We Need Higher Ventilation Rates. But How High?
The COVID-19 pandemic has changed a lot of things, including my view on higher ventilation rates. A few years ago, Joe Lstiburek of Building Science Corporation pushed hard for lower ventilation rates. He believed ASHRAE’s residential ventilation standard (62.2) resulted in homes being overventilated. That is, he thought the ventilation rates resulting from following ASHRAE 62.2 wasted energy because there was little to no epidemiological data to support higher ventilation rates. He made some really good points, and I was in his camp.
I wrote a lot about this topic about eight years ago. If you care to go back, you can read my article about Building Science Corporation’s ventilation guidance, which they called standard BSC-01. Perhaps the best place to understand the arguments on both sides of higher versus lower ventilation rates is the interviews I did with Joe Lstiburek, Paul Francisco, and Iain Walker. My last update on the “great ventilation debate” was in 2018. (Don’t miss my rant about hyperbolic cotangents in that last one!)
But that all happened in the before-times.
Post-pandemic thinking
Although it took them way too long, the US Centers for Disease Control & Prevention (CDC) and the World Health Organization (WHO) begrudgingly admitted that COVID is airborne. The aerosol scientists and indoor air quality researchers had been saying it for a year before the CDC and WHO finally came to their senses, but the word is out now. The air you breathe can make you sick. It can put you in the hospital. It can kill you. The main problem occurs indoors when you have multiple people breathing and rebreathing the same air.
The researchers I follow often talk about the need for really high ventilation rates, 5 or 6 air changes per hour. They’re mainly focusing on commercial and institutional buildings with those recommendations, though. But, as they’ll tell you, you can get some of those air changes with high-MERV filtration.
For homes, though, you don’t need as much ventilation because you don’t have as many people indoors as in a school, store, or office building. It’s also really difficult to get that much ventilation in a home because often there’s just not enough space for the ducts and the equipment to do it.
Air changes, cfm, and cfm per person
Let’s get the units out of the way first. I mentioned air changes per hour already, and that one is easy to visualize. One air change is when a volume of air equal to the volume of the house moves through the building enclosure into the house. That can happen through a combination of mechanical ventilation (fans), natural ventilation (open windows), and infiltration (air leakage). Factoring in the time span over which the air changes happen, you can specify a ventilation rate in terms of air changes per hour (ACH).
Air changes are a nice way to visualize ventilation. But mechanical ventilation happens with fans that measure the air flow rate in units like cubic feet per minute (cfm), cubic meters per hour, or liters per second. That’s what you need to know when you’re specifying the ventilation equipment. That’s also the number you get from following a standard like ASHRAE 62.2 or the ventilation requirements in the International Residential Code (IRC).
Air changes per hour and cubic feet per minute measure the same thing: ventilation rate. Sometimes people talk in ACH and other times it’s cfm, but there’s an important difference. Air changes per hour scale with the size of the house; a rate given in cfm doesn’t. You can take a ventilation rate in air changes per hour and apply it to any house. If you see a ventilation rate in cfm, it applies to a specific size house with a certain number of people in it. It’s possible to generalize cfm rates, though, by specifying them in terms of cfm per person. And that takes us to…
Recommended ventilation rates
One possible starting point for deciding on the ventilation rate is to go by what the codes and standards require. The IRC and the ASHRAE 62.2 residential ventilation standard both use formulas based on the conditioned floor area of the house and the number of bedrooms. The IRC, which uses the formula from Lstiburek’s BSC-01, says you need 1 cfm for each 100 square feet of conditioned floor area plus 7.5 cfm per person, with the number of people defined as the number of bedrooms plus one.
ASHRAE 62.2 is the same format with one change: it uses 3 cfm per 100 sf of floor area. ASHRAE 62.2 lets you take credit for infiltration and use a lower ventilation rate if the house gets a blower door test, but let’s ignore that for this discussion. In tight houses, the infiltration credit is small. In leaky houses, infiltration varies a lot with conditions. Your average infiltration rate may be 0.2 ACH, but sometimes it might be 1 ACH and other times 0.01 ACH. If you’re going to add mechanical ventilation to a leaky house, go ahead and put in a system that can bring in all the outdoor air you need. That also covers you for making the house more airtight.
Using those two formulas, we can calculate that a 3,000 sf house with 3 bedrooms would need 60 cfm under the IRC rule and 120 cfm using ASHRAE 62.2. Another way to look at the ventilation rate would be in terms of air flow per person. With the four hypothetical people in this house, that would be 15 cfm person (IRC) or 30 cfm per person (62.2). For reference, the historical range of recommended ventilation rates is 4 to 60 cfm per person.
We can also specify the ventilation rate in air changes per hour. For that 3,000 sf, 3-bedroom house that would need 60 cfm (IRC) or 120 cfm (ASHRAE 62.2), we can find the equivalent rates in air changes per hour. Assuming a 9 foot ceiling height, those ventilation rates would be 0.13 ACH (IRC) and 0.27 ACH (ASHRAE 62.2).
Using the house of our example, we can find that the historical range of recommended ventilation rates is 0.04 ACH (4 cfm per person) to 0.53 ACH. The 1989 version of ASHRAE’s residential ventilation standard required a minimum rate of 0.35 ACH. Some ventilation designers use that as their continuous ventilation rate. On the high end, 0.5 ACH is about the limit of what’s reasonable in homes. Table 1 above shows the various rates and metrics discussed here.
How much ventilation is enough?
The amount of ventilation you need in a house is, of course, a moving target. When no one’s home, you might not need to ventilate at all. With just the family at home, you’ll need some ventilation. During that big 120th birthday celebration, you’ll probably need a lot (unless people are outdoors).
One way to handle changing ventilation needs is to have a ventilation system that can operate at different rates. The Zehnder energy recovery ventilator (ERV) that I’m installing at my house can do that, as can the Renewaire ERV installed and the Broan ERV being installed in the homes of Energy Vanguarders Jeffrey Sauls and Luke Bertram. You can commission these ERVs to run at the continuous rate you choose during normal conditions. If you need more ventilation, you can hit the boost button, and the ERV ramps up to a higher rate.
Focusing on the normal, continuous ventilation rate, though, what number should you choose? As I said at the beginning, I’m in favor of higher ventilation rates now. I like the 0.35 ACH rate for ventilation systems with recovery of heat (HRV) or heat and moisture (ERV). If you’re using an ERV with high sensible and latent heat recovery, it allows you to ventilate at a higher rate with impunity. So do it! And get a system that can boost up to about 0.5 ACH.
But you have to consider other factors when deciding on your ventilation rate. Balanced ventilation with recovery (ERVs and HRVs) aren’t the only type out there. With supply-only or exhaust-only ventilation, you may want to go with a lower rate and the ASHRAE 62.2 rate is a good number to shoot for. When outdoor pollution is high, whether it’s particulates from wildfires or ground-level ozone, you’ll want to pull in less outdoor air. And if you have a really good high-MERV or HEPA filtration system, you can ventilate less.
The COVID-19 pandemic has been an eye-opener. Indoor air quality is critical in helping us get through it (as is the vaccine), but let’s not let it stop there. Let’s use the pandemic as a springboard to make permanent, long-term changes in how we deal with indoor air quality. The answer, as I’ve said before, isn’t electronic air cleaners of dubious effectiveness. It’s source control, ventilation, and filtration. And to get the most out of mechanical ventilation in our homes, we need higher ventilation rates.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.
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Hi Allison!
It might be easy for one to assume that the mere presence of a mechanical ventilation system with heat recovery (and moisture recovery), designed to meet any of the standards mentioned in your blog and commissioned to avoid pressure imbalance, would automatically produce acceptable indoor air quality.
However, the duct distribution system is important. If fresh, filtered supply air is delivered to one central location (hall, great room, etc), then bedrooms (especially with closed doors) can still see unacceptably high levels of CO2.
And continuously running ERVs connected to the ductwork of a furnace, while economical, creates the dilemma of having to run the furnace blower 24/7 to maintain pressure in the ductwork, as well as ducts sized for the heating/cooling loads, not the ventilation loads.
Optimally, a standalone, dedicated supply and return duct work for a whole house ERV can: – provide the intended ACH;
– direct fresh, filtered air to sleeping areas and other closed-off rooms;
– extract the required amount of stale/moist air from “wet” rooms;
– boost when needed for “high-pollution events”;
– be controlled on a schedule according to occupancy;
– use as little energy as possible to achieve desired flow rates.
I wonder if you will monitor and publish CO2 levels in various places in your house to compare pre-ERV and post-ERV installation conditions. It would be interesting to compare, especially as you tighten your building enclosure.
Best,
John Rockwell
John, those are all excellent points. Yes, I’ll be monitoring CO2 levels around the house. And when I monitor stuff, I usually write about it.
I would strongly recommend using the acoustic CO2 sensors. A sealed house with natural materials can have low VOC, not feel stale, but still have high CO2. Cheaper CO2 meters will use VOC levels as a proxy for lack of outside air, or use FLIR and are erratic and super sensitive. I was working on CO2 mitigation at work, and had picked up one of those new acoustic chip CO2 meters to meet some stringent certification code, decided to try it out at home and was shocked. Calibrated at work with a known 750 ppm source, was reading 1090 to 1200 in bedrooms. I had taken so much care to seal and insulate, I had robbed myself of any fresh air in the winter. I am installing a standalone ventilator and exhaust, HRU, and a 1.5 KW variable output in-duct heater. I have a veris CO2 in duct meter to go into the exhaust. It’s 0-10 VDC output will turn on a RIB sequencing relay to start fans, heat will control to a base 65F discharge temp if needed after HRU coil. I am using a pair of variable speed AC infinity fans to balance out 80CFM of outside air, 20 CFM per occupant, and will trim it slightly positively pressured to maintain that zero infiltration. The Veris and RIB should keep it +/- 50 on 500PPM. Filters on supply and exhaust to keep the coils clean.
Thanks, I didn’t know there were different kinds. Did some quick googling and it’s not easy to tell what tech they are. Can you list a couple of examples of ones with acoustic CO2? I tried a veris since you mentioned it and it was Dual beam on-dispersive infrared (NDIR), diffusion sampling.
I’ve airsealed my 1907, 1300 sq ft home w/ 3 bedrooms, 2 adults and 1 dog down to just under 7 ACH50. We have great spot ventilation, but no whole home mechanical ventilation. If the windows and doors are closed for over 24 hours our C02 hovers around 1250 and easily goes over 1500 at times. Today I decided to do an experiment where I just leave the bath fan on (66 cfm) all day. So far our C02 has stayed under 800. Given that we’re also operating at slightly net positive energy, I think we can afford the energy hit and should likely just leave that fan on 24/7!
Hi Ryan. I assume your CO2 readings are in the bedroom where 2 adults sleep?
At 1,300 SF and 66cfm, your ACH (assuming a “breathing zone” height of 2.5m) is 0.37. But it is unlikely that rate applies to all areas of your home. If your bath fan exhausts 66cfm, then you have uncontrolled infiltration of 66cfm/3,960 cubic feet per hour/95,040 cubic feet per day. Perhaps most of that comes in via a chimney, dryer vent or range hood? Or maybe areas where air sealing was inaccessible?
I don’t know your climate zone, but if you are in a humid climate, you might consider the possibility of warm, moist (and possibly dirty) air infiltrating and condensing on surfaces that are at or below dew point – possibly within inaccessible wall assemblies.
It’s great that you are net positive, but also be aware of the effects of uncontrolled infiltration.
Hey John, The C02 readings are actually taken from the common areas of the home. We’re in the Portland, OR so we’re in a mixed humid climate zone (4C) which is unique in that we have dry summers and humid winters. Our overnight lows in the winters tend to only go down into the 30’s and our summertime highs in the summer have been getting hotter (115F is the new record we just set!) The infiltration always comes from the path of least resistance and I haven’t bothered to trace it down.
Allison, did you switch camps because of Covid-19 (awareness of virus spread), or because you measured unacceptable CO2 levels with a less aggressive ventilation rate, or perhaps both? I’m still firmly in Joe’s camp on this. My house tested out at 0.52 ACH50, but with only 2 people and 3300 ft2 and I’ve never felt the need to ventilate, except for the initial months after construction when I used window fans to eliminate odors and chemicals that outgas from construction materials.
OTOH, I don’t have a C02 monitor. I noticed the C02 readout on the UltraAir display at the top of your article. I wasn’t aware that’s a thing now. I wonder how accurate it is (or any or the new wave home IAQ monitors that are out there). Of course, that’s a rhetorical question until someone publishes a study based on comparisons with a laboratory grade instrument. That’s what I’m waiting on before I spring for one.
David, yes, COVID-19 is definitely a big part of the reason for my change of position on ventilation rates. But it’s not just that because high-MERV filtration can remove a lot of the virus-laden particles. Since I’ve started watching CO2 rates, I’ve realized how difficult it is to get air changes in a house. Even leaky houses don’t do it well because you’re relying a lot on wind and stack effect.
Yeah, the need for ventilation varies with the situation. Two people in a large house don’t need much, as is the case in your house. But what if you have a dinner party? Or sell the house to a family of 6? In smaller houses or apartments with several people, ventilation is more important because there’s less total air in the house and a higher density of pollutants.
Regarding the accuracy of CO2 monitors, yeah, they’re not laboratory-grade instruments. But it’s easy to check and calibrate them by taking them outdoors. If you leave it outdoors for a while and it’s reading 600 ppm, that’s not good. If it’s close to 400 ppm, it’s OK.
Allison wrote:
> In smaller houses or apartments with several people, ventilation is more important because there’s less total air in the house and a higher density of pollutants.
Absolutely. And when the time comes to sell my home, I intend to discuss ventilation options with the buyer, and it will be covered in my yet-to-be written House Operation Manual (shouldn’t every house have one?). My point was not that ventilation is never necessary, but simply that I remain in Joe’s camp in terms of degree.
More than that, I agree with Todd’s point… that we need smart ventilation that can react to conditions, e.g., C02 at a minimum. Designing 24/7 ventilation to address the worst possible occupancy or contaminant scenarios works against all the work we’ve done to reduce the energy loads and address HVAC oversizing.
> 1 cfm for each 100 square feet of conditioned floor area plus 7.5 cfm per person
Used in a small closed off room, this will result in > 1500 ppm of CO2. Which has well documented negative effects on sleep and cognitive performance.
What I am seeing in the comments but also relevant to the writeup is that ventilation needs to be “smarter”. For example, some ventilation systems have the ability to measure CO2, VOC’s and humidity … and then leverage the sensor readings to drive ventilation rates (volume). Running a ventilation system all the time, e.g. in the winter, or on a set schedule of x minutes per hour, etc. can be and often is a real waste of energy. For example, if everyone living at the house have left the home (work, school, errands, vacations, etc), ventilation is not necessary as the sensor readings are likely going to be lower than a set threshold or when ventilation is needed. “Smart” ventilation leverages the sensor readings to tell the ventilation system to bring in fresh air only when fresh air is needed and not all the time. Think about schools … the CO2 in a classroom can skyrocket when kids are in class lowering the cognition capability of the children within the classroom. However, the classroom is only occupied for 7ish hours a day. Smart ventilation will clear out the CO2 when it is needed and not 24/7. All manufacturers need to consider how they are going to solve this problem with smart sensors. There are already companies that have this capability to include BuildEquinox in IL.
Todd Collins is right on the mark. Thanks to Allison I got an Awair monitor and found that CO2 in our bedroom would go from 600 to 1600 overnight. I may have done too good a job sealing my 1903 Victorian. Now, if I could control ventilation with my Awair I might fix this. Now my option is endless manual control. Progress is partly the number of things you do not have to think about
About “smart” ventilation…
Another article about BSC-001 summarizes: “6. There’s no provision for ventilation based on sensors of CO2, relative humidity, or any other indoor air components.”
In BSC info sheet 001, I saw: “..Performance Criteria stipulates that the 62.2 ventilation flow rate be delivered at least one-third of the time and that whole-house distribution is required. ”
Are there known arguments AGAINST smart ventilation? Perhaps related to unintended consequences, sensor reliability, or practicalities of getting them implemented correctly?
Hi Allison,
Just wondering if you have a make and model of the type of Ultra Aire controller that you display at the top of the of this article.
Hi Allison,
I’ve been suggesting higher rates for sometime now. I’ve been suggesting to my customers that based on performance and absentee rates both in schools and in the work place that they should consider a system that can run on low to meet .35 ach and on high up to 1 ACH. It is interesting to note that John Billings, father of the ASHVE standard, thought that 60 cfm per person was a rate to consider if one wanted to be healthy. He thought that not less than 30 cfm p/person would be minimum. That’s back in 1893. Yet in the industry I’ve just gotten tired of arguing with the engineers that too much ventilation is bad. Figured when we got enough dead people that things would start to change. Maybe, just maybe, we have enough dead people now. To answer the question said in large meetings “Where are all the dead people?”, well, we bury them Joe, so we don’t hear from them. Those costs are priceless…. 🙁
There’s more to the story than just CO2 though. There’s humidity and condensation. And basic outgassing of chemicals. Those were a significant part of why I wanted whole house ventilation, so I didn’t have to worry so much about what was brought into the house, and to ensure there is no condensation with a very tight house. Admittedly with no people in the house you will be generating less humidity, and a humidity measurement could be used along with the CO2. But I’d still want some air flow when not home to keep ubiquitous chemicals from building up.
I would love to monitor it when I’m away for 1-2 months, but I still don’t think I’d turn it all the way off.