high CO2 in a leaky house

Hi, folks! I've been lurking on this forum for some time without a membership because my Efficiency Kansas certification ran out years ago and I never pursued other certification. I haven't done a full energy audit myself in years, but I try to stay current with the field, so thank you for letting me lurk.

After doing a bunch of air sealing and insulation in my previous house in Kansas, I was concerned the house might be too tight, so I bought a uHoo air quality monitor (uhooair.com) which monitors temperature, humidity, air pressure, CO, CO2, TVOCs, particulates, NO2, and ozone. It found nothing interesting in that house (aside from raising a red alert when I once brought home a ripe cantaloupe!), so I had it in my in-laws' house for a year or so, but without any control over the situation there, I learned to ignore it.

When we moved to Omaha last year, I put the uHoo in the bedroom of our drafty 100-year-old apartment, and immediately it started freaking out about TVOCs and CO2 spiking each night when my wife and I and our two dogs would come into the bedroom. Whenever we would leave the room in the morning, these two factors would go back to baseline levels (below 400 ppb for TVOC and around 400 ppm for CO2). So we had met the enemy and it was us, but we figured when we moved to a house and had more space, that would resolve.

So we moved to our 95 year old house in June, and initially I put the uHoo in the finished attic because it smelled like dust and varnish to me up there, and I was curious about the particulates and VOCs. But it was unimpressed by the particulates; it continued to detect spikes in TVOC (as high as 1200 ppb) and CO2 (regularly 1000 but as high as 1800 ppm) every evening around the time my wife got home from work (I work from home) and every morning when we wake up, in spite of being sealed upstairs with no ductwork connecting it to downstairs.

I had a local energy auditor come and check the place over; he found plenty of air leakage and suggested that maybe the water heater was venting into the attic (brilliant!), so I had that checked out, but no, it is not. A friend suggested nocturnal animals might be living in the attic, but there's been no evidence to back that up. After a few more weeks of puzzling over the data from upstairs, I moved the uHoo to the living room, where I work during the day. Now it's showing high CO2 and sometimes TVOC all day long, as long as I'm in the house. As soon as all people leave the house, even if the dogs and cat are still here, the CO2 goes back toward baseline. If we go away for a weekend, the CO2 stays low until we return. The presence and absence of people in the house seems to be the one consistent factor in these readings.

Now, the alerts that I get from the monitor say I should open windows or add ventilation, but the energy auditor says the house is too leaky to benefit from ventilation and I should seal existing leaks first. I'm hesitant to seal any leaks when I'm already swimming in CO2. I've experimented with opening windows when it's not too hot out, but while that gets the CO2 under control, it lets in a ton of humidity that the AC won't remove unless I turn the thermostat way down.

I'm curious to know what you think about this situation. What am I missing?

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  • Thanks for circling back Ben! We all wanted to know the solution. :)


  • Hi, folks! I'm just following up on this post because our problem is now solved! It was the basement floor drain after all. We had the drain replaced, and both the high VOC and high CO2 readings stopped immediately. Thanks to all of you for your feedback on the question.

  • Simple solution is to just add some plants.
    Building Science forgets about the wonders of nature. NASA has a list of the best plants for IAQ. Many plants will also aid with many potential IAQ issues like voc''s. I have been recommending and preaching the benifits into all my trainings over the years. The best air filtration device on the planet and is 100% energy free.
    This is the real GREEN new deal. Have fun with you new science experiment.
    • As I've said above, I already have plants. They only take in net CO2 when the sun is shining. At night they exhale CO2 just like we do. They may in fact be contributing to the nighttime high CO2 levels in the house.

  • I don't know if I would buy more costly monitors, your readings are in-line with CO2 levels from being in a closed bedroom with people living and no ventilation. Put your money into a ventilation system and making your house tight.

    • What is discordant is that the CO2 in the Living Room is high even when the OP is home alone during the day, yet quickly goes down to normal levels when people leave. If the CO2 level drops fairly quickly, then the house must be well ventilated.

      Everyone is saying to put in more ventilation, but how much ventilation is needed? What is going to control the ventilation rate?

      • One  of the issues with the jump in readings - may also be related to the instrument design and its sensors.   If the CO2 reading is via an electrochemical sensor and not a nondispersive infrared sensor (NDIR), then its possible that it is really responding to deodorants, perfumes, or food - even a cup of coffee that is sitting within a few feet of it.   For example I have an IAQ sensor that is indeed chemical sensing (VOC) - but reports the output as CO2... (because that's what people want --  goes circular real fast - people want CO2 because VOC sensing is hard)  anyway - it will respond and start climbing as I enter the room if I've used a couple of swipes of Right Guard deodorant/antiperspirant.   It climbs if I open up a can and it sits on the other side of the desk. 

