make. The question isn't whether we want to pay for it, or even which type of ventilation, it's when can we do it. Any mechanical upgrade, any repair, any renovation... that's when you put in your ventilation system. If you are finishing your basement, VENTILATE. If your house is on a slab, when you replace your roofing make yourself a conditioned attic and VENTILATE. Some of us don't have to wait. If you have a good place for the ducts, or if your ducted heating/cooling system is well suited for blending ventilation air, do it now. It really doesn't matter how air tight or leaky your building is, we all need a way to control air quality.
    I have a special regret over the multifamily buildings that were designed and built on my watch with code-compliant exhaust-only ventilation. Most of these buildings provide "fresh" replacement air to the units through a gap under the door to the corridor. This is a compartmentalization nightmare and a very efficient way to spread communicable disease in a multifamily building. No more new buildings, and no more renovations without effective ventilation. No more willful disregard for a healthy indoor environment.

    1. GBA Editor
      Kiley Jacques | | #9

      I really appreciate your comments here, Li Ling. Thank you for sharing. The focus on indoor air quality we are seeing is one of few silver linings to come out of the pandemic. I think many people are thinking seriously about ventilation now.

  8. dbaerg | | #11

    In Doug's house, I wonder if upgrading the whole house system really is the full answer to high CO2 levels in the bedroom. It sounds like the issue is more one of changing the air in the bedroom when the door is closed. Is there a cold air return in the bedroom? Probably not in a 1950s house. Is there an adequate door undercut? Installing an HRV or ERV might help. But that fresh air needs to be introduced to the bedrooms.

  9. pioneerbuilders | | #12

    I hope it's ok to share this source:

    I was interviewed therein. Note that we build exclusively with a combination of ductless and ducted minisplits (with the exception of an upcoming project with Trane).

    This system of systems seems to work well.

  10. RecoveringEngineer | | #14

    Great article, two thoughts that haven't been covered:

    1. Are we fully characterizing the energy benefits of centralized HRVs? I have noticed in my home when the HRV is not running that my house is significantly less comfortable to the extent I need my set point raised at least 2 degrees C higher to counter cold air radiating from windows. The rooms only get warm air when the heating system is running. With continuous air flowing from my HRV, the rooms are now getting warmer air constantly and air is being transferred more evenly through the house that I can tolerate at least 2 degrees C lower temperatures. It would be interesting to model the relative energy savings possible when the heat loss from the increased ventilation is factored in.

    2. In new homes, bathroom fans are still being installed where there are centralized HRVs, controllers/thermostats (my tekmar557) don't allow boosting -- not a feature, they only have on or off! The need to still installed individual bathroom fans seems crazy and eliminates any opportunity for avoided cost. How is it boosting isn't core feature?

    1. Yeldog | | #15

      That's one of the nice things about the new multi stage equipment -- even in a house with zoning you can set them up with constant circulation -- the systems have a setting for a very low fan speed. It really balances out the temps and keeps the air moving around. Even in my house with radiant -- use it in the winter. That house is zoned and the thermostats have an interesting operation -- they open the zones a bit more when a particular zone is falling out of range. The system will cycle a bit more to that zone ... it's almost always enough that the actual heat source never starts. As the sun moves around -- this happens through the day. Same in the summer -- the super low fan with lowest AC compressor operation keeps the place comfortable and no humidity. Agree on the HRV and bathrooms -- dodged one the first time I did a complex HRV .. they assured me it would work. Thankfully -- put in the duct to an external fan (fantech). The HRV did not work to clear the bathroom when it was needed .... what it did do is pull colder air into the bathroom throughout the day .. defeating my desire for warmer bathrooms. IMO -- they are not the way to go .... pipe the make up are to the HVAC system and let it move the air around. My thermostat tracks the energy use and even with almost .20KW -- it's $4-5 a month for constant fan.

  11. DBGrover | | #18

    Hello! I have a 1797 farmhouse in Central VT. The house has been sprayfoamed and seems relatively tight (have not yet done any blower door). There is zero ventilation (no bath fans, no kitchen ventilation) and we're aware of a radon issue (6 in the living space). The house has a bit of a musty, rodenty smell and a damp basement. So, I'm aware that I need ventilation but I'm not sure which direction to go to address the various needs. Seems like the low-hanging fruit is to just point ventilate the bathrooms and kitchen. Is that a good place to start? And as far as Radon, what sort of radon mitigation / ventilation is recommended for a retrofit on an old house? Finally, should I be considering a HRV/ERV unit? The house has wood (masonry heater) and forced hot water (oil) heat so there is no duct work. In that situation, where in the home would it make sense to install a HRV/ERV exchanger? Is it enough to put one in one place and expect it to ventilate the entire home? And, here is a random, related thought -- there is an unused second story chimney flue (we removed and do not plan to replace a woodstove there). Any sense in trying to use that flue for ventilation somehow?

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |