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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, 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…

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  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. Doug McEvers | | #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. Dan Phillips | | #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. Nathan Shirai | | #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. Matt Muller | | #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. Li Ling Young | | #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. David Baerg | | #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. user-7149375 | | #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.

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