        What is lacking in the description of his problem are things like the size of the room,  the distance to the sensor,  other activities that could raise the sensor,  how leaky the envelop is... how does the sensor respond when window is open (say 1")...

        Unfortunately - we don't know exactly which sensors are used in the uHoo,  but I do know when I've tested and experimented with some of the "MQ" sensors that are out there and often used in the lowest cost consumer products - the variation from device to device is significant -- AND -- worse the response to gases other than the specific one they are sold to monitor can be nearly as large as that specific gas they were sold for.  They are often sensitive to temperature and humidity changes... making the design of the air flow through the device important.

        Dr Walker pointed out some of the tests they've seen when compared to calibrated laboratory equipment -- and from what I can see... it looks like the accuracy of the device is really suspect.

        The largest benefit is to essentially warn potential users of any of the consumer IAQ is its buyer beware -- unless they specify accuracy, sensitivity, types of sensors, and expected drift...very few do that.

        Given all the unknowns for the arrangement in the house, the accuracy of the sensor package and the concern about the IAQ ... really the biggest focus should be on just getting more fresh air - that would allow Ben to compare before and after changes with the same device - understanding - that the drift on the instrument is still not determined.    But he could use it to see if there is progress.

        Controlling the ventilation in a house might be possible if the sensor is indeed one of the newer stable versions.  Sensirion  makes a newer RH/T and NDIR compbined sensor that is pretty stable... but I haven't seen in made available to control ventilation equipment for homes - in part because demand responsive ventilation has not yet been accepted as part of 62.2.... it could be done,  but the proof that it works in all cases is not yet there.   Worse is that you'd get low end sensor folks use the MQ series stuff that would be worse than just opening up a window...

        Every case where i've been able to experiment and make it works - depends on using an HRV/ERV with a better quality sensor (like the sensirion) to adjust the airflow and balance.   Which seems to defeat the "low cost" option in Ben's comments.

      • I guess the word "quickly" may be misleading. It looks like a precipitous drop on the daily graph, but for example yesterday the CO2 readings went from 1078 ppm when we woke to 911 in under an hour when my wife left the house, but then after I left at noon it took 4 hours to drop to a low of 643 ppm. When we got back at 8pm it quickly jumped from 654 to 922 at 10pm.

        I was thinking I would manually control the ventilation rate. I'd just open a window if it wouldn't let in all the humidity and pollen.

        • I keep going back and pushing an ERV,  because they give you the ventilation, and you can have filtering on them to keep the pollen and dust outside... while also more or less maintaining the current RH balance.  If you tighten up the building envelop -- which would make a big difference in the winter time heating bills - you'd definitely  need some kind of fresh air system (ERV/HRV)..

          Furnace is forced air?   Hot water heater is a draft gas heater?  And where is the clothes dryer located - inside living space or garage? A good fresh air ventilation system can bring in air across a high MERV filter to remove nearly all of the pollen, and if you add carbon activated filter it can help reduce some of the smells coming in.  That will also reduce the perception of smells in the house --- dusty, etc. 

  • There is some good information in these posts, mixed in with a lot of misinformation, conjecture and speculation that does not consider the facts provided by the OP.

    I looked on the Uhoo website and could not find any information on what sensors are in the unit. I do a lot of IAQ testing with calibrated sensors. I bought two Foobots last year, which is similar to the Uhoo and roughly the same price range. The Foobot measures CO2 using an algorithm. I have found the CO2 readings to be very erratic and very different from the reading of a calibrated CO2 meter. I suspect that the Uhoo has the same problem. For the price, the Uhoo cannot have an individual sensor for each of its readings.

    My 1,100 sq.ft. house in Central Vermont (8,300 HDD avg./year) has an average leakage rate (about 10 ACH50 or 1 ACHnat). In the middle of winter when only I am home all day, the CO2 may rise to about 800-1,000 ppm, which is normal and within any guidelines. As one post noted, RH is another indicator of air leakiness, assuming that moisture sources in the house are reasonable (basement is not a notable source, some plants, a few people and a pet or two).

    It appears that a single source of CO2 has been ruled out. If you are still concerned, buy a dedicated, calibrated CO2 meter.

    There are no published standards for CO2, that I know of, only guidelines. In a classroom setting, levels should be kept below about 1,000 ppm. Levels above roughly 1,500 ppm can make you feel a bit drowsy, especially with longer exposure times.

    "If you don't test, you guess"

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